There is no consensus of opinion on the origin of the large rate accelerations observed for enzyme-catalyzed hydride transfer. The interpretation of recent results from studies on hydride transfer reactions catalyzed by alcohol dehydrogenase (ADH) focus on the proposal that the effective barrier height is reduced by quantum-mechanical tunneling through the energy barrier. This interpretation contrasts sharply with the notion that enzymatic rate accelerations are obtained through direct stabilization of the transition state for the nonenzymatic reaction in water. The binding energy of the dianion of substrate DHAP provides 11 kcal mol−1 stabilization of the transition state for the hydride transfer reaction catalyzed by glycerol-3-phosphate dehydrogenase (GPDH). We summarize evidence that the binding interactions between (GPDH) and dianion activators are utilized directly for stabilization of the transition state for enzyme-catalyzed hydride transfer. The possibility is considered, and then discounted, that these dianion binding interactions are utilized for the stabilization of a tunnel ready state (TRS) that enables efficient tunneling of the transferred hydride through the energy barrier, and underneath the energy maximum for the transition state. It is noted that the evidence to support the existence of a tunnel-ready state for the hydride transfer reactions catalyzed by ADH is ambiguous. We propose that the rate acceleration for ADH is due to the utilization of the binding energy of the cofactor NAD+/NADH in the stabilization of the transition state for enzyme-catalyzed hydride transfer.

The stabilization of the transition state for hlGPDH-catalyzed reduction of DHAP due to the action of the phosphodianion of DHAP and the cationic side chain of R269 is between 12.4 and 17 kcal/mol. The R269A mutation of glycerol-3-phosphate dehydrogenase (hlGPDH) results in a 9.1 kcal/mol destabilization of the transition state for enzyme-catalyzed reduction of dihydroxyacetone phosphate (DHAP) by NADH, and there is a 6.7 kcal/mol stabilization of this transition state by 1.0 M guanidine cation (Gua+) [J. Am. Chem. Soc. 2015, 137, 5312–5315]. The R269A mutant shows no detectable activity toward reduction of glycolaldehyde (GA), or activation of this reaction by 30 mM HPO32–. We report the unprecedented self-assembly of R269A hlGPDH, dianions (X2– = FPO32–, HPO32–, or SO42–), Gua+ and GA into a functioning catalyst of the reduction of GA, and fourth-order reaction rate constants kcat/KGAKXKGua. The linear logarithmic correlation (slope = 1.0) between values of kcat/KGAKX for dianion activation of wildtype hlGPDH-catalyzed reduction of GA and kcat/KGAKXKGua shows that the electrostatic interaction between exogenous dianions and the side chain of R269 is not significantly perturbed by cutting hlGPDH into R269A and Gua+ pieces. The advantage for connection of hlGPDH (R269A mutant + Gua+) and substrate pieces (GA + HPi) pieces, (ΔGS‡)HPi+E+Gua = 5.6 kcal/mol, is nearly equal to the sum of the advantage to connection of the substrate pieces, (ΔGS‡)GA+HPi = 3.3 kcal/mol, for wildtype hlGPDH-catalyzed reaction of GA + HPi, and for connection of the enzyme pieces, (ΔGS‡)E+Gua = 2.4 kcal/mol, for Gua+ activation of the R269A hlGPDH-catalyzed reaction of DHAP.

Primary deuterium kinetic isotope effects (1°DKIE) on (kcat/KGA, M–1 s–1) for dianion (X2–) activated hydride transfer from NADL to glycolaldehyde (GA) catalyzed by glycerol-3-phosphate dehydrogenase were determined over a 2100-fold range of enzyme reactivity: (X2–, 1°DKIE); FPO32–, 2.8 ± 0.1; HPO32–, 2.5 ± 0.1; SO42–, 2.8 ± 0.2; HOPO32–, 2.5 ± 0.1; S2O32–, 2.9 ± 0.1; unactivated; 2.4 ± 0.2. Similar 1°DKIEs were determined for kcat. The observed 1°DKIEs are essentially independent of changes in enzyme reactivity with changing dianion activator. The results are consistent with (i) fast and reversible ligand binding; (ii) the conclusion that the observed 1°DKIEs are equal to the intrinsic 1°DKIE on hydride transfer from NADL to GA; (iii) similar intrinsic 1°DKIEs on GPDH-catalyzed reduction of the substrate pieces and the whole physiological substrate dihydroxyacetone phosphate. The ground-state binding interactions for different X2– are similar, but there are large differences in the transition state interactions for different X2–. The changes in transition state binding interactions are expressed as changes in kcat and are proposed to represent changes in stabilization of the active closed form of GPDH. The 1°DKIEs are much smaller than observed for enzyme-catalyzed hydrogen transfer that occurs mainly by quantum-mechanical tunneling.

The side chains of R269 and N270 interact with the phosphodianion of dihydroxyacetone phosphate (DHAP) bound to glycerol 3-phosphate dehydrogenase (GPDH). The R269A, N270A, and R269A/N270A mutations of GPDH result in 9.1, 5.6, and 11.5 kcal/mol destabilization, respectively, of the transition state for GPDH-catalyzed reduction of DHAP by the reduced form of nicotinamide adenine dinucleotide. The N270A mutation results in a 7.7 kcal/mol decrease in the intrinsic phosphodianion binding energy, which is larger than the 5.6 kcal/mol effect of the mutation on the stability of the transition state for reduction of DHAP; a 2.2 kcal/mol stabilization of the transition state for unactivated hydride transfer to the truncated substrate glycolaldehyde (GA); and a change in the effect of phosphite dianion on GPDH-catalyzed reduction of GA, from strongly activating to inhibiting. The N270A mutation breaks the network of hydrogen bonding side chains, Asn270, Thr264, Asn205, Lys204, Asp260, and Lys120, which connect the dianion activation and catalytic sites of GPDH. We propose that this disruption dramatically alters the performance of GPDH at these sites.

Kinetic parameters are reported for the reactions of whole substrates (kcat/Km, M–1 s–1) (R)-glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP) and for the substrate pieces [(kcat/Km)E·HPi/Kd, M–2 s–1] glycolaldehyde (GA) and phosphite dianion (HPi) catalyzed by the I172A/L232A mutant of triosephosphate isomerase from Trypanosoma brucei brucei (TbbTIM). A comparison with the corresponding parameters for wild-type, I172A, and L232A TbbTIM-catalyzed reactions shows that the effect of I172A and L232A mutations on ΔG⧧ for the wild-type TbbTIM-catalyzed reactions of the substrate pieces is nearly the same as the effect of the same mutations on TbbTIM previously mutated at the second side chain. This provides strong evidence that mutation of the first hydrophobic side chain does not affect the functioning of the second side chain in catalysis of the reactions of the substrate pieces. By contrast, the effects of I172A and L232A mutations on ΔG⧧ for wild-type TbbTIM-catalyzed reactions of the whole substrate are different from the effect of the same mutations on TbbTIM previously mutated at the second side chain. This is due to the change in the rate-determining step that determines the barrier to the isomerization reaction. X-ray crystal structures are reported for I172A, L232A, and I172A/L232A TIMs and for the complexes of these mutants to the intermediate analogue phosphoglycolate (PGA). The structures of the PGA complexes with wild-type and mutant enzymes are nearly superimposable, except that the space opened by replacement of the hydrophobic side chain is occupied by a water molecule that lies ∼3.5 Å from the basic side chain of Glu167. The new water at I172A mutant TbbTIM provides a simple rationalization for the increase in the activation barrier ΔG⧧ observed for mutant enzyme-catalyzed reactions of the whole substrate and substrate pieces. By contrast, the new water at the L232A mutant does not predict the decrease in ΔG⧧ observed for the mutant enzyme-catalyzed reactions of the substrate piece GA.

The kinetic parameters for activation of yeast triosephosphate isomerase (ScTIM), yeast orotidine monophosphate decarboxylase (ScOMPDC), and human liver glycerol 3-phosphate dehydrogenase (hlGPDH) for catalysis of reactions of their respective phosphodianion truncated substrates are reported for the following oxydianions: HPO32–, FPO32–, S2O32–, SO42– and HOPO32–. Oxydianions bind weakly to these unliganded enzymes and tightly to the transition state complex (E·S‡), with intrinsic oxydianion Gibbs binding free energies that range from −8.4 kcal/mol for activation of hlGPDH-catalyzed reduction of glycolaldehyde by FPO32– to −3.0 kcal/mol for activation of ScOMPDC-catalyzed decarboxylation of 1-β-d-erythrofuranosyl)orotic acid by HOPO32–. Small differences in the specificity of the different oxydianion binding domains are observed. We propose that the large −8.4 kcal/mol and small −3.8 kcal/mol intrinsic oxydianion binding energy for activation of hlGPDH by FPO32– and S2O32–, respectively, compared with activation of ScTIM and ScOMPDC reflect stabilizing and destabilizing interactions between the oxydianion −F and −S with the cationic side chain of R269 for hlGPDH. These results are consistent with a cryptic function for the similarly structured oxydianion binding domains of ScTIM, ScOMPDC and hlGPDH. Each enzyme utilizes the interactions with tetrahedral inorganic oxydianions to drive a conformational change that locks the substrate in a caged Michaelis complex that provides optimal stabilization of the different enzymatic transition states. The observation of dianion activation by stabilization of active caged Michaelis complexes may be generalized to the many other enzymes that utilize substrate binding energy to drive changes in enzyme conformation, which induce tight substrate fits.

