Extracellular PHA Degradation

Extracellular PHA depolymerases are ubiquitous in the environment (Tokiwa and Calabia 2004). The earliest discovery of PHA-degrading bacteria belongs to 

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Intracellular degradation of PHA by depolymerases has not been studied in as much depth as compared to extracellular PHA degradation. C. necator, an organism able to accumulate PHB, was the organism of research to study the intracellular mobilization. This is mediated by a cyclic metabolic route from... 

Due to the differences between the physical structures of the intracellular native granules and extracellular denatured PHA, intracellular PhaZ is unable to hydrolyse extracellular PHA, and extracellular PhaZ cannot hydrolyse intracellular PHA [14]. The enzymatic degradation of P(3HB) is known as a heterogeneous reaction because the polyhydroxybutyrate (PHB) depolymerase is water-soluble, whereas the P(3HB) polymer is water-insoluble. Therefore, the enzymatic degradation of P(3HB) involves two steps, namely adsorption and hydrolysis. During adsorption, the enzyme is attached to the surface of P(3HB) via the binding domain of the polymer. This is followed by hydrolysis of the polymer chain at the active site of the enzyme [5]. 

The PHA degradation pathway as described in most bacteria studied begins with the depolymerization of PHA to D-3-hydroxybutyrate monomers by PHA depolymerase (encoded by phaZ). Extracellular and intracellular PHA degradation have been described (Jendrossek and Handrick 2002) for utilization of PHAs present in the environment or accumulated in PHA granules, respectively (Tanio et al. 1982 Saegusa et al. 2001 Jendrossek and Handrick 2002 Pbtter and Steinbiichel 2005). 

The mechanism of extracellular degradation of PHA is not yet well understood. A wide variety of microorganisms (fungi, aerobic and anaerobic bacteria) have been implicated but only Pseudomonas lemoignei, Alcaligenes faecalis, and Comamonas sp. have been studied in detail. Production of extracellular PHA depolymerases is induced by the presence of PHA as the sole carbon source. Generally, a PHB depolymerase acts from the hydroxyl terminus to form dimers (or trimers, depending on the depolymerase) and a trace amoimt of monomer. An... 

Jendrossek D. Extracellular polyhydroxyalkanoate (PHA) depolymerases The key enzymes of PHA degradation. In Doi Y, Steinbiichel A, editors. Biopolymers. Hoboken Wiley-VCH 2002. p 41-83. 

Degradation of PHA is important for two practical reasons it affects the yield and quality of PHA during commercial production, and it affects the disposal of consumer goods manufactured from PHA. Degradation of PHA can occur as a nonbiological process or as the result of extracellular and intracellular activities of PHA-degrading enzymes. This topic has been reviewed by Dawes and Senior (1973), Anderson and Dawes (1990), Hocking and Marchessault (1994), Steinbuchel (1996), and Jendrossek et al. (1996). 

Biodegradation in the environment involves extracellular enzymes secreted by microorganisms, and these enzymes have been collectively described as PHA depolymerases. Of the characterized depolymerases, most of them hydrolyze the PHA polymer into dimers, and the dimers are further hydrolyzed to the monomers by an extracellular or intracellular dimer hydrolase. The depolymerase from a Comamonas species, however, appears to hydrolyze poly(3HB) directly to 3-hydroxybutyrate (reviewed by Hocking and Marchessault 1994). Not surprisingly, the degree of crystallinity significantly affects the rate of PHA degradation, and it was concluded that poly(3HB) molecules in the amorphous state are more easily hydrolyzed than poly(3HB) in the crystalline state (reviewed by Steinbiichel 1996). 

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. 

PHA synthesis has been advanced as an energy-producing mechanism in anaerobic syntrophic (H2-producing, acetogenic) bacteria [159]. Extracellular PHBV degradation concurrent with sulfate reduction in anoxic lake-sediment samples was seen by Urmeneta et al. [160] as an indication that PHAs could serve as carbon and electron sources. 

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