Extracellular polymerization of 3-hydroxyalkanoate monomers with the polymerase of Alcaligenes eutrophus

Structure of polyhydroxyalkanoate (PHA) synthase PhaC from Chromobacterium sp. USM2, producing biodegradable plastics

Polyhydroxyalkanoate (PHA) is a promising candidate for use as an alternative bioplastic to replace petroleum-based plastics. Our understanding of PHA synthase PhaC is poor due to the paucity of available three-dimensional structural information. Here we present a high-resolution crystal structure of the catalytic domain of PhaC from Chromobacterium sp. USM2, PhaC _Cs -CAT. The structure shows that PhaC _Cs -CAT forms an α/β hydrolase fold comprising α/β core and CAP subdomains. The active site containing Cys291, Asp447 and His477 is located at the bottom of the cavity, which is filled with water molecules and is covered by the partly disordered CAP subdomain. We designated our structure as the closed form, which is distinct from the recently reported catalytic domain from Cupriavidus necator (PhaC _Cn -CAT). Structural comparison showed PhaC _Cn -CAT adopting a partially open form maintaining a narrow substrate access channel to the active site, but no product egress. PhaC _Cs -CAT forms a face-to-face dimer mediated by the CAP subdomains. This arrangement of the dimer is also distinct from that of the PhaC _Cn -CAT dimer. These findings suggest that the CAP subdomain should undergo a conformational change during catalytic activity that involves rearrangement of the dimer to facilitate substrate entry and product formation and egress from the active site.

Enzymes as Green Catalysts for Precision Macromolecular Synthesis

The present article comprehensively reviews the macromolecular synthesis using enzymes as catalysts. Among the six main classes of enzymes, the three classes, oxidoreductases, transferases, and hydrolases, have been employed as catalysts for the in vitro macromolecular synthesis and modification reactions. Appropriate design of reaction including monomer and enzyme catalyst produces macromolecules with precisely controlled structure, similarly as in vivo enzymatic reactions. The reaction controls the product structure with respect to substrate selectivity, chemo-selectivity, regio-selectivity, stereoselectivity, and choro-selectivity. Oxidoreductases catalyze various oxidation polymerizations of aromatic compounds as well as vinyl polymerizations. Transferases are effective catalysts for producing polysaccharide having a variety of structure and polyesters. Hydrolases catalyzing the bond-cleaving of macromolecules in vivo, catalyze the reverse reaction for bond forming in vitro to give various polysaccharides and functionalized polyesters. The enzymatic polymerizations allowed the first in vitro synthesis of natural polysaccharides having complicated structures like cellulose, amylose, xylan, chitin, hyaluronan, and chondroitin. These polymerizations are "green" with several respects; nontoxicity of enzyme, high catalyst efficiency, selective reactions under mild conditions using green solvents and renewable starting materials, and producing minimal byproducts. Thus, the enzymatic polymerization is desirable for the environment and contributes to "green polymer chemistry" for maintaining sustainable society.

A multi-functional polyhydroxybutyrate nanoparticle for theranostic applications

One-step enzymatic synthesis of theranostic PHB nanoparticles using PHA synthase fused with A33scFv and GFP.

New insights into activation and substrate recognition of polyhydroxyalkanoate synthase from Ralstonia eutropha

The polyhydroxyalkanoate synthase of Ralstonia eutropha (PhaC(Re)) shows a lag time for the start of its polymerization reaction, which complicates kinetic analysis of PhaC(Re). In this study, we found that the lag can be virtually eliminated by addition of 50 mg/L TritonX-100 detergent into the reaction mixture, as well as addition of 2.5 g/L Hecameg detergent as previously reported by Gerngross and Martin (Proc Natl Sci USA 92: 6279-6283, 1995). TritonX-100 is an effective lag eliminator working at much lower concentration than Hecameg. Kinetic analysis of PhaC(Re) was conducted in the presence of TritonX-100, and PhaC(Re) obeyed Michaelis-Menten kinetics for (R)-3-hydroxybutyryl-CoA substrate. In inhibitory assays using various compounds such as adenosine derivatives and CoA derivatives, CoA free acid showed competitive inhibition but other compounds including 3'-dephospho CoA had no inhibitory effect. Furthermore, PhaC(Re) showed a considerably reduced reaction rate for 3'-dephospho (R)-3-hydroxybutyryl CoA substrate and did not follow typical Michaelis-Menten kinetics. These results suggest that the 3'-phosphate group of CoA plays a critical role in substrate recognition by PhaC(Re).

