Biodegradability ofPolyhydroxyalkanoate (PHAs)PHAs are natural biodegradablethermoplastics which different from petroleum based plastics is theirbiodegradability. The extent of PHAspolymer biodegradation is dependent upon various factors including physical andchemical properties but solely dependent on the chemical structure of thepolymer, molecular weight (Mn), polymer composition and crystallinity (Lee, 1996) and the nature of its monomer units and otherimportant factor such as the type of microbiota involved and their easycolonization.
In addition to environmental factors, such as the exposure of surfacearea, pH, moisture, temperature, electrochemical reaction, biological oxygendemand and supply of other nutrients (nitrogen, phosphorus), are also important(Holmes 1988, Anderson and Dawes 1990, Brandl et al. 1990). PHAs and PHB-degrading microbes are widelydistributed in environment among Bacillus, Pseudomonas and Streptomyces(Chowdhury, 1963)and the families of Pseudonocardiaceae and related genera, Micromonosporaceae,Thermonosporaceae, Streptosporangiaceae and Streptomycetaceae (Tokiwa &Jarerat 2003). PHA degrading enzymeshave been reported in various microorganisms (bacteria, fungi andactinomycetes) and they are ubiquitous in nature. They are susceptible toenzymatic degradation because the enzymatic polymerization is responsible fortheir synthesis (Lenz,1993). Biodegradation of the polymermaterial occurs in various steps under the influence of either cellular orextra-cellular enzymes.
The enzyme activity depends on the PHA composition andthe environmental conditions (Choi et al., 2004; Jendrossek and Handrick, 2002).Thesemicroorganisms secrete enzymes like PHA hydrolases and PHA depolymerases thatbinds to the polymer substrate then subsequently catalyzes a hydrolyticcleavage gradually leads to that break down the PHAs into its shortened monomerichydroxyacids and finally degradation of the polymer which are utilized as acarbon source for growth. Biodegradation of PHA under aerobic conditionsresults in the release of carbon dioxide and water, whereas in anaerobicconditions the degradation products are carbon dioxide and methane (Santhanam andSasidharan, 2010; Lee, 1996; Jendrossek etal. 1996). Microorganisms secrete enzymes that break down thepolymer into its molecular building blocks, called hydroxyacids, which areutilized as a carbon source for growth. Copolymers containing PHB monomer unitshave been found to be degraded more rapidly than either PHB or 3HB-co-3HVcopolymers.
The principal enzyme for the degradation of PHB and oligomersderived from the polymer is PHB depolymerase. Studies on the extracellular PHBdepolymerase of Alcaligenes faecalis have indicated it to be an endo-typehydrolase. Other prominent organisms in which PHB depolymerase has beenidentified and worked upon are Rhodospirillum rubrum, B. megaterium, A.beijerinckii, and Pseudomonas lemoignei. Biodegradation of PHA under aerobicconditions results in carbon dioxide and water, whereas in anaerobicc onditionsthe degradation products are carbon dioxide and methane.
Ideally, the biopolymerwill be disposed in a bio waste collection, and later composted. This processwill ultimately leave behind carbon dioxide and water, which areenvironmentally friendly byproducts.
