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.
从嗜热放线菌 S. thermocyaneoviolaceus IFO 14271 中纯化了两种
NADPH 依赖的 α-
酮酯还原酶(STreptomyces thermocyaneoviolaceus
酮酯还原酶 STKER-II 和 -III)。通过凝胶过滤色谱法,测得原形态 STKER-II 和 -III 的分子量分别为 60 kDa 和 70 kDa。根据
SDS 聚
丙烯酰胺凝胶电泳,这两种酶都是同源二聚体,亚单位分子量分别为 29 kDa 和 30 kDa。STKER-II 和 -III 在 pH 7.0 到 10.0 及 pH 5.5 到 9.0 的范围内都表现出稳定性。乙基 3-甲基-2-氧代
丁酸酯被两种分离的酶还原为相应的 (R)-氢氧基酯,且具有优异的对映体过量。STKER-III 对体积较大的底物的还原具有较高的立体选择性,而 STKER-II 催化的还原选择性较低,除了乙基 3-甲基-2-羟基
丁酸酯。两种酶对具有长烷基链的脂肪族
酮酯的 Km 值较小。