In the Absence of RidA, Endogenous 2-Aminoacrylate Inactivates Alanine Racemases by Modifying the Pyridoxal 5′-Phosphate Cofactor

Members of the RidA (YjgF/YER057c/UK114) protein family are broadly conserved across the domains of life. In vitro, these proteins deaminate 3- or 4-carbon enamines that are generated as mechanistic intermediates of pyridoxal 5'-phosphate (PLP)-dependent serine/threonine dehydratases. The three-carbon enamine 2-aminoacrylate can inactivate some enzymes by forming a covalent adduct via a mechanism that has been well characterized in vitro. The biochemical activity of RidA suggested that the phenotypes of ridA mutant strains were caused by the accumulation of reactive enamine metabolites. The data herein show that in ridA mutant strains of Salmonella enterica, a stable 2-aminoacrylate (2-AA)/PLP adduct forms on the biosynthetic alanine racemase, Alr, indicating the presence of 2-aminoacrylate in vivo. This study confirms the deleterious effect of 2-aminoacrylate generated by metabolic enzymes and emphasizes the need for RidA to quench this reactive metabolite.