Now PHA can also be produced efficiently and with novel properties in genetically engineered bacteria. Recombinant E.coli harboring multicopy plasmid carrying A.eutrophus PHA biosynthesis genes is developed. PHB concentration of 80 g.l-1 and 2.0 g.l-1h-1 is obtained. Metabolix researchers have transferred proprietary genes for PHA production into the safe and widely used industrial microbial strain, Escherichia coli, where the new genes result in rapid production of high levels of polymer. This strain, E. coli, K12, is already the source of a number of FDA approved food additives and medical products. The raw materials for the transgenic fermentations are widely available sugars like glucose, which are derived from renewable plant crops. Transgenic fermentation systems have several advantages over the non-engineered systems. As a result, the economics of PHA production are now more favourable than at any previous time. Better polymer yields, easier recovery, and production of new copolymers are all enabled by the use of E.coli -- the tried and true workhorse of the biotechnology industry. Faster growth means that engineered E.coli can produce PHA in just 24 hours, c ompared to three days or more for non-engineered strains. High levels of polymer, up to 90% of cell weight, are also achievable, so less carbon substrate is wasted to make extraneous biomass, and polymer isolation becomes more straightforward. Finally, E.coli has the best understood genetics and biochemistry of any organism in the world, so that further metabolic engineering (e.g. to produce new polymer compositions) is possible.
Advantages of using recombinant E.coli for production of PHA :
Price factor -
The major drawback in commercialisation of PHA is the high cost of production. The cost of raw material itself accounts for 40%-50% of the total production cost. Looking to this various systems using different carbon sources are being explored. This includes use of whey, mollasses, acids from wastes by anaerobic treatment and then their conversion to PHA etc. The use of recombinant E.coli or genetically engineered plants will have their own economics reducing the price.
Commercial applications and wide use of PHAs is hampered due to its price. Today price with natural producer like A.eutrophus is US $16 per kg. This is about 18 time more expens ive than polypropylene. For PHA to be commercially viable price should come to US $3-5 per kg. With recombinant E.coli as producer of PHA, price can be reduced to US $4 per kg. which is close to other biodegradable plastic materials such as polylactides an d aliphatic polyesters. With transgenic plants producing PHA price comes to only US 20 cents per kg. Most important aspect is its biodegradable nature helps to overcome the problem of environmental hazard. And the value of clean environment will otherwise also outweigh conventional plastics on price factor in favour of more and more use of PHA.
Advantages of using recombinant E.coli for production of PHA :
- Wide range of substrates (lactose, xylose, sucrose) can be used. Therefore whey, agricultural wastes and byproducts and molasses which are cheaper raw materials can be used.
- Engineered E.coli can produce PHB in 24 hours while non-engineered producers take 3 days.
- It is easier and less costly to purify polymers from recombinant E.coli than from A.eutrophus, since E.coli becomes fragile due to accumulation of biopolymer.
- Recombinant E.coli does not possess PHA depolymerase as what natural PHA producers possess. Hence synthesized PHA is not degraded by producer recombinant E.coli.
- Molecular weight of PHB produced by fermentation of recombinant E.coli can be controlled by modulating activity of PHA synthase enzyme. Average Molecular weight of PHA produced by recombinant E.coli is 4 X 103 kDa, Average Molecular weight of PHA produced by A.eutrophus is 6 X 102 to 1.2 X 103 kDa. Molecular weight decreases by increase in enzyme activity.
- Polydispersity index can be controlled by modulation of synthase activity.
Price factor -
The major drawback in commercialisation of PHA is the high cost of production. The cost of raw material itself accounts for 40%-50% of the total production cost. Looking to this various systems using different carbon sources are being explored. This includes use of whey, mollasses, acids from wastes by anaerobic treatment and then their conversion to PHA etc. The use of recombinant E.coli or genetically engineered plants will have their own economics reducing the price.
Commercial applications and wide use of PHAs is hampered due to its price. Today price with natural producer like A.eutrophus is US $16 per kg. This is about 18 time more expens ive than polypropylene. For PHA to be commercially viable price should come to US $3-5 per kg. With recombinant E.coli as producer of PHA, price can be reduced to US $4 per kg. which is close to other biodegradable plastic materials such as polylactides an d aliphatic polyesters. With transgenic plants producing PHA price comes to only US 20 cents per kg. Most important aspect is its biodegradable nature helps to overcome the problem of environmental hazard. And the value of clean environment will otherwise also outweigh conventional plastics on price factor in favour of more and more use of PHA.
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