Microbial Synthesis and Properties of Polyhydroxy-alkanoates
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Microbial Synthesis and Properties of Polyhydroxyalkanoates Yoshiharu Doi Introduction A wide variety of microorganisms accumulate an optically active polymer of D(-)-3-hydroxybutyric acid, P(3HB), as an intracellular storage material of carbon and energy.1 The P(3HB) was first isolated from Bacillus megaterium in 1925 by Lemoigne.2 Many prokaryotic organisms, such as bacteria and cyanobacteria, have been found to accumulate P(3HB) up to 80% of their cellular dry weight when growth is limited by the depletion of an essential nutrient such as nitrogen, oxygen, phosphorus, or magnesium.1-3 Recently, many bacteria have been found to accumulate copolymers of D(-)3-hydroxyalkanoic acids with a chain length ranging from three to 14 carbon atoms.*~6 In addition, 4-hydroxybutyric acid was found as a constituent of bacterial polyhydroxyalkanoates (PHA).4 Bacterial PHA polymers have attracted much attention as environmentally degradable thermoplastics for a wide range of agricultural, marine, and medical applications.7 PHA is degraded in soil, sludge, or sea water. Some microorganisms such as bacteria and fungi secrete extracellular PHA depolymerases to degrade environmental PHA and utilize the decomposed compounds as nutrients. In this article, I report the metabolism, production, and properties of bacterial PHA. PHA Metabolism Many bacteria synthesize PHA as an intracellular storage material of carbon and energy, and PHA is accumulated as granules within the cytoplasm. The pathway and regulation of PHA synthesis have MRS BULLETIN/NOVEMBER 1992
been studied extensively in Alcaligenes eutrophus.^9 The biosynthesis and degradation of P(3HB) in bacterial cells occur via a cyclic metabolic process, which is shown in Figure 1. P(3HB) is synthesized from acetyl-coenzyme A (acetyl-CoA) by a sequence of three enzymatic reactions. 3-ketothiolase catalyzes the reversible condensation reaction of two acetyl-CoA molecules into acetoacetyl-CoA. The intermediate is reduced to D(-)-3hydroxybutyryl-CoA by NADPH-linked acetoacetyl-CoA reductase. P(3HB) polymer is then produced by the polymerization of D(-)-3-hydroxybutyryl-CoA by the action of PHA polymerase (synthase). The degradation of P(3HB) is initiated by PHA depolymerase to form D(-)3hydroxybutyric acid. NAD-specific dehydrogenase oxidizes the acid to acetoacetate, which is then converted to acetoacetyl-CoA. Thus, acetoacetyl-CoA is an intermediate compound common to the biosynthesis and degradation of P(3HB). A key regulatory enzyme in P(3HB) synthesis is 3-ketothiolase8,9 which is inhibited by high concentrations of free coenzyme A. Under balanced growth conditions in the presence of excess oxygen, acetyl-CoA enters the tricarboxylic acid (TCA) cycle for energy generation and the formation of amino acids. As a result, the concentration of free coenzyme A is high and the synthesis of P(3HB) is inhibited. Under growth-limiting conditions in the presence of an excess carbon source, citrate synthesis is inhibited by high concentrations of NADH and acetyl-CoA
levels increase, which le
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