This paper describes the synthesis and structure of protein nanotube (NT) with a lectin interior surface and its size-dependent dextran loading ability in aqueous medium. The NTs were prepared using an alternating layer-by-layer build-up assembly of poly-l-arginine (PLA) and human serum albumin (HSA) in a track-etched polycarbonate (PC) membrane (pore diameter, 400 nm), subsequently coating concanavalin A (ConA) as the last layer. Dissolution of the PC template yielded (PLA/HSA)₂PLA/ConA NTs with 419 ± 14 nm outer diameter and 50 ± 7 nm wall thickness. In a 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES) buffered solution, the NTs captured fluorescein isothiocyanate (FITC)-labeled dextran [molecular weight 4 kDa, FITC-Dex(4k)] efficiently into the pores. The ratio of the bound FITC-Dex(4kDa)/ConA was estimated to be 2.1 (mol/mol). Two of four glucosyl-residue binding sites of ConA on the wall presumably faced to the aqueous inner phase of the tube, and they can bind FITC-Dex(4k). On the one hand, only half amount of FITC-Dex was loaded into the channels when the molecular weight of the dextran is greater than 20 kDa. Small-angle X-ray scattering measurements revealed that the radius of gyration (R_g) of the FITC-Dex(4k) is 1.45 nm (5.0 mg/ml), which is satisfactorily small to interact with the each binding site of ConA independently. In contrast, the R_g values of FITC-Dex(20k) and FITC-Dex(40k) were 3.75 nm (5.0 mg/ml) and 6.62 nm (4.0 mg/ml), respectively. These large dextrans probably formed an equivalent complex with ConA on the tube wall. Copyright © 2014 John Wiley & Sons, Ltd.

The superoxide radical anion (O₂^.−) is biologically toxic and contributes to the pathogenesis of various diseases. Here we describe the superoxide dismutase (SOD) activity of human serum albumin (HSA) complexed with a single Cu^II ion at the N-terminal end (HSA–Cu complex). The structure of this naturally occurring copper-coordinated blood serum protein has been characterized by several physicochemical measurements. The O₂^.− dismutation ability of the HSA–Cu (1:1) complex is almost the same as that of the well-known SOD mimics, such as Mn^III-tetrakis(N-methylpyridinium)porphyrin. Interestingly, the HSA–Cu complex does not induce a subsequent Fenton reaction to produce the hydroxyl radical (OH^.), which is one of the most harmful reactive oxygen species.

A nanocylindrical wall structure was obtained by layer-by-layer (LbL) assembly of poly-L-arginine (PLA) and human serum albumin (HSA) and characterized by scanning electron microscopy (SEM), scanning force microscopy (SFM), and cryogenic transmission electron microscopy (cryo-TEM). SEM and SFM measurements of a lyophilized powder of (PLA/HSA)₃ nanotubes yielded images of round, chimney-like architectures with approximately 100 nm wall thickness. Cryo-TEM images of the hydrated sample revealed that the tube walls are composed of densely packed HSA molecules. Moreover, when small-angle X-ray scattering was used to characterize the individual PLA and HSA components in aqueous solutions, maximum diameters of approximately 28 nm and 8 nm were obtained, respectively. These values indicate the minimum thickness of wall layers consisting of PLA and HSA. It can also be concluded from SEM as well as from cryo-TEM images that the protein cylinders are considerably swollen in the presence of water. Furthermore, HSA retains esterase activity if assembled in nanotubes, as indicated by measurements of para-nitrophenyl acetate hydrolysis under semi-physiological conditions (pH 7.4, 22 °C). The enzyme activity parameters (Michaelis constant, K_m, and catalytic constant, k_cat) were comparable to those of free HSA.

This communication describes the synthesis and structure of multilayered protein nanotubes bearing a magnetite (Fe₃O₄) surface exterior, and their binding capability for zinc(II)-protoporphyrin IX (ZnPP) in aqueous medium. The nanotubes were fabricated using an alternating layer-by-layer (LbL) assembly of human serum albumin (HSA) and poly-L-arginine (PLA) in a track-etched polycarbonate (PC) membrane (pore size, 400 nm), which had been precoated in advance with Fe₃O₄ nanoparticles. Dissolution of the PC template yielded Fe₃O₄(PLA/HSA)₃ nanotubes. SEM measurements revealed the formation of uniform hollow cylinders with 417 ± 16 nm outer diameter and 56 ± 7 nm wall thickness. TEM observations confirmed the homogeneous outer surface of Fe₃O₄. In an aqueous medium, the nanotubes captured ZnPP into the swollen wall. The ZnPP-loaded protein nanotubes were collected by exposure to a magnetic field. Copyright © 2011 John Wiley & Sons, Ltd.

This report describes the synthesis and enzyme activities of multilayered protein nanotubes with an α-glucosidase (αGluD) interior surface. The nanotubes were prepared by using an alternating layer-by-layer (LbL) assembly of human serum albumin (HSA) and oppositely charged poly-L-arginine (PLA) into a track-etched polycarbonate (PC) membrane (pore size=400 nm) followed by addition of αGluD as the last layer of the wall. Subsequent dissolution of the PC template yielded (PLA/HSA)₂PLA/αGluD nanotubes. SEM measurements revealed the formation of uniform hollow cylinders with (413±17) nm outer diameter and (52±3) nm wall thickness. In aqueous media, the nanotubes captured a fluorogenic glucopyranoside, 4-methyl-umbelliferyl-α-D-glucopyranoside (MUGlc), into their one-dimensional pore space and hydrolyzed the substrate efficiently to form α-D-glucose. We determined the enzyme parameters (Michaelis constant, K_M, and catalytic constant, k_cat, values) of the protein nanotubes. The several-micrometers-long cylinders were of sufficient length to be spun down by centrifugation at 4000 g, so the product could therefore be easily separated. Similar biocatalysts were prepared by complexation of biotinylated-αGluD into HSA-based nanotubes bearing a single avidin layer as an internal surface. The obtained hybrid nanotubes also exhibited the same enzyme activity for the MUGlc hydrolysis.