The side chain cation of R269 lies at the surface of l-glycerol 3-phosphate dehydrogenase (GPDH) and forms an ion pair to the phosphodianion of substrate dihydroxyacetone phosphate (DHAP), which is buried at the nonpolar protein interior. The R269A mutation of GPDH results in a 110-fold increase in Km (2.8 kcal/mol effect) and a 41 000-fold decrease in kcat (6.3 kcal/mol effect), which corresponds to a 9.1 kcal/mol destabilization of the transition state for GPDH-catalyzed reduction of DHAP by NADH. There is a 6.7 kcal/mol stabilization of the transition state for the R269A mutant GPDH-catalyzed reaction by 1.0 M guanidinium ion, and the transition state for the reaction of the substrate pieces is stabilized by an additional 2.4 kcal/mol by their covalent attachment at wildtype GPDH. These results provide strong support for the proposal that GPDH invests the 11 kcal/mol intrinsic phosphodianion binding energy of DHAP in trapping the substrate at a nonpolar active site, where strong electrostatic interactions are favored, and obtains a 9 kcal/mol return from stabilizing interactions between the side chain cation and transition state trianion. We propose a wide propagation for the catalytic motif examined in this work, which enables strong transition state stabilization from enzyme–phosphodianion pairs.

The side chains of Y208 and S211 from loop 7 of triosephosphate isomerase (TIM) form hydrogen bonds to backbone amides and carbonyls from loop 6 to stabilize the caged enzyme–substrate complex. The effect of seven mutations [Y208T, Y208S, Y208A, Y208F, S211G, S211A, Y208T/S211G] on the kinetic parameters for TIM catalyzed reactions of the whole substrates dihydroxyacetone phosphate and d-glyceraldehyde 3-phosphate [(kcat/Km)GAP and (kcat/Km)DHAP] and of the substrate pieces glycolaldehyde and phosphite dianion (kcat/KHPiKGA) are reported. The linear logarithmic correlation between these kinetic parameters, with slope of 1.04 ± 0.03, shows that most mutations of TIM result in an identical change in the activation barriers for the catalyzed reactions of whole substrate and substrate pieces, so that the transition states for these reactions are stabilized by similar interactions with the protein catalyst. The second linear logarithmic correlation [slope = 0.53 ± 0.16] between kcat for isomerization of GAP and Kd⧧ for phosphite dianion binding to the transition state for wildtype and many mutant TIM-catalyzed reactions of substrate pieces shows that ca. 50% of the wildtype TIM dianion binding energy, eliminated by these mutations, is expressed at the wildtype Michaelis complex, and ca. 50% is only expressed at the wildtype transition state. Negative deviations from this correlation are observed when the mutation results in a decrease in enzyme reactivity at the catalytic site. The main effect of Y208T, Y208S, and Y208A mutations is to cause a reduction in the total intrinsic dianion binding energy, but the effect of Y208F extends to the catalytic site.

The caged complex between orotidine 5′-monophosphate decarboxylase (ScOMPDC) and 5-fluoroorotidine 5′-monophosphate (FOMP) undergoes decarboxylation ∼300 times faster than the caged complex between ScOMPDC and the physiological substrate, orotidine 5′-monophosphate (OMP). Consequently, the enzyme conformational changes required to lock FOMP at a protein cage and release product 5-fluorouridine 5′-monophosphate (FUMP) are kinetically significant steps. The caged form of ScOMPDC is stabilized by interactions between the side chains from Gln215, Tyr217, and Arg235 and the substrate phosphodianion. The control of these interactions over the barrier to the binding of FOMP and the release of FUMP was probed by determining the effect of all combinations of single, double, and triple Q215A, Y217F, and R235A mutations on kcat/Km and kcat for turnover of FOMP by wild-type ScOMPDC; its values are limited by the rates of substrate binding and product release, respectively. The Q215A and Y217F mutations each result in an increase in kcat and a decrease in kcat/Km, due to a weakening of the protein–phosphodianion interactions that favor fast product release and slow substrate binding. The Q215A/R235A mutation causes a large decrease in the kinetic parameters for ScOMPDC-catalyzed decarboxylation of OMP, which are limited by the rate of the decarboxylation step, but much smaller decreases in the kinetic parameters for ScOMPDC-catalyzed decarboxylation of FOMP, which are limited by the rate of enzyme conformational changes. By contrast, the Y217A mutation results in large decreases in kcat/Km for ScOMPDC-catalyzed decarboxylation of both OMP and FOMP, because of the comparable effects of this mutation on rate-determining decarboxylation of enzyme-bound OMP and on the rate-determining enzyme conformational change for decarboxylation of FOMP. We propose that kcat = 8.2 s–1 for decarboxylation of FOMP by the Y217A mutant is equal to the rate constant for cage formation from the complex between FOMP and the open enzyme, that the tyrosyl phenol group stabilizes the closed form of ScOMPDC by hydrogen bonding to the substrate phosphodianion, and that the phenyl group of Y217 and F217 facilitates formation of the transition state for the rate-limiting conformational change. An analysis of kinetic data for mutant enzyme-catalyzed decarboxylation of OMP and FOMP provides estimates for the rate and equilibrium constants for the conformational change that traps FOMP at the enzyme active site.

Values of (kcat/Km)GAP for triosephosphate isomerase-catalyzed reactions of (R)-glyceraldehyde 3-phosphate and kcat/KHPiKGA for reactions of the substrate pieces glycolaldehyde and HPO32– have been determined for wild-type and the following TIM mutants: I172V, I172A, L232A, and P168A (TIM from Trypanosoma brucei brucei); a 208-TGAG for 208-YGGS loop 7 replacement mutant (L7RM, TIM from chicken muscle); and, Y208T, Y208S, Y208A, Y208F and S211A (yeast TIM). A superb linear logarithmic correlation, with slope of 1.04 ± 0.03, is observed between the kinetic parameters for wild-type and most mutant enzymes, with positive deviations for L232A and L7RM. The unit slope shows that most mutations result in an identical change in the activation barriers for the catalyzed reactions of whole substrate and substrate pieces, so that the two transition states are stabilized by similar interactions with the protein catalyst. This is consistent with a role for dianions as active spectators, which hold TIM in a catalytically active caged form.

The mechanism for activation of orotidine 5′-monophosphate decarboxylase (OMPDC) by interactions of side chains from Gln215 and Try217 at a gripper loop and R235, adjacent to this loop, with the phosphodianion of OMP was probed by determining the kinetic parameters kcat and Km for all combinations of single, double, and triple Q215A, Y217F, and R235A mutations. The 12 kcal/mol intrinsic binding energy of the phosphodianion is shown to be equal to the sum of the binding energies of the side chains of R235 (6 kcal/mol), Q215 (2 kcal/mol), Y217 (2 kcal/mol), and hydrogen bonds to the G234 and R235 backbone amides (2 kcal/mol). Analysis of a triple mutant cube shows small (ca. 1 kcal/mol) interactions between phosphodianion gripper side chains, which are consistent with steric crowding of the side chains around the phosphodianion at wild-type OMPDC. These mutations result in the same change in the activation barrier to the OMPDC-catalyzed reactions of the whole substrate OMP and the substrate pieces (1-β-d-erythrofuranosyl)orotic acid (EO) and phosphite dianion. This shows that the transition states for these reactions are stabilized by similar interactions with the protein catalyst. The 12 kcal/mol intrinsic phosphodianion binding energy of OMP is divided between the 8 kcal/mol of binding energy, which is utilized to drive a thermodynamically unfavorable conformational change of the free enzyme, resulting in an increase in (kcat)obs for OMPDC-catalyzed decarboxylation of OMP, and the 4 kcal/mol of binding energy, which is utilized to stabilize the Michaelis complex, resulting in a decrease in (Km)obs.

