DFT and MP2 studies on the C2C2α bond cleavage in thiamin catalysis

Thiamin diphosphate (ThDP), the biologically active derivative of vitamin B1, is an important cofactor of several enzymes that catalyze the oxidative and non-oxidative conversion of α-keto acids. The final step of non-oxidative decarboxylation of pyruvate by pyruvate decarboxylase – the liberation of acetaldehyde – requires deprotonation of the α-hydroxyl group and cleavage of the C2–C2α bond of the transitory 2-(1-hydroxyethyl)-ThDP intermediate. It has been proposed that the cofactor 4′-amino/imino function is essentially involved in the deprotonation of the α-hydroxyl group. Proton transfer and C2–C2α cleavage may occur in a stepwise manner, or, alternatively in a concerted mechanism. Here, density functional theory (DFT) calculations as well as second order Møller–Plesset perturbation theory (MP2) studies were performed on a simple model for the enzyme using the program package Gaussian 03. Calculations favor a stepwise mechanism with initial formation of the C2α alkoxide, followed by C2–C2α bond cleavage.