Mechanistic study of the reaction of thiol-containing enzymes with alpha,beta-unsaturated carbonyl substrates by computation and chemoassays.

We investigated the reactions between substituted alpha,beta-unsaturated carbonyl compounds (Michael systems) and thiols by computations as well as chemoassays. The results give insight into variations in the underlying mechanisms as a function of the substitution pattern. This is of interest for the mechanisms of inhibition of the SARS coronavirus main protease (SARS-CoV M(pro)) by etacrynic acid derivatives as well as for the excess toxicity of substituted alpha,beta-unsaturated carbonyl compounds. This study compares possible reaction courses including 1,4-addition followed by a ketonization step, and underscores the importance of a base-catalyzed step for the reactivity of thiol groups in enzymes. Phenyl and methyl substituents at the Michael system decrease the reactivity of the electrophilic compound, but chlorophenyl substituents partly recover the reactivity. Computations also indicate that electron-pushing substituents lead to a change in the reaction mechanism. The conformation of the Michael system is also found to significantly influence reactivity: the s-cis conformation leads to higher reactivity than the s-trans conformation. The computed data explain the trends in measured inhibition potencies of substituted alpha,beta-unsaturated carbonyl compounds and of reaction rates in chemical assays. They also indicate that the reversibility of inhibition does not stand in contrast to the formation of a new covalent bond between inhibitor and protease.