Insight in the role of bovine serum albumin for promoting the in situ surface growth of polyhydroxybutyrate (PHB) on patterned surfaces via enzymatic surface-initiated polymerization

Polyhydroxyalkanoates (PHAs) are a family of aliphatic polyesters produced by a variety of microorganisms as a reserve of carbon and energy. Enzymes involved in the synthesis of PHAs can be utilized to produce polymers in vitro, both in bulk and on solid surfaces. Here, site-specific attachment of the key catalytic enzyme, PHA synthase, on lithographically patterned surfaces and subsequent addition of (R)-3-hydroxybutyryl-CoA substrate allowed us to fabricate spatially ordered polyhydroxybutyrate (PHB) polymeric structures via an in situ enzymatic surface-initiated polymerization (ESIP). By varying the reaction conditions, we enhanced the growth of PHB on solid surfaces and analyzed the resulting structures by fluorescence microscopy, atomic force microscopy (AFM), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and gel permeation chromatography (GPC). We found that stabilization of smaller PHB granule structures by an addition of bovine serum albumin (BSA) was the most important factor for a successful synthesis of a PHB layer up to 1mum in thickness, consisting mainly of larger cluster assemblies of PHB granules that cover the entire patterned area. Immunofluorescence detection and surface contact angle analysis revealed that BSA was physically bound to the PHB polymer all through the cluster, and reduced the overall hydrophobicity of the polymer surface. Based on information obtained from AFM, kinetic measurements and various polymer characterization methods, a plausible model for roles of BSA in the enhancement of PHB formation on surfaces is discussed. Furthermore, by using biotinylated BSA conjugates, we were able to incorporate biotin groups into the PHB polymer matrix, thus generating a bioactive surface that can be used for displaying other functional biomolecules through streptavidin-biotin interaction on the PHB structures. Because of its versatility, our fabrication strategy is expected to be a useful surface modification tool for numerous biomedical and biotechnological applications.

Gamma-ray crosslinking of poly(3-hydroxyoctanoate-co-undecenoate)

The gamma-ray crosslinking of films made of poly(3-hydroxyoctanoate) containing undecenoate moieties (up to 33 mole%) were studied. X-ray diffraction, thermal analysis, dynamic mechanical analysis (DMA), solid state nuclear magnetic resonance (NMR) spectroscopy and degree of crosslinking (swelling analysis) as a function of irradiation dose were evaluated for treatments in air or in N(2) atmosphere. After uncrosslinked material was isolated by CHCl(3) extraction, solid state NMR data suggested that only a small percentage of the double bonds took part in the formation of irradiation crosslinks. Crosslinking in N(2) was more efficient than in air and a 20 kGy dose was sufficient for optimal crosslinking. The X-ray diffraction patterns of the polymer films were unaffected by moderate irradiation. The use of sodium hypochlorite to isolate poly(3-hydroxyoctanoate-co-undecenoate) samples resulted in partial chlorination of the double bonds and considerable depolymerization.

In vitro biosynthesis of polyesters

In vitro synthesis of polyesters using isolated enzymes as catalyst via non-biosynthetic pathways is reviewed. In most cases, lipase was used as catalyst and various monomer combinations, typically oxyacids or their esters, dicarboxylic acids or their derivatives/glycols, and lactones, afforded the polyesters. The enzymatic polymerization often proceeded under mild reaction conditions in comparison with chemical processes. By utilizing characteristic properties of lipases, regio- and enantioselective polymerizations proceeded to give functional polymers, most of which are difficult to synthesize by conventional methodologies.