Lipase chemoselectivity towards alcohol and thiol acyl acceptors in a transacylation reaction

The lipase chemoselectivity towards an alcohol and a thiol was investigated for the two lipases Candida antarctica lipase B (CalB) and Rhizomucor miehei lipase (Rml). Hexanol and hexanethiol were used as acyl acceptors in a transacylation reaction with ethyl octanoate in cyclohexane. CalB showed the highest chemoselectivity ratio (kcat/KM)OH/(kcat/KM)SH, of 88,000 while the ratio for Rml was 1200. That could be compared with the ratio, kOH/kSH, of 120 for the non-catalyzed reaction. Thus, the enzyme contribution to the chemoselectivity between hexanol and hexanethiol was 730 for CalB and 10 for Rml. High KM values displayed towards hexanethiol (above 1.8 M) were the largest contribution to the selectivity. No saturation was achieved. The KM values were more than two orders of magnitude higher than those of hexanol.