Stereochemical Course of the Generation of 3-Mercaptohexanal and 3-Mercaptohexanol by β-Lyase-Catalyzed Cleavage of Cysteine Conjugates

The product resulting from the reaction between E-2-hexenal and L-cysteine was shown to be a diastereoisomeric mixture of 2-(2-S-L-cysteinylpentyl)-1,3-thiazolidine-4-carboxylic acid 1. Treatment of the conjugate with two sources of cysteine-S-conjugate β-lyase (tryptophanase from E. coli and a crude enzyme extract prepared from Eubacterium limosum) resulted in the formation of 3-mercaptohexanal. The reaction proceeded with a slight preference for the (S)-configured product, however, with low conversion rate. The role of 3-S-L-cysteinylhexanal 2 as substrate for β-lyases was demonstrated by in situ generation of 2 from 3-S-(N-acetyl-L-cysteinyl)hexanal using acylase. Opposite enantioselectivity was observed for the liberation of 3-mercaptohexanol from 3-S-L-cysteinylhexanol 5 by the enzyme preparations from Eubacterium limosum and tryptophanase. Various yeasts produced 3-mercaptohexanol starting from 1 as well as from 5. The reactions proceeded without preferential formation of one of the enantiomers.