抄録
Two NADPH-dependent α-keto ester reductases (Streptomyces thermocyaneoviolaceus keto ester reductase, STKER-II and -III) were purified from S. thermocyaneoviolaceus IFO 14271, one of thermophilic actinomycetes. The molecular masses of native STKER-II and -III were estimated to be 60 kDa and 70 kDa by gel filtration chromatography, respectively. These enzymes were both homodimers, with 29-kDa and 30-kDa subunit molecular masses based on SDS polyacrylamide gel electrophoresis. STKER-II and -III were stable from pH 7.0 to 10.0 and pH 5.5 to 9.0, respectively. Ethyl 3-methyl-2-oxobutanoate was reduced by both enzymes isolated to the corresponding (R)-hydroxy ester with excellent enantiomeric excess. STKER-III showed high stereoselectivity for the reduction of bulky substrates, while the selectivity of the STKER-II-catalyzed reduction was low except for ethyl 3-methyl-2-hydroxybutanoate. Both enzymes had small Km values toward aliphatic keto esters having a long alkyl chain.
