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Intracellular Degradation

Intracellular degradation takes place when carbon limitation conditions are stressed upon the bacterium. Accumulated PHA in the cells undergoes hydrolysis as carbon and energy sources. Further, it breaks down to 3-hydroxy alkanoic acid, a monomeric component by PHA depolymerase and oligomer hydrolase [42]. If the PHA is made up of one kind of monomer, such as 3-hydroxybutyrate, the resulting PHA is called poly(3-hydroxybutyrate) [P(3HB)] homopolymer, and it is the most common type of PHA which is synthesized by various bacteria naturally. P(3HB) is further degraded to 3-hydroxybutyric acid, which is oxidized by a dehydrogenase to acetoacetyl-CoA, converted into acetyl-CoA by beta ketothiolase [43]. All these breakdown products of PHA are naturally found in animals. So, biodegradation of PHA does not lead to toxic products and can be termed biocompatible material [44, 45].

Extracellular Degradation

Extracellular depolymerase hydrolyzes partially crystallized P(3HB). These depolymerases comprises of a single peptide (22–58 amino acids) and three functional domains, catalytic domain (320–400 amino acids), linker domain (50–100 amino acids), and substrate-binding domain (40–60 amino acids) from N-terminal to C-terminal. [46, 47] The catalytic domain is further classified into two types of depolymerases, i.e., Type I and Type II, differing on the order of the sequential order of active amino acids forming a catalytic triad. Apart from these depolymerases, any lipases also possess the ability to hydrolyze poly(ω-hydroxyalkanoates) such as poly(6-hydroxyhexanoate) [P(6HHx)] and poly(4-hydroxybutyrate) [P(4HB)].

Specific enzymes, PHA depolymerases, present in the soil and aquatic microorganisms degrade the PHAs. Until this time, the identification of 600 PHA depolymerases from the wide society of microorganisms has been made. Various microorganisms in the soil, fresh waters, compost, and marine environments help in the degradation of PHAs. Bacteria present in marine environments such as Pseudoalteromonas sp. NRRL B-30083, Marinobacter sp. NK-1, Alcaligenes faecalis AE122, actinobacteria Nocardiopsis aegyptia, and Streptomyces sp. SNG9 are the few microorganisms that are known to be PHA degraders [46–48].