Extracellular depolymerase

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. 

In general, both P2HAs and P3HAs, are subjected to acid-based catalyst hydrolytic decomposition in a low-temperature range, and only biotechnologically produced PHAs are subjected in addition to enzymatic decomposition due to extracellular enzymes. Some microorganisms excrete extracellular depolymerases to degrade microbial PHAs and to utilize the decomposed compounds as nutrients. 

The polydispersity of sample A is rather high. This is due to a fimgal infection of the extracted polymer. It seems that the polymer chains did undergo some cleaving by extracellular depolymerases from the fungi, which led to increased PD and a rather low value of Mn. This is a mechanism also responsible for the biodegradation of these polymers in a natural envirorunent. 

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. 

Although extracellular depolymerases act on semicrystalline PHA, the degradation rate has been shown to decrease with increasing crystallinity of PHB films (45). For a blow-molded, P(HB-co-16%HV) product (a 20-/xm cross section was exposed to partially purified PHB hydrolase from Penicillium funiculosum), the degradation rate is faster at the less crystalline surface than at the more crystalline core (42). Furthermore, it has been shown that the amorphous regions on the surface of PHB films are degraded preferentially whereas microbial colonization takes place at spherical holes which tend to be at the crystal centers and at the boundary between spherulites (46). 

Scherer T.M., Clinton Fuller R., Lenz R.W., Goodwin S. Hydrolase activity of an extracellular depolymerase from Aspergillus famigatus with bacterial and synthethic polyesters, Polym. Deg. Stab. 64(1999)267. 

Poly(hydroxyalkanoates) are polyesters that exist inside of many bacterial cells as storage compounds of carbon and energy, and they also exist outside of the cells. Enzymes that degrade the intracellular (native) polyesters are different from those that degrade the extracellular polyesters [81]. This discussion will focus on the enzymes that depolymerize the extracellular poly(hydroxyalkanoates), which are more crystalline than the intracellular polyesters. Of course, man-made fibers and films made of poly(hydroxyalkanoates) exist outside of microbial cells, and therefore extracellular depolymerases are required for their biodegradation. 

On the other hand, the biological method of depolymerisation uses either the intracellular [246] or extracellular depolymerases [214]. It has also been shown that the intracellular PHA depolymerisation mechanisms can be exploited in vivo to generate (R)- monomers from PHAscl and PHAmcl accumulated by several bacteria such as A. latus, R. eutropha, P. oleovorans and P. aeruginosa [247]. 

Polymer blends containing poly(3 hydroxybutyrate-co 16%-3-hydroxyvalerate, (P(HB-coOHV), were investigated by incorporating poly(E-caprolactone), (PCD, poly(styrene-co-35%-acrylonitrile), (SAN), or poly(styrene), (PS), as the second component. Blend concentrations of 20/80, 50/50 and 80/20 weight percents were solvent cast from chloroform and analyzed for miscibility using DSC. Biodegradation studies were conducted using an exposure to the extracellular depolymerase from Penicillium funiculosum at 28 C for 48 and 96 hours. 

PHA-degrading enzymes (extracellular depolymerase) are excreted by a number of bacteria and fungi in the environment (soil [182-185, freshwater [186], sludge [187], seawater [188, 189], hot-springs [190], compost [178], air [191]). Electron microscopy analysis of PHA films revealed that degradation occurs at the surface by enzymic hydrolysis. The degradation is therefore a function of the surface area available for microbial colonisation. 

Several kinds of extracellular depolymerases have been purified and characterised from various microorganisms [187, 189,191, 195-197], All the depolymerases are comprised of an N-terminal catalytic domain, a C-terminal substrate binding domain, and a linker region connecting the two domains. Similar catalytic and binding domains have also been identified in other depolymerising enzymes that hydrolyse water-insoluble polysaccharides such as cellulose [198], xylan [198,199], and chitin [200], The catalytic domain contains a lipase box pentapeptide [Gly-Xi-Ser-X2-Gly] as the active site, which is common for serine hydrolase [201]. Further detailed aspects on the structure and mechanisms of PHA depolymerase (EC 3.1.1.76) can be found elsewhere [202]. 

Ramsay et al. [179] developed a method for the specific isolation of microorganisms producing extracellular depolymerases for medium-side-chain PHAs. A sterile, colloidal suspension of a copolyester of 3HB, 3HHx, 3HO and 3HD monomers was used as enrichment medium, and six bacterial species capable of growing in it were isolated. Not surprisingly, all were identified as pseudomonads or members of related genera. 

Nishida and Tokiwa s [211] observations suggested two different methods of microbial attack on PHB a preferential degradation of amorphous regions of the polymer by extracellular depolymerase, followed by colonization by bacteria of the film surface and subsequent localized degradation. 

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