Polyester depolymerase

Figure 2 shows good depolymerase activity between pH 7 and pH 11, with an optimum at pH 8.5. This is in sharp contrast to a polyester depolymerase produced by Pseudomonas aeruginosa, which had a distinct pH optimum at 7.3 (unpublished data). 

Polyesters, such as microbially produced poly[(P)-3-hydroxybutyric acid] [poly(3HB)], other poly[(P)-hydroxyalkanoic acids] [poly(HA)] and related biosynthetic or chemosynthetic polyesters are a class of polymers that have potential applications as thermoplastic elastomers. In contrast to poly(ethylene) and similar polymers with saturated, non-functionalized carbon backbones, poly(HA) can be biodegraded to water, methane, and/or carbon dioxide. This review provides an overview of the microbiology, biochemistry and molecular biology of poly(HA) biodegradation. In particular, the properties of extracellular and intracellular poly(HA) hydrolyzing enzymes [poly(HA) depolymerases] are described. 

With findings of Doi and coworkers who proposed a two-step reaction for enzymatic polyester hydrolysis, namely adsorption of the depolymerase to the polymer and subsequent hydrolysis [62,63]. 

Although there is evidence that all poly(HA) depolymerases cleave the polyesters by the same mechanism (catalytic triad), the poly(3HO) depolymerase differs considerably from poly(HASCL) depolymerases in terms of primary sequence and polymer-binding. This might be due to different approaches of these enzymes to get access to the polymers reflecting the distinctive physicochemical properties of poly(HASCL) and poly(HMCLA) rather than coevolution. 

Scherer, T. M., Fuller, R. C., Lenz, R. W. Goodwin, S. (1999). Hydrolase activity of an extracellular depolymerase from Aspergillus fumigatus with bacterial and synthetic polyesters. Polymer Degradation and Stability, 64, 267-75. 

At the time of writing, the applications of biodegradable polymers are confined mostly to the field of agriculture, where they are used in products with limited lifetimes, such as mulch films and pellets for the controlled release of herbicides. The synthetic polyesters used in medical applications, principally polylactide and poly(lactide-co-glycolide), while claimed to be biodegradable, are degraded in the body mainly, if not entirely, by chemical hydrolysis. There is little evidence that the hydrolysis of these polyesters of a-hydroxyacids can be catalyzed by hydrolase or depolymerase enzymes. 

Biodegradable, aliphatic polyesters can be synthesized or produced by bacteria and cyanobacteria. These organisms accumulate poly(3-hydroxy butyrate) (PHB) up to 80 wt% of the dry cellular material. Long branching has been produced by modifying the culture medium [Ramsay et ah, 1991]. PHB is degraded by the enzyme poly(3-hydroxybutyrate depolymerase). 

Another basic issue to be considered was represented by the influence of PHB on the biodegradability of PVA in the presence of specific PVA-degrading microorganims, by considering that biodegradation of the polyester and of the polyhydroxylated polymer is strictly mediated by PHB-depolymerase and PVA-oxidase specific enzymes, respectively. In this connection, the biodegradability of both a graft copolymer and a cast blend was ascertained in the presence of a select bacterial culture able to utilize PVA. 

Focarete, M.L., Ceccorulli, G., Scandola, M., and Kowalczuk, M., 1998, Further evidence of crystallinity-induced biodegradation of synthetic atactic poly(3-hydfoxybutyrate) by PHB-depolymerase A from Pseudomonas lemoignei. Blends of atactic poly(3-hydroxybutyrate) with ciystalline polyesters. Macromolecules 31 8485-8492. 

The adsorption of PHA depolymerase to P(3HB) single crystals has been investigated using immunogold labeling techniques.It has been found that PHA depolymerase adsorbed homogeneously on the surfaces of single crystals without site specificity. PHA depolymerases are inactive toward rubbery amorphous polyesters such as native amorphous PHA... 

In recent years, biodegradable plastics have attracted as environmentally fHendly materials to solve the problem of the waste plastics. This paper reports the microbial degradation of polyesters in the naarine environments and the properties of extracellular depolymerases from some polyester-degrading microorganisms. 

Numata K, Sato S, Fujita M, Tsuge T, Iwata T, Doi Y (2007) Adsorption effects of poly(hydroxybutyric add) depolymerase on chain-folding surface of polyester single crystals revealed by mutant enzyme and frictional force microscopy. Polym Degrad Stab 92 176-183 Ohura T, Kasuya K, Doi Y (1999) Cloning and characterization of the polyhydroxybutyrate depolymerase gene of Pseudomonas stutzeri and analysis of the function of substrate-binding domains. Appl Environ Microbiol 65 189-197... 

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