BIOPOL - PHB Accumulation in Micro-organisms - General structure & Properties of PHA

PHB material was first described in 1926 by Lemoigne. PHB is a very common and widespread storage material in many micro-organisms. PHB has been found to be a very basic polymer of variety of chemically similar polymers, the polyhydroxyalkonates. Poly - beta hydroxy butyrate (PHB) accumulates as energy reserve material in many micro-organisms like Alcaligenes, Azotobacter, Bacillus, Nocardia, Pseudomonas, Rhizobium etc. PHB has physical properties comparable with polypropylene (PP). Poly - beta hydroxy butyrate (PHB) consists repeat units of CH(CH₃)-CH₂ -CO-O. The difference is that PP shows insignificant degradation while PHB shows complete degradation. PHB sinks while PP floats. Therefore, degradation is easy at sediment. Alcaligenes eutrophusand Azotobacter beijerinckii can accumulate upto 70% of their dry weight of PHB. These micro-o rganisms can produce the polymer in environment of N and P limitation. Minimum 40-50% of the dry weight of this polymer is required for making the process commercially viable. Extraction of PHB is done by using solvents like halogenated hydrocarbons and purification is done. Moulding and extrusion of dried cells directly is possible when PHB contents are high. A lot of work is done on engineering polymeric properties of PHB. However PHB is suitable for specialized areas like biomedical use and speciality coatings.

PHB Accumulation in Micro-organisms

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.

Fig.1 -

Fig.2 -

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 (Fig.4). Metabolism of PHB occurs as -

Fig.3 - -

Fig.4 -

Properties of PHB -

1. PHB is water insoluble and relative ly resistant to hydrolytic degradation. This differentiates PHB from most other currently available biodegradable plastics, which are either water soluble or moisture sensitive.
2. PHB shows good oxygen permeability.
3. PHB has good ultra-violet resistance but has poor resistance to acids and bases.
4. PHB is soluble in chloroform and other chlorinated hydrocarbons.
5. PHB is biocompatible and hence is suitable for medical applications.
6. PHB has melting point 175⁰C., and glass transition temperature 15⁰C.
7. PHB has tensile strength 40 MPa which is close to that of polypropylene.
8. PHB sinks in water while polypropylene floats. But sinking of PHB facilitates its anaerobic biodegradation in sediments.
9. PHB is nontoxic.

Bioplastics are making commercial and scientific progress continuously. W. R. Grace was the American company which carried out t he work on PHB as early as 1960s. The work was then curtailed because at that time the techniques available for extraction were not able to provide a product thermally stable in processing. The development of PHB was begun by ICI in 1975-6 as a response t o increase in oil prices. ICI started marketing BIOPOL in 1982. ICI, the UK chemical group, has opened a plant at Billingham in North-east of England to make 300 tonnes of Biopol a year, which it says is the first fully biodegradable commercial plastic. The company plans to raise its annual production of this Nature's plastic to 5000 tonnes very soon. At present, Biopol costs about pounds 10 per kg., 20 times more than conventional plastic. Costs can be reduced to some extent by scaling up of the product ion. Even at its current price, ICI has plenty of buyers for limited amounts of Biopol hat they produce.

Australia's A$1 billion raw sugar industry is going to follow Brazilian researchers into new industry producing plastic from sugarcane. Australia's sugar to plastic plans are based on technology held by Procter & Gamble Co. which uses sugarcane genes to produce a plant which produces the polymer poly(3-hydroxybutyrate) (PHB).

Brazilian sugar industry, through the largest industry co-operative Copersucar, is well advanced in an ambitious non-genetic project to produce PHB by using bacteria to convert sugar to plastic.

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