Reasoning enantioselectivity and kinetics of seleno-subtilisin from the subtilisin template

The active-site serine (Ser221) of subtilisin Carlsberg(from Bacillus licheniformis) and subtilisin BPN' (fromBacillus amyloliquefaciens) was chemically converted into a selenocystein. Contrary to subtilisin's protease activity the semisynthetic seleno-subtilisin catalyzed the reduction of hydroperoxides. Enantioselectivity and kinetics of this reaction were studied by kinetic resolution of five racemic alkyl aryl hydroperoxides catalyzed by the seleno-subtilisin variants. Due to the identical tertiary structure of subtilisin and seleno-subtilisin, the enzymes have comparable substrate binding properties. Thus, a rational screening for suitable peroxidase substrates featuring structural characteristics of known subtilisin substrates was enabled. The enantioselective recognition of (S)-configured alkyl aryl hydroperoxides by seleno-subtilisin was comprehensible by subtilisin's preference for comparable (S)-alkyl aryl amines or alcohols. The analysis of chiral products by multidimensional gas chromatography revealed enantiomeric excesses up to 98%. Kinetics of seleno-subtilisin were rationalized on the basis of the established substrate-catalyst interactions of the subtilisin framework. The Carlsberg and BPN' peroxidase variants revealed typical differences in turnover numbers (kcat) and Michaelis-Menten affinity constants (Km) already known from subtilisin variants. Turnover numbers of seleno-subtilisin BPN' were lower and Km values were higher in comparison to Carlsberg variant. Substrate affinity of several substituted 1-arylethyl hydroperoxides to seleno-subtilisin was reasonable in comparison to corresponding aryl boronic acid inhibitors of subtilisin.