Polylactate (PLA) is a typical biodegradable polymer originated from lactate, which has a variety of applications as a common or a medical polymer. At present, PLA is being prepared by polymerizing lactate which is produced by fermenting microorganisms, but only low molecular weight PLA (1000-5000 dalton) is produced by direct polymerization of lactate. To synthesize high molecular weight (>100,000 dalton) of PLA, a method polymerizing low molecular weight PLA obtained by direct polymerization of lactate with a chain coupling agent can be used. However, it has disadvantages like that the process for preparing PLA of high molecular weight is complicated due to the addition of a solvent or a chain coupling agent, and also it isn't easy to remove them. At present, in the process for preparing commercially available PLA of high molecular weight, a method, in which lactate is converted into lactide to synthesize PLA by cyclodehydration of the lactide ring, is being used.
PLA homopolymer can be easily obtained from chemical synthesis method using lactate, but lactate compolymer having various monomer units is difficult to be produced and its commercial availability is very low.
Meanwhile, polyhydroxyalkanoate (PHA) is a polyester which microorganisms accumulate therein as a carbon and energy storage compound when other nutritive elements, for example, phosphorus, nitrogen, magnesium, oxygen, are deficient while the carbon source is in excess. PHA is recognized as an alternative material for synthesized plastics since it has similar properties to synthetic polymers originating from petroleum, and, at the same time, shows an excellent biodegradation property.
To produce PHA in microorganisms, an enzyme which converts microorganisms' metabolites into a PHA monomer and PHA synthase which synthesizes the PHA polymer using the PHA monomers are required. When producing PLA and lactate copolymer with microorganisms, the same system is needed and an enzyme being able to provide lactyl-CoA also is needed in addition to an enzyme providing hydroxyacyl-CoA, original substrate of PHA synthase.
On this account, the present inventors developed a system using propionyl-CoA transferase originated from Clostridium propionicum to provide lactyl-CoA and succeeded the production of PLA and lactate copolymer (Korean Patent Application laid-open No. 10-2006-0121555). However, it has little PHA synthase activity on hydroxyalkanoate which is hydroxylated at the 2-position. There have been reports of PHA synthase activity on lactyl-CoA measured in vitro, but PHA synthase activity on lactyl-CoA is reported to be very weak as said above (Zhang et al., Appl. Microbiol. Biotechnol., 56:131, 2001; Valentin and Steinbuchel, Appl. Microbiol. Biotechnol., 40:699, 1994;  Yuan et al. Arch Biochem Biophys. 394:87, 2001). Therefore, if a PHA synthase can not use lactyl-CoA efficiently and the PHA synthase is used to produce PLA and lactate copolymer, the synthesis efficiency must be low. That is, because lactate, hydroxyalkanoate which is hydroxylated at the 2-carbon position, is not a suitable substrate for PHA synthase, PHA synthase being able to use lactyl-CoA efficiently is very important to synthesize PLA and lactate copolymer efficiently.
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