Entropic and enthalpic contributions to the enantioselectivity of quinohaemoprotein alcohol dehydrogenases from Acetobacter pasteurianus and Comamonas testosteroni in the oxidation of primary and secondary alcohols

Quinohaemoprotein alcohol dehydrogenases, QH-ADHs, show appreciable enantioselectivity in the oxidation of certain chiral primary and secondary alcohols. We determined the effect of temperature on the enantiomeric ratio of structurally related enzymes, QH-ADH, Type I, from Comamonas testosteroni and QH-ADH, Type II, from Acetobacter pasteurianus, in the kinetic resolution of racemic 2,2-dimethyl-4-(hydroxymethyl)-1,3-dioxolane (solketal) and 2-butanol, respectively. It appears that entropic contributions to the stabilization of the enantioselectivity-determining transition state play an important role in the enantiopreference. Consequences of these findings for the formulation of models that can be used to summarise the observed enantioselectivities are discussed.