•Lytic polysaccharide monooxygenases are newly discovered copper oxygenases.•LPMOs open up new thinking about oxidation mechanisms.•Copper histidine brace active site is challenging precepts in bioinorganic chemistry.•The catalytic mechanism of LPMOs is an area of intense study.•LPMOs are important factors in commercial biofuel production.Lytic polysaccharide monooxygenases (LPMOs) are recently discovered copper-containing oxygenases. LPMOs oxidise recalcitrant polysaccharides such as chitin and cellulose, thereby making these substrates more tractable to canonical chitinase or cellulase action. As such, LPMOs are attracting much attention not only for their capacity to greatly increase the efficiency of production of cellulosic-based biofuels, but also for the new questions they pose about the mechanisms of biological oxidation of recalcitrant substrates. This review draws together the current thinking on the catalytic mechanisms of LPMOs and other copper catalysed oxygenations and provides a blueprint for further investigation into the mechanisms of action of these intriguing enzymes.

The capacity of metal-dependent fungal and bacterial polysaccharide oxygenases, termed GH61 and CBM33, respectively, to potentiate the enzymatic degradation of cellulose opens new possibilities for the conversion of recalcitrant biomass to biofuels. GH61s have already been shown to be unique metalloenzymes containing an active site with a mononuclear copper ion coordinated by two histidines, one of which is an unusual τ-N-methylated N-terminal histidine. We now report the structural and spectroscopic characterization of the corresponding copper CBM33 enzymes. CBM33 binds copper with high affinity at a mononuclear site, significantly stabilizing the enzyme. X-band EPR spectroscopy of Cu(II)-CBM33 shows a mononuclear type 2 copper site with the copper ion in a distorted axial coordination sphere, into which azide will coordinate as evidenced by the concomitant formation of a new absorption band in the UV/vis spectrum at 390 nm. The enzyme’s three-dimensional structure contains copper, which has been photoreduced to Cu(I) by the incident X-rays, confirmed by X-ray absorption/fluorescence studies of both aqueous solution and intact crystals of Cu-CBM33. The single copper(I) ion is ligated in a T-shaped configuration by three nitrogen atoms from two histidine side chains and the amino terminus, similar to the endogenous copper coordination geometry found in fungal GH61.

Reaction of cis-[RuCl2(DMSO-S)3(DMSO-O)] with cis-1,3,5-triaminocyclohexane (tach) results in the formation of [RuCl(tach)(DMSO-S)2]Cl, a valuable precursor for a wide range of other tach-containing Ru complexes. Reaction of [RuCl(tach)(DMSO-S)2]Cl with the chelating nitrogen-based ligands (N–N = bipyridine, phenanthroline, and ethylenediamine) affords [Ru(N–N)(DMSO-S)2(tach)][Cl]2. A similar reaction between [RuCl(tach)(DMSO-S)]Cl with the chelating phosphorus-based ligands (P–P = dppm, dppe, dppp, dppb, dppv, and dppben) leads to the formation of [RuCl(P–P)(tach)]Cl. The structures of 10 examples of the tach-containing complexes have been determined by single crystal X-ray diffraction. An examination of the structural metrics obtained from these studies indicates that the tach ligand is a strong sigma donor. In addition, the presence of the NH2 groups in the tach ligand allow for participation in hydrogen bonding further modulating the coordinative properties of the ligand.