ATP binding to human serine racemase is cooperative and modulated by glycine

The N-methyl D-aspartate (NMDA) receptors play a key role in excitatory neurotransmission, and control learning, memory and synaptic plasticity. Their activity is modulated by the agonist glutamate and by the co-agonists D-serine and glycine. In the human brain, D-serine is synthesized from L-serine by the dimeric pyridoxal 5′-phosphate-dependent enzyme serine racemase, which also degrades L- and D-serine to pyruvate and ammonia. The dependence of L- and D-serine b-elimination and L-serine racemization activities on ATP concentration was characterized, and was found to be strongly cooperative, with Hill coefficients close to 2 and apparent ATP dissociation constants ranging from 0.22 to 0.41 mM. ATP binding to the holo-enzyme, monitored by the fluorescence changes of the coenzyme, was also determined to be cooperative, with an apparent dissociation constant of 0.24 mM. Glycine, an active-site ligand, increased the serine racemase affinity for ATP by ~ 22-fold, abolishing cooperativity. Conversely, ATP increased the noncooperative glycine binding15-fold. These results indicate cross-talk between allosteric and active sites, leading to the stabilization of two alternative protein conformations with ATP affinities of ~ 10 lM and 1.8 mM, as evaluated within the Monod, Wyman and Changeux model. Therefore, intracellular ATP and glycine control D-serine homeostasis, and, indirectly, NMDA receptor activity. Because hyper- and hypo-activation of NMDA receptors are associated with neuropathologies, the development of allosteric drugs modulating serine racemase activity is a promising therapeutic strategy.