Polyhydroxy Alkonates (PHAs)
PHAs are the polymer of hydroxyalkanoates which are accumulated as a carbon and energy storage material in various microorganisms usually under unfavorable growth conditions, such as limitation of N, P, S, Mg, or O2 in the presence of excess carbon source. Numerous bacteria such as Ralstonia eutropha, Alcaligenes latus, Azotobacter vinelandii, Azotobacter chroococcum, Methylotrophs, Pseudomonads, Rhodobacter sphaeroides, and recombinant Escherichia coli synthesize and accumulate PHA. Polyhydroxy-alkanoates or PHAs is synthesized from simple metabolic intermediates and polymerized for minimal disruption to the osmotic balance of the cell.
In 1925, polyhydroxybutyrate became the first known PHA, and in the 1970s polyhydroxyoctanoate and a copolymer of 3-hydroxybutyrate and 3-hydroxyvalerate (PHBV) were identified. By the mid-1990s, almost 100 microbial PHAs had been found.
Many microorganisms can utilize PHAs as an energy source using secreted depolymerases to break down the polymer into water-soluble monomers. In fact, more than 200 microorganisms from a wide range of environments are known to exploit PHAs as an energy source. PHAs are therefore entirely biodegradable in a wide range of microbial enriched conditions ranging from soil to sewage
Recently, the problems concerning the global environment and solid waste management have created much interest in the development of biodegradable plastics. PHAs have been considered one of the most promising biodegradable plastics because of its similar physical properties to petrochemical plastics and complete biodegradability. However, one of the problems hindering commercialization of PHA is the high production cost of PHA compared with petrochemically based polymers. Much effort has recently been made to develop an economical process for the production of PHA. Improving productivity of the fermentation process and developing an efficient recovery procedure will make it economically more feasible.
The biotechnological use of these thermoplastic compounds was first exploited in the 1980s using bulk production of PHAs by expensive bacterial fermentation of either natural or genetically engineered polymer-accumulating bacteria, Ralstonia eutropha. Although the most useful PHA derived plastic, polyhydroxybutyrate-co-valerate copolymer (PHB/V), was used under the name BiopolTm for a time in some commercial products such as shampoo bottles etc., the expense of the bacterial fermentation production severely limited its competitiveness with oil based plastics. A cheaper source of PHAs was obviously needed.
Biopol is technically a family of linear polyesters of three hydroxybutyric and three hydroxyvaleric acids produced in nature from the fermentation of sugars by the bacterium Alcaligenes Eutrophus.
General structure of PHA and some representative members:
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)