General structure of PHA and some representative members

Polyhydroxyalkonates (PHAs) are polyesters of various hydroxyalkonates that are synthesized and intracellularly accumulated by num erous micro-organisms as energy reserve material. More than 100 different monomer units have been identified as the constituents of PHAs. This creates the possibility of producing biodegradable polymers with wide range of properties. PHB, Poly (3-hydroxybutyrate) is the best characterized PHA. PHB has lowest molecular weight and is most common in nature. Their molecular weight can be upto 2 million (i.e. 20000 monomers per polymer molecule). The monomer units of PHA are all in D-(-) configuration owing to the stereospecificity of the biosynthetic enzymes.

n = 1	R = hydrogen	Poly (hydroxy propionate)
	R = methyl	Poly (3-hydroxybutyrate)
	R = ethyl	Poly (3-hydroxyvalerate)
	R = propyl	Poly (3-hydroxyhexanoate)
	R = pentyl	Poly (3-hydoxyoctanoate)
	R = nonyl	Poly (3-hydroxy doedcanoate)
n = 2	R = hydrogen	Poly (4-hydroxybutyrate)
	R = methyl	Poly (4-hydroxyvalerate)
n = 3	R = hydrogen	Poly (5-hydroxyvalerate)
	R = methyl	Poly (5-hydroxyhexanoate)
n = 4	R = hexyl	Poly (6-hydroxydodecanoate)

There are three enzymes present in A.eutrophus for PHA biosynthesis (Fig.3). These are - PHA synthase, beta ketothiolase and reductase. Natural producers also have PHA depolymerase that degrades the polymer and uses the breakdown metabolites for cell growth