Two mutations of the phosphodianion gripper loop in chicken muscle triosephosphate isomerase (cTIM) were examined: (1) the loop deletion mutant (LDM) formed by removal of residues 170–173 [Pompliano, D. L., et al. (1990) Biochemistry 29, 3186–3194] and (2) the loop 6 replacement mutant (L6RM), in which the N-terminal hinge sequence of TIM from eukaryotes, 166-PXW-168 (X = L or V), is replaced by the sequence from archaea, 166-PPE-168. The X-ray crystal structure of the L6RM shows a large displacement of the side chain of E168 from that for W168 in wild-type cTIM. Solution nuclear magnetic resonance data show that the L6RM results in significant chemical shift changes in loop 6 and surrounding regions, and that the binding of glycerol 3-phosphate (G3P) results in chemical shift changes for nuclei at the active site of the L6RM that are smaller than those of wild-type cTIM. Interactions with loop 6 of the L6RM stabilize the enediolate intermediate toward the elimination reaction catalyzed by the LDM. The LDM and L6RM result in 800000- and 23000-fold decreases, respectively, in kcat/Km for isomerization of GAP. Saturation of the LDM, but not the L6RM, by substrate and inhibitor phosphoglycolate is detected by steady-state kinetic analyses. We propose, on the basis of a comparison of X-ray crystal structures for wild-type TIM and the L6RM, that ligands bind weakly to the L6RM because a large fraction of the ligand binding energy is utilized to overcome destabilizing electrostatic interactions between the side chains of E168 and E129 that are predicted to develop in the loop-closed enzyme. Similar normalized yields of DHAP, d-DHAP, and d-GAP are formed in LDM- and L6RM-catalyzed reactions of GAP in D2O. The smaller normalized 12–13% yield of DHAP and d-DHAP observed for the mutant cTIM-catalyzed reactions compared with the 79% yield of these products for wild-type cTIM suggests that these mutations impair the transfer of a proton from O-2 to O-1 at the initial enediolate phosphate intermediate. No products are detected for the LDM-catalyzed isomerization reactions in D2O of [1-13C]GA and HPi, but the L6RM-catalyzed reaction in the presence of 0.020 M dianion gives a 2% yield of the isomerization product [2-13C,2-2H]GA.

•Structure–function relationships for the TIM-barrel fold.•Mechanism of action of eponymous triosephosphate isomerase (TIM).•Role of front loops in TIM-catalyzed isomerization.•Design and mechanism of action of a monomeric variant of TIM.•Utilization of dianion binding energy to “mold” distorted loops of mutant TIMs.The TIM-barrel fold is described and its propagation throughout the enzyme universe noted. The functions of the individual front loops of the eponymous TIM-barrel of triosephosphate isomerase are presented in a discussion of: (a) electrophilic catalysis, by amino acid side chains from loops 1 and 4, of abstraction of an α-carbonyl hydrogen from substrate dihydroxyacetone phosphate (DHAP) or d-glyceraldehyde 3-phosphate (DGAP). (b) The engineering of loop 3 to give the monomeric variant monoTIM and the structure and catalytic properties of this monomer. (c) The interaction between loops 6, 7 and 8 and the phosphodianion of DHAP or DGAP. (d) The mechanism by which a ligand-gated conformational change, dominated by motion of loops 6 and 7, activates TIM for catalysis of deprotonation of DHAP or DGAP. (e) The conformational plasticity of TIM, and the utilization of substrate binding energy to “mold” the distorted active site loops of TIM mutants into catalytically active enzymes. The features of the TIM-barrel fold that favor effective protein catalysis are discussed.Graphical abstract

Glu-167 of triosephosphate isomerase from Trypanosoma brucei brucei (TbbTIM) acts as the base to deprotonate substrate to form an enediolate phosphate trianion intermediate. We report that there is a large ∼6 pK unit increase in the basicity of the carboxylate side chain of Glu-167 upon binding of the inhibitor phosphoglycolate trianion (I3-), an analog of the enediolate phosphate intermediate, from pKEH ≈ 4 for the protonated free enzyme EH to pKEHI ≈ 10 for the protonated enzyme–inhibitor complex EH•I3-. We propose that there is a similar increase in the basicity of this side chain when the physiological substrates are deprotonated by TbbTIM to form an enediolate phosphate trianion intermediate and that it makes an important contribution to the enzymatic rate acceleration. The affinity of wildtype TbbTIM for I3- increases 20 000-fold upon decreasing the pH from 9.3 to 4.9, because TbbTIM exists mainly in the basic form E over this pH range, while the inhibitor binds specifically to the rare protonated enzyme EH. This reflects the large increase in the basicity of the carboxylate side chain of Glu-167 upon binding of I3- to EH to give EH•I3-. The I172A mutation at TbbTIM results in an ∼100-fold decrease in the affinity of TbbTIM for I3- at pH < 6 and an ∼2 pK unit decrease in the basicity of the carboxylate side chain of Glu-167 at the EH•I3- complex, to pKEHI = 7.7. Therefore, the hydrophobic side chain of Ile-172 plays a critical role in effecting the large increase in the basicity of the catalytic base upon the binding of substrate and/or inhibitors.

Orotidine 5′-monophosphate decarboxylase catalyzes the decarboxylation of truncated substrate (1-β-d-erythrofuranosyl)orotic acid to form (1-β-d-erythrofuranosyl)uracil. This enzyme-catalyzed reaction is activated by tetrahedral oxydianions, which bind weakly to unliganded OMPDC and tightly to the enzyme-transition state complex, with the following intrinsic oxydianion binding energies (kcal/mol): SO32–, −8.3; HPO32–, −7.7; S2O32–, −4.6; SO42–, −4.5; HOPO32–, −3.0; HOAsO32–, no activation detected. We propose that the oxydianion and orotate binding domains of OMPDC perform complementary functions in catalysis of decarboxylation reactions: (1) The orotate binding domain carries out decarboxylation of the orotate ring. (2) The activating oxydianion binding domain has the cryptic function of utilizing binding interactions with tetrahedral inorganic oxydianions to drive an enzyme conformational change that results in the stabilization of transition states at the distant orotate domain.

The syntheses of two novel truncated analogs of the natural substrate orotidine 5′-monophosphate (OMP) for orotidine 5′-monophosphate decarboxylase (OMPDC) with enhanced reactivity toward decarboxylation are reported: 1-(β-d-erythrofuranosyl)-5-fluoroorotic acid (FEO) and 5′-deoxy-5-fluoroorotidine (5′-dFO). A comparison of the second-order rate constants for the OMPDC-catalyzed decarboxylations of FEO (10 M–1 s–1) and 1-(β-d-erythrofuranosyl)orotic acid (EO, 0.026 M–1 s–1) shows that the vinyl carbanion-like transition state is stabilized by 3.5 kcal/mol by interactions with the 5-F substituent of FEO. The OMPDC-catalyzed decarboxylations of FEO and EO are both activated by exogenous phosphite dianion (HPO32–), but the 5-F substituent results in only a 0.8 kcal stabilization of the transition state for the phosphite-activated reaction of FEO. This provides strong evidence that the phosphite-activated OMPDC-catalyzed reaction of FEO is not limited by the chemical step of decarboxylation of the enzyme-bound substrate. Evidence is presented that there is a change in the rate-limiting step from the chemical step of decarboxylation for the phosphite-activated reaction of EO, to closure of the phosphate gripper loop and an enzyme conformational change at the ternary E•FEO•HPO32– complex for the reaction of FEO. The 4′-CH3 and 4′-CH2OH groups of 5′-dFO and orotidine, respectively, result in identical destabilizations of the transition state for the unactivated decarboxylation of 2.9 kcal/mol. By contrast, the 4′-CH3 group of 5′-dFO and the 4′-CH2OH group of orotidine result in very different 4.7 and 8.3 kcal/mol destabilizations of the transition state for the phosphite-activated decarboxylation. Here, the destabilizing effect of the 4′-CH3 substituent at 5′-dFO is masked by the rate-limiting conformational change that depresses the third-order rate constant for the phosphite-activated reaction of the parent substrate FEO.

