Bio plastics or biodegradable plastics are by chemical nature polyhydroxy alkanoates or PHAs. They are currently being produced in large amount by microbial fermentation process in industries. Among all the polyhydroxy alkanoates, polyhydroxy butyrate or PHB is the most important one as bio plastics.
The conventional plastics, made from coal or oil are not biodegradable. They survive 100s of years and are a major source of environmental pollution, often resulting in ecological imbalance. A heavy demand for biodegradable plastic materials has generated in the modern world. There are some attempts to chemically synthesise biodegradable polyesters such as polylactic acid and polyglycolic acid. The production of polyhydroxy alkanoates by fermentation is the preferred process for production of biodegradable plastics.
PHB Production Processes
There several production processes of polyhydroxy butyrate. Some among them are currently undergoing research to produce a bulk amount of bio plastics in plants.
PHB Production in Cytoplasm
Starting from acetyl CoA, production of polyhydroxy butyrate is a three stage pathway, involving the enzymes like 3-Ketothiolase, acetoacetyl CoA reductase, and PHB synthase.
The three genes coding for the respective enzymes have been isolated from Alcaligenes eutrophus and cloned. The cytoplasm of plant cell from Arabidopsis thaliana contains 3-ketothiolase. Therefore in the initial experiments only two genes phaB and phaC coding for acetoacetyl CoA reductase and PHB synthase respectively were transferred to the developing Arabidopsis thaliana.
Although the process is quite flawless in producing PHB it has two main disadvantages. The quantity of PHB produced was very less and the growth of the plant was stunted.
PHB Production in Plastids
The problems faced in the production of PHB in cytoplasm was solved by transferring all the three genes of the enzymes involved in PHB production namely phbA, phbB, phbC to chloroplast. Each gene was separately fused with a coding sequence of transit peptide bound to N-terminal fragment of RUBISCO, (Ribulose 1, 5 biphosphate carboxylase) subunit protein.
The genes expression was carried out by CaMV355 promoter. Transgenic Arabidopsis thaliana plants with each gene construct was first developed. Then a series of sexual crossings were carried out between the individual transgenic plants. The transgenic plants developed by this approach produced a good quantity of PHB or bio plastic around 14% dry weight of the plant. In addition to the good yield there was no noticeable effect of stunted growth of the plants or any discrepancy in fertility of the plant.
PHB Production in Cotton Fibres
Cotton fibres contain the enzymes β-ketothiolase. Thus the genes for the other two enzymes of PHB production pathway such as phaB for acetoacetyl CoA reductase and phaC for PHB synthase from Alcaligenes eutrophus were transferred into meristem of cotton plants by particles bombardment method. Adequate synthesis of PHB was reported in the fibres of transgenic cotton plants.
Application of PHB
Polyhydroxy butyrate can be used as a biomaterial in implantation in medicine. It can be used as a biodegradable material for implantation inside human body. PHB does not produce any immune response and the body does not reject the implantation. Thus PHB is biocompatible and well suited for implantation medical uses.
PHB has several medical applications such as durable bone implants, for dressing of wounds. Now a days PHB is attempted to be used as sutures in operations and as implantation. But these processes did not have much success as PHB degraded very slowly.
The most important bio plastic of commercial importance is polyhydroxy butyrate. But there are many disadvantages of PHB production as it is found as stiff and bristle plastics and it forms highly crystalline polymers. The other notable bio plastics composed of polyhydroxy alkanoates or PHA copolymers are polymers made up of longer monomers. It is less crystalline and more flexible material compared to PHB.
Polyhydroxy alkanoates or PHA polymers are intracellular carbon and energy storage compounds, produced by many microorganisms. They are biodegradable polymers and are elastic in nature depending on the polymer composition. PHAs are well suited for the synthesis of bio plastics, and biodegradable packing materials.
The polyhydroxy alkanoates polymers are produced from the intermediates of β oxidation of fatty acids notably 3-hydroxyacyl CoA. Some success has been reported in the production of polyhydroxy alkanoates or PHAs by genetic manipulations of peroxisomes and glyoxisomes.
Application of PHA
PHAs are mainly used for biodegradable packing materials. PHA is further found to be used as implantation and biocompatible materials in cardiovascular diseases, bone regeneration. Another use of PHA is as micro particulate carrier of drugs. Further uses are like wound dressing, as sutures in medical applications.
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