Microbially Originated Polyhydroxyalkanoate (PHA) Biopolymers: An Insight into the Molecular Mechanism and Biogenesis of PHA Granules

Abstract

Microorganisms especially bacteria and cyanobacteria have the ability to synthesize polyhydroxyalkanoates (PHAs) granules as carbon and energy storage compounds within their cells. Owing to eco-friendly, biodegradability, modifiable mechanical properties, non-toxicity, biocompatibility, hydrophobicity, cellular growth support, piezoelectricity, attachment without carcinogenic effects, optical purity and desired surface modifications, the PHAs have received substantial attention towards research as well as commercial ventures and comparable to non-biodegradable conventional plastics presently in use. Microbial PHA biosynthetic pathways are grouped into four types, where PHA synthases are the main enzymes. The PHA synthases exploit the hydroxyacyl-CoAs as substrates and catalyze the covalent bond formation among the hydroxyl group of one along with the carboxyl group of other hydroxyalkanoate that result into the formation of PHAs. Depending on the specificity of substrate as well as components of subunit, PHA synthases are grouped into four types, i.e., class I synthesizing Short-Chain-Length (SCL) PHAs (represented by the bacterium Cupriavidus necator), class II synthesizing Medium-Chain-Length (MCL) PHAs (represented by the bacterium Pseudomonas putida), class III (represented by bacterial species such as Allochromatium vinosum), and class IV PHA synthases (so far represented only by Bacillus sp., B. megaterium). Interestingly, these PHA synthases have a preserved cysteine residue as a catalytic active site to which the resulting PHA chain is linked through covalent bond. Overall, this chapter gives an overview on the structure and genes of PHA synthases including PHA biosynthetic routes, mechanism of PHAs polymerization together with biogenesis of PHA granules and phasins as major PHA granule-associated proteins.