Linus Pauling proposed that the large rate accelerations for enzymes are caused by the high specificity of the protein catalyst for binding the reaction transition state. The observation that stable analogues of the transition states for enzymatic reactions often act as tight-binding inhibitors provided early support for this simple and elegant proposal. We review experimental results that support the proposal that Pauling’s model provides a satisfactory explanation for the rate accelerations for many heterolytic enzymatic reactions through high-energy reaction intermediates, such as proton transfer and decarboxylation. Specificity in transition state binding is obtained when the total intrinsic binding energy of the substrate is significantly larger than the binding energy observed at the Michaelis complex. The results of recent studies that aimed to characterize the specificity in binding of the enolate oxygen at the transition state for the 1,3-isomerization reaction catalyzed by ketosteroid isomerase are reviewed. Interactions between pig heart succinyl-coenzyme A:3-oxoacid coenzyme A transferase (SCOT) and the nonreacting portions of coenzyme A (CoA) are responsible for a rate increase of 3 × 1012-fold, which is close to the estimated total 5 × 1013-fold enzymatic rate acceleration. Studies that partition the interactions between SCOT and CoA into their contributing parts are reviewed. Interactions of the protein with the substrate phosphodianion group provide an ∼12 kcal/mol stabilization of the transition state for the reactions catalyzed by triosephosphate isomerase, orotidine 5′-monophosphate decarboxylase, and α-glycerol phosphate dehydrogenase. The interactions of these enzymes with the substrate piece phosphite dianion provide a 6–8 kcal/mol stabilization of the transition state for reaction of the appropriate truncated substrate. Enzyme activation by phosphite dianion reflects the higher dianion affinity for binding to the enzyme–transition state complex compared with that of the free enzyme. Evidence is presented that supports a model in which the binding energy of the phosphite dianion piece, or the phosphodianion group of the whole substrate, is utilized to drive an enzyme conformational change from an inactive open form EO to an active closed form EC, by closure of a phosphodianion gripper loop. Members of the enolase and haloalkanoic acid dehalogenase superfamilies use variable capping domains to interact with nonreacting portions of the substrate and sequester the substrate from interaction with bulk solvent. Interactions of this capping domain with the phenyl group of mandelate have been shown to activate mandelate racemase for catalysis of deprotonation of α-carbonyl carbon. We propose that an important function of these capping domains is to utilize the binding interactions with nonreacting portions of the substrate to activate the enzyme for catalysis.

Triosephosphate isomerase (TIM) catalyzes the isomerization of dihydroxyacetone phosphate to form d-glyceraldehyde 3-phosphate. The effects of two structural mutations in TIM on the kinetic parameters for catalysis of the reaction of the truncated substrate glycolaldehyde (GA) and the activation of this reaction by phosphite dianion are reported. The P168A mutation results in similar 50- and 80-fold decreases in (kcat/Km)E and (kcat/Km)E·HPi, respectively, for deprotonation of GA catalyzed by free TIM and by the TIM·HPO32– complex. The mutation has little effect on the observed and intrinsic phosphite dianion binding energy or the magnitude of phosphite dianion activation of TIM for catalysis of deprotonation of GA. A loop 7 replacement mutant (L7RM) of TIM from chicken muscle was prepared by substitution of the archaeal sequence 208-TGAG with 208-YGGS. L7RM exhibits a 25-fold decrease in (kcat/Km)E and a larger 170-fold decrease in (kcat/Km)E·HPi for reactions of GA. The mutation has little effect on the observed and intrinsic phosphodianion binding energy and only a modest effect on phosphite dianion activation of TIM. The observation that both the P168A and loop 7 replacement mutations affect mainly the kinetic parameters for TIM-catalyzed deprotonation but result in much smaller changes in the parameters for enzyme activation by phosphite dianion provides support for the conclusion that catalysis of proton transfer and dianion activation of TIM take place at separate, weakly interacting, sites in the protein catalyst.

The side chain cation of Arg235 provides a 5.6 and 2.6 kcal/mol stabilization of the transition states for orotidine 5′-monophosphate (OMP) decarboxylase (OMPDC) from Saccharomyces cerevisiae catalyzed reactions of OMP and 5-fluoroorotidine 5′-monophosphate (FOMP), respectively, a 7.2 kcal/mol stabilization of the vinyl carbanion-like transition state for enzyme-catalyzed exchange of the C-6 proton of 5-fluorouridine 5′-monophosphate (FUMP), but no stabilization of the transition states for enzyme-catalyzed decarboxylation of truncated substrates 1-(β-d-erythrofuranosyl)orotic acid and 1-(β-d-erythrofuranosyl) 5-fluorouracil. These observations show that the transition state stabilization results from formation of a protein cation–phosphodianion pair, and that there is no detectable stabilization from an interaction between the side chain and the pyrimidine ring of substrate. The 5.6 kcal/mol side chain interaction with the transition state for the decarboxylation reaction is 50% of the total 11.2 kcal/mol transition state stabilization by interactions with the phosphodianion of OMP, whereas the 7.2 kcal/mol side chain interaction with the transition state for the deuterium exchange reaction is a larger 78% of the total 9.2 kcal/mol transition state stabilization by interactions with the phosphodianion of FUMP. The effect of the R235A mutation on the enzyme-catalyzed deuterium exchange is expressed predominantly as a change in the turnover number kex, whereas the effect on the enzyme-catalyzed decarboxylation of OMP is expressed predominantly as a change in the Michaelis constant Km. These results are rationalized by a mechanism in which the binding of OMP, compared with that for FUMP, provides a larger driving force for conversion of OMPDC from an inactive open conformation to a productive, active, closed conformation.

The E. coli isopentenyl diphosphate isomerase (IDI) catalyzed reaction of isopentenyl diphosphate (IPP) in D2O gives a 66% yield of dimethylallyl diphosphate labeled with deuterium at the (E)-methyl group (d-DMAPP) and a 34% yield of IPP labeled with 1 mol of deuterium at C-2 (d-IPP). This shows that the release to D2O of the initial product of the IDI-catalyzed reaction (d-DMAPP) is slower than its conversion to d-IPP. Product dissociation is therefore rate determining for isomerization of IPP with a rate constant kdis ≈ kcat = 0.08 s–1. The data provide an estimated rate constant of kas = 6 × 103 M–1 s–1 for binding of DMAPP to E. coli IDI that is similar to rate constants determined for the binding of N-protonated 2-amino ethyl diphosphate intermediate analogs to IDI from yeast [Reardon, J. E.; Abeles, R. H. Biochemistry1986, 25, 5609–5616]. We propose that ligand binding to IDI is relatively slow because there is a significant kinetic barrier to reorganization of the initial encounter complex between enzyme, substrate, and an essential Mg2+ to form the Michaelis complex where the metal cation bridges the protein and the substrate diphosphate group.

The exchange for deuterium of the C-6 protons of uridine 5′-monophosphate (UMP) and 5-fluorouridine 5′-monophosphate (F-UMP) catalyzed by yeast orotidine 5′-monophosphate decarboxylase (ScOMPDC) at pD 6.5–9.3 and 25 °C was monitored by 1H NMR spectroscopy. Deuterium exchange proceeds by proton transfer from C-6 of the bound nucleotide to the deprotonated side chain of Lys-93 to give the enzyme-bound vinyl carbanion. The pD–rate profiles for kcat give turnover numbers for deuterium exchange into enzyme-bound UMP and F-UMP of 1.2 × 10–5 and 0.041 s–1, respectively, so that the 5-fluoro substituent results in a 3400-fold increase in the first-order rate constant for deuterium exchange. The binding of UMP and F-UMP to ScOMPDC results in 0.5 and 1.4 unit decreases, respectively, in the pKa of the side chain of the catalytic base Lys-93, showing that these nucleotides bind preferentially to the deprotonated enzyme. We also report the first carbon acid pKa values for proton transfer from C-6 of uridine (pKCH = 28.8) and 5-fluorouridine (pKCH = 25.1) in aqueous solution. The stabilizing effects of the 5-fluoro substituent on C-6 carbanion formation in solution (5 kcal/mol) and at ScOMPDC (6 kcal/mol) are similar. The binding of UMP and F-UMP to ScOMPDC results in a greater than 5 × 109-fold increase in the equilibrium constant for proton transfer from C-6, so that ScOMPDC stabilizes the bound vinyl carbanions, relative to the bound nucleotides, by at least 13 kcal/mol. The pD–rate profile for kcat/Km for deuterium exchange into F-UMP gives the intrinsic second-order rate constant for exchange catalyzed by the deprotonated enzyme as 2300 M–1 s–1. This was used to calculate a total rate acceleration for ScOMPDC-catalyzed deuterium exchange of 3 × 1010 M–1, which corresponds to a transition-state stabilization for deuterium exchange of 14 kcal/mol. We conclude that a large portion of the total transition-state stabilization for the decarboxylation of orotidine 5′-monophosphate can be accounted for by stabilization of the enzyme-bound vinyl carbanion intermediate of the stepwise reaction.

