Two-water-assisted racemization of the succinimide intermediate formed in proteins. A computational model study

Racemization of aspartic acid (Asp) residues in proteins plays an important role in the molecular biology of aging. In the widely accepted mechanism of the Asp racemization, a succinimide (SI) intermediate is the species which actually undergo the direct racemization. In the present study, a two-water-assisted mechanism of the SI racemization was computationally investigated using a model compound in which an aminosuccinyl (Asu) residue is capped with acetyl and NMe groups on the Nand C-termini, respectively. The two water molecules catalyze the enolization of the Hα−Cα−C=O portion in the Asu residue by mediating proton relay from the α-carbon atom to the carboxyl oxygen atom. After the enolization, migration of the water molecules and conformational change lead to the mirror image of the initially formed enol two-water complex, and the racemization is completed by the following ketonization. The overall activation barrier (28.2 kcal·mol) corresponds to the enolization and ketonization steps, and falls within the available experimental activation energies (21.4 29.0 kcal·mol). Therefore, the two-water-assisted mechanism investigated here is plausible for the in vivo and in vitro racemization reactions of the SI intermediates formed in peptides and proteins.