Suicide substrate reaction-like modification of mouse serine racemase with L-serine

A pyridoxal 5'-phosphate-dependent fold-type II serine racemase (SR) is responsible for the synthesis of D-Ser, which serves as a co-agonist of N-methyl-D-aspartate glutamate receptor. In addition to racemization, SR catalyzes the dehydration of D- and L-Ser. SR is suggested to be involved in the D-Ser degradation in vivo, but this has not been confirmed. In this study, we found that mouse SR (mSR) underwent a suicide substrate reaction-like modification with its substrate, resulting in a remarkable change in its reaction specificity. mSR gradually lost its activity by the incubation with L- and D-Ser, but not completely. mSR was labelled with [14C]-L-Ser. ESI-MS analysis revealed that the molecular mass of SR increased by 84 Da by the incubation with L-Ser. Taken together with the results of previous crystallographic studies of fission yeast SR, we concluded that the active site lysine residue of mSR was modified with an α-aminoacrylate intermediate generated from L-Ser and converted to a lysinoalanine residue. The modification significantly decreased the racemization and L-Ser dehydration activities, while dramatically increased the D-Ser dehydration activity by the ~100 times reduction of the Km value. This is probably advantageous for the D-Ser degradation by mSR under physiological conditions.