The role of the hydrophobic side chains of Ile-172 and Leu-232 in catalysis of the reversible isomerization of R-glyceraldehyde 3-phosphate (GAP) to dihydroxyacetone phosphate (DHAP) by triosephosphate isomerase (TIM) from Trypanosoma brucei brucei (Tbb) has been investigated. The I172A and L232A mutations result in 100- and 6-fold decreases in kcat/Km for the isomerization reaction, respectively. The effect of the mutations on the product distributions for the catalyzed reactions of GAP and of [1-13C]-glycolaldehyde ([1-13C]-GA) in D2O is reported. The 40% yield of DHAP from wild-type Tbb TIM-catalyzed isomerization of GAP with intramolecular transfer of hydrogen is found to decrease to 13% and to 4%, respectively, for the reactions catalyzed by the I172A and L232A mutants. Likewise, the 13% yield of [2-13C]-GA from isomerization of [1-13C]-GA in D2O is found to decrease to 2% and to 1%, respectively, for the reactions catalyzed by the I172A and L232A mutants. The decrease in the yield of the product of intramolecular transfer of hydrogen is consistent with a repositioning of groups at the active site that favors transfer of the substrate-derived hydrogen to the protein or the oxygen anion of the bound intermediate. The I172A and L232A mutations result in (a) a >10-fold decrease (I172A) and a 17-fold increase (L232A) in the second-order rate constant for the TIM-catalyzed reaction of [1-13C]-GA in D2O, (b) a 170-fold decrease (I172A) and 25-fold increase (L232A) in the third-order rate constant for phosphite dianion (HPO32–) activation of the TIM-catalyzed reaction of GA in D2O, and (c) a 1.5-fold decrease (I172A) and a larger 16-fold decrease (L232A) in Kd for activation of TIM by HPO32– in D2O. The effects of the I172A mutation on the kinetic parameters for the wild-type TIM-catalyzed reactions of the whole substrate and substrate pieces are consistent with a decrease in the basicity of the carboxylate side chain of Glu-167 for the mutant enzyme. The data provide striking evidence that the L232A mutation leads to a ca. 1.7 kcal/mol stabilization of a catalytically active loop-closed form of TIM (EC) relative to an inactive open form (EO).

Triosephosphate isomerase (TIM) catalyzes the stereospecific 1,2-proton shift at dihydroxyacetone phosphate (DHAP) to give (R)-glyceraldehyde 3-phosphate through a pair of isomeric enzyme-bound cis-enediolate phosphate intermediates. The chemical transformations that occur at the active site of TIM were well understood by the early 1990s. The mechanism for enzyme-catalyzed isomerization is similar to that for the nonenzymatic reaction in water, but the origin of the catalytic rate acceleration is not understood. We review the results of experimental work that show that a substantial fraction of the large 12 kcal/mol intrinsic binding energy of the nonreacting phosphodianion fragment of TIM is utilized to activate the active site side chains for catalysis of proton transfer. Evidence is presented that this activation is due to a phosphodianion-driven conformational change, the most dramatic feature of which is closure of loop 6 over the dianion. The kinetic data are interpreted within the framework of a model in which activation is due to the stabilization by the phosphodianion of a rare, desolvated, loop-closed form of TIM. The dianion binding energy is proposed to drive the otherwise thermodynamically unfavorable desolvation of the solvent-exposed active site. This reduces the effective local dielectric constant of the active site, to enhance stabilizing electrostatic interactions between polar groups and the anionic transition state, and increases the basicity of the carboxylate side chain of Glu-165 that functions to deprotonate the bound carbon acid substrate. A rebuttal is presented to the recent proposal [Samanta, M., Murthy, M. R. N., Balaram, H., and Balaram, P. (2011) ChemBioChem 12, 1886–1895] that the cationic side chain of K12 functions as an active site electrophile to protonate the carbonyl oxygen of DHAP.

Mutants of orotidine 5′-monophosphate decarboxylase containing all possible single (Q215A, Y217F, and R235A), double, and triple substitutions of the side chains that interact with the phosphodianion group of the substrate orotidine 5′-monophosphate have been prepared. Essentially the entire effect of these mutations on the decarboxylation of the truncated neutral substrate 1-(β-d-erythrofuranosyl)orotic acid that lacks a phosphodianion group is expressed as a decrease in the third-order rate constant for activation by phosphite dianion. The results are consistent with a model in which phosphodianion binding interactions are utilized to stabilize a rare closed enzyme form that exhibits a high catalytic activity for decarboxylation.

First-order rate constants, determined by 1H NMR, are reported for deuterium exchange between solvent D2O and the α-amino carbon of glycine in the presence of increasing concentrations of carbonyl compounds (acetone, benzaldehyde, and salicylaldehyde) and at different pD and buffer concentrations. These rate data were combined with 1H NMR data that define the position of the equilibrium for formation of imines/iminium ions from addition of glycine to the respective carbonyl compounds, to give second-order rate constants kDO for deprotonation of α-imino carbon by DO−. The assumption that these second-order rate constants lie on linear structure−reactivity correlations between log kOL and pKa was made in estimating the following pKa’s for deprotonation of α-imino carbon: pKa = 22, glycine−acetone iminium ion; pKa = 27, glycine−benzaldehyde imine; pKa ≈ 23, glycine−benzaldehyde iminium ion; and, pKa = 25, glycine−salicylaldehyde iminium ion. The much lower pKa of 17 [Toth, K.; Richard, J. P. J. Am. Chem. Soc. 2007, 129, 3013−3021 ] for carbon deprotonation of the adduct between 5′-deoxypyridoxal (DPL) and glycine shows that the strongly electron-withdrawing pyridinium ion is unique in driving the extended delocalization of negative charge from the α-iminium to the α-pyridinium carbon. This favors carbanion protonation at the α-pyridinium carbon, and catalysis of the 1,3-aza-allylic isomerization reaction that is a step in enzyme-catalyzed transamination reactions. An analysis of the effect of incremental changes in structure on the activity of benzaldehyde in catalysis of deprotonation of glycine shows the carbonyl group electrophile, the 2-O− ring substituent and the cation pyridinium nitrogen of DPL each make a significant contribution to the catalytic activity of this cofactor analogue. The extraordinary activity of DPL in catalysis of deprotonation of α-amino carbon results from the summation of these three smaller effects.

Orotidine 5′-monophosphate decarboxylase (OMPDC) catalyzes the exchange for deuterium from solvent D2O of the C-6 proton of 1-(β-d-erythrofuranosyl)-5-fluorouracil (FEU), a phosphodianion truncated product analog. The deuterium exchange reaction of FEU is accelerated 1.8 × 104-fold by 1 M phosphite dianion (HPO32−). This corresponds to a 5.8 kcal/mol stabilization of the vinyl carbanion-like transition state, which is similar to the 7.8 kcal/mol stabilization of the transition state for OMPDC-catalyzed decarboxylation of a truncated substrate analog by bound HPO32−. These results show that the intrinsic binding energy of phosphite dianion is used in the stabilization of the vinyl carbanion-like transition state common to the decarboxylation and deuterium exchange reactions.

The L232A mutation in triosephosphate isomerase (TIM) from Trypanosoma brucei brucei results in a small 6-fold decrease in kcat/Km for the reversible enzyme-catalyzed isomerization of glyceraldehyde 3-phosphate to give dihydroxyacetone phosphate. In contrast, this mutation leads to a 17-fold increase in the second-order rate constant for the TIM-catalyzed proton transfer reaction of the truncated substrate piece [1-13C]glycolaldehyde ([1-13C]-GA) in D2O, a 25-fold increase in the third-order rate constant for the reaction of the substrate pieces GA and phosphite dianion (HPO32–), and a 16-fold decrease in Kd for binding of HPO32– to the free enzyme. Most significantly, the mutation also results in an 11-fold decrease in the extent of activation of the enzyme toward turnover of GA by bound HPO32–. The data provide striking evidence that the L232A mutation leads to a ca. 1.7 kcal/mol stabilization of a catalytically active loop-closed form of TIM (Ec) relative to an inactive open form (Eo). We propose that this is due to the relief, in L232A mutant TIM, of unfavorable steric interactions between the bulky hydrophobic side chain of Leu-232 and the basic carboxylate side chain of Glu-167, the catalytic base, which destabilize Ec relative to Eo.

