Biodegradability Of PHA
Environmental degradability of PHA materials
One of the unique properties of biological PHA materials is their biodegradability in various environments. The rate of biodegradation of PHA materials depends on many factors, notably those related to the environment (temperature, moisture level, pH, and nutrient supply) and those related to the PHA materials themselves (composition, crystallinity, additives, and surface area). Electron microscopy has revealed that degradation occurs at the surface by enzymatic hydrolysis (surface erosion). The molecular weights of PHA samples remained almost unchanged during the course of biodegradation. A number of microorganisms such as bacteria and fungi in soil, sludge, and sea water excrete extracellular PHA-degrading enzymes to hydrolyze the solid PHA into water-soluble oligomers and monomer, and subsequently utilize the resulting products as nutrients within cells.
Biodegradability of P(3HB-co-3HV) film in tropical mangrove ecosystem (after 0, 1, 2, 3 or 4 weeks)
Enzymatic degradability of PHA materials
Analysis of structural genes of extracellular PHB depolymerases has shown that all enzymes are comprised of N-terminal catalytic domain, C-terminal substrate-binding domain, and a linker region connecting the two domains. It has been suggested that the C-terminal domain acts as a substrate-binding domain for water-insoluble P(3HB) substrate. This is because the PHB depolymerase without the C-terminal domain lost the hydrolyzing activity toward the water-insoluble P(3HB), while it retained the activity toward water-soluble (R)-3HB oligomers . On the other hand, the linker regions may play a structural role in maintaining an optimal distance between the catalytic domain and substrate-binding domain.
The rate of enzymatic erosion of P(3HB) by PHB depolymerase is strongly dependent on the concentration of enzyme. The rates of enzymatic hydrolysis increased to a maximum value along with the concentration of PHB depolymerase, and then was followed by a gradual decrease [46, 47-49]. Solid P(3HB) polymer is a water-insoluble substrate, while PHB depolymerases are soluble in water. The enzymatic degradation of P(3HB) material by PHB depolymerase therefore is a heterogeneous reaction involving two steps, namely, adsorption and hydrolysis. The first step is adsorption of the enzyme onto the surface of P(3HB) material by the binding domain of enzyme, and the second step is hydrolysis of polymer chains by the active site of the enzyme.