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Polyhydroxyalkanoates or PHAs are linear polyesters produced in nature by bacterial fermentation of sugar or lipids. They are produced by the bacteria to store carbon and energy. More than 150 different monomers can be combined within this family to give materials with extremely different properties. These plastics are biodegradeable and are used in the production of bioplastics. They can be either thermoplastic or elastomeric materials, with melting points ranging from 40 to 180 °C. The mechanical and biocompatibility of PHA can also be changed by blending, modifying the surface or combining PHA with other polymers, enzymes and inorganic materials, making it possible for a wider range of applications.〔(Polyhydroxyalkanoates for tissue engineering )〕 ==Biosynthesis== To produce PHA, a culture of a micro-organism such as ''Alcaligenes eutrophus'' is placed in a suitable medium and fed appropriate nutrients so that it multiplies rapidly. Once the population has reached a substantial level, the nutrient composition is changed to force the micro-organism to synthesize PHA. The yield of PHA obtained from the intracellular inclusions can be as high as 80% of the organism's dry weight. The biosynthesis of PHA is usually caused by certain deficiency conditions (e.g. lack of macro elements such as phosphorus, nitrogen, trace elements, or lack of oxygen) and the excess supply of carbon sources. Polyesters are deposited in the form of highly refractive granules in the cells. Depending upon the microorganism and the cultivation conditions, homo- or copolyesters with different hydroxyalkanic acids are generated. PHA granules are then recovered by disrupting the cells. Recombinants ''Bacillus subtilis'' str. pBE2C1 and ''Bacillus subtilis'' str. pBE2C1AB were used in production of polyhydroxyalkanoates (PHA) and it was shown that they could use malt waste as carbon source for lower cost of PHA production. PHA synthases are the key enzymes of PHA biosynthesis. They use the coenzyme A - thioester of (r)-hydroxy fatty acids as substrates. The two classes of PHA synthases differ in the specific use of hydroxy fatty acids of short or medium chain length. The resulting PHA is of the two types: *Poly (HA SCL) from hydroxy fatty acids with short chain lengths including three to five carbon atoms are synthesized by numerous bacteria, including ''Ralstonia eutropha'' and ''Alcaligenes latus'' (PHB). *Poly (HA MCL) from hydroxy fatty acids with middle chain lengths including six to 14 carbon atoms, can be made for example, by ''Pseudomonas putida''. A few bacteria, including ''Aeromonas hydrophila'' and ''Thiococcus pfennigii'', synthesize copolyester from the above two types of hydroxy fatty acids, or at least possess enzymes that are capable of part of this synthesis. Another even larger scale synthesis can be done with the help of soil organisms. For lack of nitrogen and phosphorus they produce a kilogram of PHA per three kilograms of sugar. The simplest and most commonly occurring form of PHA is the fermentative production of poly-beta-hydroxybutyrate (poly-3-hydroxybutyrate, P3HB), which consists of 1000 to 30000 hydroxy fatty acid monomers. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Polyhydroxyalkanoates」の詳細全文を読む スポンサード リンク
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