Product yields for the reactions of (R)-glyceraldehyde 3-phosphate (GAP) in D2O at pD 7.9 catalyzed by wildtype triosephosphate isomerase from Trypanosoma brucei brucei (Tbb TIM) and a monomeric variant (monoTIM) of this wildtype enzyme were determined by 1H NMR spectroscopy and were compared with the yields determined in earlier work for the reactions catalyzed by TIM from rabbit and chicken muscle [O’Donoghue, A. C., Amyes, T. L., and Richard, J. P. (2005) , Biochemistry44, 2610−2621]. Three products were observed from the reactions catalyzed by TIM: dihydroxyacetone phosphate (DHAP) from isomerization with intramolecular transfer of hydrogen, d-DHAP from isomerization with incorporation of deuterium from D2O into C-1 of DHAP, and d-GAP from incorporation of deuterium from D2O into C-2 of GAP. The yield of DHAP formed by intramolecular transfer of hydrogen decreases from 49% for the muscle enzymes to 40% for wildtype Tbb TIM to 34% for monoTIM. There is no significant difference in the ratio of the yields of d-DHAP and d-GAP for wildtype TIM from muscle sources and Trypanosoma brucei brucei, but partitioning of the enediolate intermediate of the monoTIM reaction to form d-DHAP is less favorable ((kC1)D/(kC2)D = 1.1) than for the wildtype enzyme ((kC1)D/(kC2)D = 1.7). Product yields for the wildtype Tbb TIM and monoTIM-catalyzed reactions of glycolaldehyde labeled with carbon-13 at the carbonyl carbon ([1-13C]-GA) at pD 7.0 in the presence of phosphite dianion and in its absence were determined by 1H NMR spectroscopy [Go, M. K., Amyes, T. L., and Richard, J. P. (2009) Biochemistry48, 5769−5778]. There is no detectable difference in the yields of the products of wildtype muscle and Tbb TIM-catalyzed reactions of [1-13C]-GA in D2O. The kinetic parameters for phosphite dianion activation of the reactions of [1-13C]-GA catalyzed by wildtype Tbb TIM are similar to those reported for the enzyme from rabbit muscle [Amyes, T. L. and Richard, J. P. (2007) Biochemistry46, 5841−5854], but there is no detectable dianion activation of the reaction catalyzed by monoTIM. The engineered disruption of subunit contacts at monoTIM causes movement of the essential side chains of Lys-13 and His-95 away from the catalytic active positions. We suggest that this places an increased demand that the intrinsic binding energy of phosphite dianion be utilized to drive the change in the conformation of monoTIM back to the active structure for wildtype TIM.

d-Xylose isomerase (XI) and triosephosphate isomerase (TIM) catalyze the aldose–ketose isomerization reactions of d-xylose and d-glyceraldehyde 3-phosphate (DGAP), respectively. d-Glyceraldehyde (DGA) is the triose fragment common to the substrates for XI and TIM. The XI-catalyzed isomerization of DGA to give dihydroxyacetone (DHA) in D2O was monitored by 1H nuclear magnetic resonance spectroscopy, and a kcat/Km of 0.034 M–1 s–1 was determined for this isomerization at pD 7.0. This is similar to the kcat/Km of 0.017 M–1 s–1 for the TIM-catalyzed carbon deprotonation reaction of DGA in D2O at pD 7.0 [Amyes, T. L., O’Donoghue, A. C., and Richard, J. P. (2001) J. Am. Chem. Soc. 123, 11325–11326]. The much larger activation barrier for XI-catalyzed isomerization of d-xylose (kcat/Km = 490 M–1 s–1) versus that for the TIM-catalyzed isomerization of DGAP (kcat/Km = 9.6 × 106 M–1 s–1) is due to (i) the barrier to conversion of cyclic d-xylose to the reactive linear sugar (5.4 kcal/mol) being larger than that for conversion of DGAP hydrate to the free aldehyde (1.7 kcal/mol) and (ii) the intrinsic binding energy [Jencks, W. P. (1975) Adv. Enzymol. Relat. Areas Mol. Biol. 43, 219–410] of the terminal ethylene glycol fragment of d-xylose (9.3 kcal/mol) being smaller than that of the phosphodianion group of DGAP (∼12 kcal/mol). The XI-catalyzed isomerization of DGA in D2O at pD 7.0 gives a 90% yield of [1-1H]DHA and a 10% yield of [1-2H]DHA, the product of isomerization with incorporation of deuterium from solvent D2O. By comparison, the transfer of 3H from the labeled hexose substrate to solvent is observed only once in every 109 turnovers for the XI-catalyzed isomerization of [2-3H]glucose in H2O [Allen, K. N., Lavie, A., Farber, G. K., Glasfeld, A., Petsko, G. A., and Ringe, D. (1994) Biochemistry 33, 1481–1487]. We propose that truncation of the terminal ethylene glycol fragment of d-xylose to give DGA results in a large decrease in the rate of XI-catalyzed isomerization with hydride transfer compared with that for proton transfer. An ultra-high-resolution (0.97 Å) X-ray crystal structure was determined for the complex obtained by soaking crystals of XI with 50 mM DGA. The triose binds to XI as the unreactive hydrate, but ligand binding induces metal cofactor movement and conformational changes in active site residues similar to those observed for XI·sugar complexes.

The reaction of 2-methoxyphenylethyl tosylate (MeO-1-Ts) is first-order in [N3–]. A carbon-13 NMR analysis of the products of the reactions of MeO-1-[α-13C]Ts shows the formation of MeO-1-[β-13C]OH and MeO-1-[β-13C]N3 from the trapping of a symmetrical 4-methoxyphenonium ion reaction intermediate 2+. An analysis of the rate and product data provides a value of kaz/ks = 83 M–1 for partitioning of 2+ between addition of azide ion and solvent. These data set a limit for the lifetime of 2+ in aqueous solution.

Experimental probes of the acidity of weak carbon acids have been developed and used to determine the carbon acid pKas of glycine, glycine derivatives and iminium ion adducts of glycine to the carbonyl group, including 5′-deoxypyridoxal (DPL). The high reactivity of the DPL-stabilized glycyl carbanion towards nucleophilic addition to both DPL and the glycine-DPL iminium ion favors the formation of Claisen condensation products at enzyme active sites. The formation of the iminium ion between glycine and DPL is accompanied by a 12-unit decrease in the pKa of 29 for glycine. The complicated effects of formation of glycine iminium ions to DPL and other aromatic and aliphatic aldehydes and ketones on carbon acid pKa are discussed. These data provide insight into the contribution of the individual pyridine ring substituents to the catalytic efficiency of DPL. It is suggested that the 5′-phosphodianion group of PLP may play an important role in enzymatic catalysis of carbon deprotonation by providing up to 12 kcal/mol of binding energy that is utilized to stabilize the transition state for the enzymatic reaction. This article is part of a Special Issue entitled: Pyridoxal Phospate Enzymology.

A product deuterium isotope effect (PIE) of 1.0 was determined as the ratio of the yields of [6-1H]-uridine 5′-monophosphate (50%) and [6-2H]-uridine 5′-monophosphate (50%) from the decarboxylation of orotidine 5′-monophosphate (OMP) in 50/50 (v/v) HOH/DOD catalyzed by orotidine 5′-monophosphate decarboxylase (OMPDC) from Saccharomyces cerevisiae, Methanothermobacter thermautotrophicus, and Escherichia coli. This unitary PIE eliminates a proposed mechanism for enzyme-catalyzed decarboxylation in which proton transfer from Lys-93 to C-6 of OMP provides electrophilic push to the loss of CO2 in a concerted reaction. We propose that the complete lack of selectivity for the reaction of solvent H and D, which is implied by the value of PIE = 1.0, is enforced by restricted C−N bond rotation of the −CH2−NL3+ group of the side chain of Lys-93. A smaller PIE of 0.93 was determined as the ratio of the product yields for OMPDC-catalyzed decarboxylation of 5-fluoroorotidine 5′-monophosphate (5-FOMP) in 50/50 (v/v) HOH/DOD. Mutations on the following important active-site residues of OMPDC from S. cerevisiae have no effect on the PIE on OMPDC-catalyzed decarboxylation of OMP or decarboxylation of 5-FOMP: R235A, Y217A, Q215A, S124A, and S154A/Q215A.

The K12G mutation at yeast triosephosphate isomerase (TIM) results in a 5.5 × 105-fold decrease in kcat/Km for isomerization of glyceraldehyde 3-phosphate, and the activity of this mutant can be successfully “rescued” by NH4+ and primary alkylammonium cations. The transition state for the K12G mutant TIM-catalyzed reaction is stabilized by 1.5 kcal/mol by interaction with NH4+. The larger 3.9 kcal/mol stabilization by CH3CH2CH2CH2NH3+ is due to hydrophobic interactions between the mutant enzyme and the butyl side chain of the cation activator. There is no significant transfer of a proton from alkylammonium cations to GAP at the transition state for the K12G mutant TIM-catalyzed reaction, because activation by a series of RNH3+ shows little or no dependence on the pKa of RNH3+. A comparison of kcat/Km = 6.6 × 106 M−1 s−1 for the wildtype TIM-catalyzed isomerization of GAP and the third-order rate constant of 150 M−2 s−1 for activation by NH4+ of the K12G mutant TIM-catalyzed isomerization shows that stabilization of the bound transition state by the effectively intramolecular interaction of the cationic side chain of Lys-12 at wildtype TIM is 6.3 kcal/mol greater than that for the corresponding intermolecular interaction of NH4+ at K12G mutant TIM.

