Molecular orbital study on the reaction mechanism of irreversible enzyme inhibitors

By means of the molecular orbital method, the reaction mechanism of the specific and irreversible enzyme inhibitors, such as cycloserine, L-2-amino-4-methoxy-trans-3-butenoic acid (AMB), and vinylglycine (2-amino-3-butenoic acid), was studied. Firstly, it was attempted to know which pathway is probable between the transamination process and the isomerization one. By comparing the energy increments for these two reactions, the transamination reaction was predicted to be energetically favorable, supporting the proposition of Rando. Upon complexing with the coenzyme-pyridoxal moiety of alanine racemase or aminotransferase, the reactivity of the inhibitors toward the nucleophile was found to be considerably increased due to the lowering of the lowest unoccupied molecular orbital (LUMO), and this was considered to be the reason why the inhibitors become bound with the enzyme irreversibly. The LUMO of aspartate, substrate of aspartate aminotransferase, is higher than those of the inhibitors in the free state, as well as in the pyridoxal-linked state. This difference in the energy of the molecular orbital between substrate and inhibitors was considered to be correlated with the difference in the complex-forming properties of these compounds toward the nucleophile in the enzyme.