Fundamental reaction pathway and free energy profile for inhibition of proteasome by epoxomicin

First-principles quantum mechanical/molecular mechanical free energy calculations have been performed to provide the first detailed computational study on the possible mechanisms for reaction of proteasome with a representative peptide inhibitor, Epoxomicin (EPX). The calculated results reveal that the most favorable reaction pathway consists of five steps. The first is a proton transfer process, activating Thr1-O ^γ directly by Thr1-N ^z to form a zwitterionic intermediate. The next step is nucleophilic attack on the carbonyl carbon of EPX by the negatively charged Thr1-O ^γ atom, followed by a proton transfer from Thr1-N ^z to the carbonyl oxygen of EPX (third step). Then, Thr1-N ^z attacks on the carbon of the epoxide group of EPX, accompanied by the epoxide ring-opening (S _N2 nucleophilic substitution) such that a zwitterionic morpholino ring is formed between residue Thr1 and EPX. Finally, the product of morpholino ring is generated via another proton transfer. Noteworthy, Thr1-O ^γ can be activated directly by Thr1-N ^z to form the zwitterionic intermediate (with a free energy barrier of only 9.9 kcal/mol), and water cannot assist the rate-determining step, which is remarkably different from the previous perception that a water molecule should mediate the activation process. The fourth reaction step has the highest free energy barrier (23.6 kcal/mol) which is reasonably close to the activation free energy (∼21-22 kcal/mol) derived from experimental kinetic data. The obtained novel mechanistic insights should be valuable for not only future rational design of more efficient proteasome inhibitors but also understanding the general reaction mechanism of proteasome with a peptide or protein.