We report that the K12G mutation in triosephosphate isomerase (TIM) from Saccharomyces cerevisiae results in (1) a ∼50-fold increase in Km for the substrate glyceraldehyde 3-phosphate (GAP) and a 60-fold increase in Ki for competitive inhibition by the intermediate analogue 2-phosphoglycolate, resulting from the loss of stabilizing ground state interactions between the alkylammonium side chain of Lys-12 and the ligand phosphodianion group; (2) a 12000-fold decrease in kcat for isomerization of GAP, suggesting a tightening of interactions between the side chain of Lys-12 and the substrate on proceeding from the Michaelis complex to the transition state; and (3) a 6 × 105-fold decrease in kcat/Km, corresponding to a total 7.8 kcal/mol stabilization of the transition state by the cationic side chain of Lys-12. The yields of the four products of the K12G TIM-catalyzed isomerization of GAP in D2O were quantified as dihydroxyacetone phosphate (DHAP) (27%), [1(R)-2H]DHAP (23%), [2(R)-2H]GAP (31%), and methylglyoxal (18%) from an enzyme-catalyzed elimination reaction. The K12G mutation has only a small effect on the relative yields of the three products of the transfer of a proton to the TIM-bound enediol(ate) intermediate in D2O, but it strongly favors catalysis of the elimination reaction to give methylglyoxal. The K12G mutation also results in a ≥14-fold decrease in kcat/Km for isomerization of bound glycolaldehyde (GA), although the dominant observed product of the mutant enzyme-catalyzed reaction of [1-13C]GA in D2O is [1-13C,2,2-di-2H]GA from a nonspecific protein-catalyzed reaction. The observation that the K12G mutation results in a large decrease in kcat/Km for the reactions of both GAP and the neutral truncated substrate [1-13C]GA provides evidence for a stabilizing interaction between the cationic side chain of Lys-12 and the negative charge that develops at the enolate-like oxygen in the transition state for deprotonation of the sugar substrate “piece”.

Bovine serum albumin (BSA) in D2O at 25 °C and pD 7.0 was found to catalyze the deuterium exchange reactions of [1-13C]glycolaldehyde ([1-13C]GA) to form [1-13C,2-2H]GA and [1-13C,2,2-di-2H]GA. The formation of [1-13C,2-2H]GA and [1-13C,2,2-di-2H]GA in a total yield of 51 ± 3% was observed at early reaction times, and at later times, [1-13C,2-2H]GA was found to undergo BSA-catalyzed conversion to [1-13C,2,2-di-2H]GA. The overall second-order rate constant for these deuterium exchange reactions [(kE)P] equals 0.25 M−1 s−1. By comparison, (kE)P values of 0.04 M−1 s−1 [Go, M. K., Amyes, T. L., and Richard, J. P. (2009) Biochemistry 48, 5769−5778] and 0.06 M−1 s−1 [Go, M. K., Koudelka, A., Amyes, T. L., and Richard, J. P. (2010) Biochemistry 49, 5377−5389] have been determined for the wild-type- and K12G mutant TIM-catalyzed deuterium exchange reactions of [1-13C]GA, respectively, to form [1-13C,2,2-di-2H]GA. These data show that TIM and BSA exhibit a modest catalytic activity toward deprotonation of the α-hydroxy α-carbonyl carbon. We suggest that this activity is intrinsic to many globular proteins, and that it must be enhanced to demonstrate meaningful de novo design of protein catalysts of proton transfer at α-carbonyl carbon.

The R235A mutation at yeast orotidine 5′-monophosphate decarboxylase (OMPDC) results in a 1300-fold increase in Km and a 14-fold decrease in kcat for decarboxylation of orotidine 5′-monophosphate, corresponding to a 5.8 kcal/mol destabilization of the transition state. There is strong activation of this mutant enzyme by added guanidinium cation (Gua+): 1 M Gua+ stabilizes the transition state by ca. 3 kcal/mol. This stabilization is due to the binding of Gua+ to the binary Emut·OMP complex, with a Kd of 50 mM, to form the 9-fold more reactive ternary Emut·OMP·Gua+ complex. The “effective molarity” of the cationic side chain of Arg-235 at the wild-type enzyme is calculated to be 160 M.

Primary kinetic isotope effects (KIEs) on a series of carboxylic acid-catalyzed protonation reactions of aryl-substituted α-methoxystyrenes (X-1) to form oxocarbenium ions have been computed using the second-order Kleinert variational perturbation theory (KP2) in the framework of Feynman path integrals (PI) along with the potential energy surface obtained at the B3LYP/6-31+G(d,p) level. Good agreement with the experimental data was obtained, demonstrating that this novel computational approach for computing KIEs of organic reactions is a viable alternative to the traditional method employing Bigeleisen equation and harmonic vibrational frequencies. Although tunneling makes relatively small contributions to the lowering of the free energy barriers for the carboxylic acid catalyzed protonation reaction, it is necessary to include tunneling contributions to obtain quantitative estimates of the KIEs. Consideration of anharmonicity can further improve the calculated KIEs for the protonation of substituted α-methoxystyrenes by chloroacetic acid, but for the reactions of the parent and 4-NO2 substituted α-methoxystyrene with substituted carboxylic acids, the correction of anharmonicity overestimates the computed KIEs for strong acid catalysts. In agreement with experimental findings, the largest KIEs are found in nearly ergoneutral reactions, ΔG° ≈ 0, where the transition structures are nearly symmetric and the reaction barriers are relatively low. Furthermore, the optimized transition structures are strongly dependent on the free energy for the formation of the carbocation intermediate, that is, the driving force ΔG°, along with a good correlation of Hammond shift in the transition state structure.

Primary product isotope effects (PIEs) on L+ and carboxylic acid catalyzed protonation of ring-substituted α-methoxystyrenes (X-1) to form oxocarbenium ions X-2+ in 50/50 (v/v) HOH/DOD were calculated from the yields of the α-CH3 and α-CH2D labeled ketone products, determined by 1H NMR. A plot of PIE against reaction driving force shows a maximum PIE of 8.7 for protonation of 4-MeO-1 by Cl2CHCOOH (ΔG° = 1.0 kcal/mol). The PIE decreases to 8.1 for protonation of 4-MeO-1 by L3O+ (ΔG° = −2.8 kcal/mol) and to 5.1 for protonation of 3,5-di-NO2-1 by MeOCH2COOH (ΔG° = 13.1 kcal/mol). The PIE maximum is around ΔG° = 0. Arrhenius-type plots of PIEs on protonation of 4-MeO-1 and 3,5-di-NO2-1 by L3O+ and on protonation of X-1 by MeOCH2COOH in 50/50 (v/v) HOH/DOD give similar slopes and intercepts. These were used to calculate values of [(Ea)H − (Ea)D] = −1.2 kcal/mol and (AH/AD) = 1.0 for the difference in activation energy for reactions of A−H and A−D and for the limiting PIE at infinite temperature, respectively. These parameters are consistent with reaction of the hydron over an energy barrier. There is no evidence for quantum mechanical tunneling of the hydron through the barrier. These PIEs suggest that the transferred hydron at the transition state lies roughly equidistant between the acid donor and base acceptor and contrast with the recently published Brønsted parameters [Richard, J. P.; Williams, K. B. J. Am. Chem. Soc. 2007, 129, 6952−6961], which are consistent with a product-like transition state. An explanation for these seemingly contradictory results is discussed.

Studies of nonenzymatic electrophilic catalysis of carbon deprotonation of glycine show that pyridoxal 5′-phosphate (PLP) strongly enhances the carbon acidity of α-amino acids, but that this is not the overriding mechanistic imperative for cofactor catalysis. Although the fully protonated PLP–glycine iminium ion adduct exhibits an extraordinary low α-imino carbon acidity (pKa = 6), the more weakly acidic zwitterionic iminium ion adduct (pKa = 17) is selected for use in enzymatic reactions. The similar α-imino carbon acidities of the iminium ion adducts of glycine with 5′-deoxypyridoxal and with phenylglyoxylate show that the cofactor pyridine nitrogen plays a relatively minor role in carbanion stabilization. The 5′-phosphodianion group of PLP likely plays an important role in catalysis by providing up to 12 kcal/mol of binding energy that may be utilized for transition state stabilization.

Schramm and coworkers have punched holes into human purine nucleoside phosphorylase by substitution of glycine for aromatic amino acid residues at a protein lid. The results of studies on the enzymes with holes illuminate hidden chemistry that occurs at the enzyme active site.

Closure of the active site phosphate gripper loop of orotidine 5′-monophosphate decarboxylase from Saccharomyces cerevisiae (ScOMPDC) over the bound substrate orotidine 5′-monophosphate (OMP) activates the bound substrate for decarboxylation by at least 104-fold [Amyes, T. L., Richard, J. P., and Tait, J. J. (2005) J. Am. Chem. Soc. 127, 15708−15709]. The 19-residue phosphate gripper loop of the mesophilic ScOMPDC is much larger than the nine-residue loop at the ortholog from the thermophile Methanothermobacter thermautotrophicus (MtOMPDC). This difference in loop size results in a small decrease in the total intrinsic phosphate binding energy of the phosphodianion group of OMP from 11.9 to 11.6 kcal/mol, along with a modest decrease in the extent of activation by phosphite dianion of decarboxylation of the truncated substrate 1-(β-d-erythrofuranosyl)orotic acid. The activation parameters ΔH⧧ and ΔS⧧ for kcat for decarboxylation of OMP are 3.6 kcal/mol and 10 cal K−1 mol−1 more positive, respectively, for MtOMPDC than for ScOMPDC. We suggest that these differences are related to the difference in the size of the active site loops at the mesophilic ScOMPDC and the thermophilic MtOMPDC. The greater enthalpic transition state stabilization available from the more extensive loop−substrate interactions for the ScOMPDC-catalyzed reaction is largely balanced by a larger entropic requirement for immobilization of the larger loop at this enzyme.

Product distributions for the reaction of glycolaldehyde labeled with carbon-13 at the carbonyl carbon ([1-13C]-GA) catalyzed by triosephosphate isomerase (TIM) in D2O at pD 7.0 in the presence of phosphite dianion and in its absence were determined by 1H NMR spectroscopy. We observe three products for the relatively fast phosphite-activated reaction (Amyes, T. L., and Richard, J. P. (2007) Biochemistry 46, 5841−5854): [2-13C]-GA from isomerization with intramolecular transfer of hydrogen (12% of products), [2-13C,2-2H]-GA from isomerization with incorporation of deuterium from D2O at C-2 (64% of products), and [1-13C,2-2H]-GA from incorporation of deuterium from D2O at C-2 (23% of products). The much slower unactivated reaction in the absence of phosphite results in formation of the same three products along with the doubly deuterated product [1-13C,2,2-2H2]-GA. The two isomerization products ([2-13C]-GA and [2-13C,2-2H]-GA) are formed in the same relative yields in both the unactivated and the phosphite-activated reactions. However, the additional [1-13C,2-2H]-GA and the doubly deuterated [1-13C,2,2-2H2]-GA formed in the unactivated TIM-catalyzed reaction are proposed to result from nonspecific reaction(s) at the protein surface. The data provide evidence that phosphite dianion affects the rate, but not the product distribution, of the TIM-catalyzed reaction of [1-13C]-GA at the enzyme active site. They are consistent with the conclusion that both reactions occur at an unstable loop-closed form of TIM and that activation of the isomerization reaction by phosphite dianion results from utilization of the intrinsic binding energy of phosphite dianion to stabilize the active loop-closed enzyme.

Mononuclear complexes between Zn2+ and the following four macrocycles were prepared: 1,4,7,10-tetraazacyclododecane (1), 1-oxa-4,7,10-triazacyclododecane (2), 1,5,9-triazacyclododecane (3) and 1-hydroxyethyl-1,4,7-triazacyclononane (4). The pH rate profiles of values of the observed second-order rate constant log (kZn)app for Zn(X)(OH2)-catalyzed cleavage (X = 1, 2, 3 and 4) of 2-hydroxypropyl-4-nitrophenyl phosphate (HpPNP) show downward breaks centered at the pKa for ionization of the respective zinc bound water. At low pH, where the rate acceleration for the catalyzed reaction is largest, the stabilizing interaction between the catalyst and the bound transition state is 5.7, 7.4, 7.4 and 5.9 kcal mol−1 for the reactions catalyzed by Zn(1)(OH2), Zn(2)(OH2), Zn(3)(OH2) and Zn(4)(OH2), respectively. The interactions between the metal cation and the macrocycle cause either a modest increase or reduction in transition state stabilization compared with 6.6 kcal mol−1 stabilization for catalysis by Zn(OH2)6. The best Zn(II)–macrocycle catalysts are those for which the interactions between the metal ion and macrocycle are the weakest. Inhibition studies show that each of the four catalysts form complexes with phosphate and oxalate dianions with a much higher affinity than diethyl phosphate monoanion, consistent with stronger interaction of the catalysts with the transition state dianion compared with the substrate monoanion HpPNP. The pH-dependence of methyl phosphate inhibition of Zn(2) catalyzed cleavage of HpPNP shows that only the Zn(2)(OH2) species binds the inhibitor. This result is consistent with a mechanism that has Zn(2)(OH2) as the active catalytic species.

The macrocycles 1,4,7-tris(carbamoylmethyl)-1,4,7,10-tetrazacyclododecane (1), 1,4,7-tris[(N-ethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane (2), 1,4,7-tris[(N,N-diethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane (3) and their Eu(III) complexes are prepared. Studies using direct Eu(III) excitation luminescence spectroscopy show that all three Eu(III) complexes exhibit only one predominant isomer with two bound waters under neutral to mildly basic conditions (Eu(X)(H2O)2 for X = 1–3). There are no detectable ligand ionizations over the pH range 5.0–8.0 for Eu(3), 5.0–8.5 for Eu(2) or 5.0–9.5 for Eu(1). The three Eu(III) complexes show a linear dependence of second-order rate constants for the cleavage of 4-nitrophenyl-2-hydroxyethylphosphate (HpPNP) on pH in the range 6.5–8.0 for Eu(3), 7.0–8.5 for Eu(2) and 7.0–9.0 for Eu(1). This pH–rate profile is consistent with the Eu(III) complex–substrate complex being converted to the active form by loss of a proton and with Eu(III) water pKa values that are higher than 8.0 for Eu(3), 8.5 for Eu(2) and 9.0 for Eu(1). Inhibition studies show that Eu(1) binds strongly to the dianionic ligand methylphosphate (Kd = 0.28 mM), and more weakly to diethylphosphate (Kd = 7.5 mM), consistent with a catalytic role of the Eu(III) complexes in stabilizing the developing negative charge on the phosphorane transition state.

Mononuclear complexes between Zn2+ and the following four macrocycles were prepared: 1,4,7,10-tetraazacyclododecane (1), 1-oxa-4,7,10-triazacyclododecane (2), 1,5,9-triazacyclododecane (3) and 1-hydroxyethyl-1,4,7-triazacyclononane (4). The pH rate profiles of values of the observed second-order rate constant log (kZn)app for Zn(X)(OH2)-catalyzed cleavage (X = 1, 2, 3 and 4) of 2-hydroxypropyl-4-nitrophenyl phosphate (HpPNP) show downward breaks centered at the pKa for ionization of the respective zinc bound water. At low pH, where the rate acceleration for the catalyzed reaction is largest, the stabilizing interaction between the catalyst and the bound transition state is 5.7, 7.4, 7.4 and 5.9 kcal mol−1 for the reactions catalyzed by Zn(1)(OH2), Zn(2)(OH2), Zn(3)(OH2) and Zn(4)(OH2), respectively. The interactions between the metal cation and the macrocycle cause either a modest increase or reduction in transition state stabilization compared with 6.6 kcal mol−1 stabilization for catalysis by Zn(OH2)6. The best Zn(II)–macrocycle catalysts are those for which the interactions between the metal ion and macrocycle are the weakest. Inhibition studies show that each of the four catalysts form complexes with phosphate and oxalate dianions with a much higher affinity than diethyl phosphate monoanion, consistent with stronger interaction of the catalysts with the transition state dianion compared with the substrate monoanion HpPNP. The pH-dependence of methyl phosphate inhibition of Zn(2) catalyzed cleavage of HpPNP shows that only the Zn(2)(OH2) species binds the inhibitor. This result is consistent with a mechanism that has Zn(2)(OH2) as the active catalytic species.

The macrocycles 1,4,7-tris(carbamoylmethyl)-1,4,7,10-tetrazacyclododecane (1), 1,4,7-tris[(N-ethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane (2), 1,4,7-tris[(N,N-diethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane (3) and their Eu(III) complexes are prepared. Studies using direct Eu(III) excitation luminescence spectroscopy show that all three Eu(III) complexes exhibit only one predominant isomer with two bound waters under neutral to mildly basic conditions (Eu(X)(H2O)2 for X = 1–3). There are no detectable ligand ionizations over the pH range 5.0–8.0 for Eu(3), 5.0–8.5 for Eu(2) or 5.0–9.5 for Eu(1). The three Eu(III) complexes show a linear dependence of second-order rate constants for the cleavage of 4-nitrophenyl-2-hydroxyethylphosphate (HpPNP) on pH in the range 6.5–8.0 for Eu(3), 7.0–8.5 for Eu(2) and 7.0–9.0 for Eu(1). This pH–rate profile is consistent with the Eu(III) complex–substrate complex being converted to the active form by loss of a proton and with Eu(III) water pKa values that are higher than 8.0 for Eu(3), 8.5 for Eu(2) and 9.0 for Eu(1). Inhibition studies show that Eu(1) binds strongly to the dianionic ligand methylphosphate (Kd = 0.28 mM), and more weakly to diethylphosphate (Kd = 7.5 mM), consistent with a catalytic role of the Eu(III) complexes in stabilizing the developing negative charge on the phosphorane transition state.