Regulation and dynamics of human serine racemase

Human serine racemase (hSR), predominantly localized in neu-rons and astrocytes, is a dimeric pyridoxal 5’-phosphate (PLP) enzyme that catalyzes the reversible racemization of L-serine toD-serine, the physiologically relevant co-agonist of N-methyl-D-aspartate receptors. Because both high and low D-serine levelsare associated with neuropathologies, hSR is a promising drugtarget. We have shown that that:- ATP, an allosteric effector of hSR, binds in a strongly coop-erative fashion, with a Hill coefficient close to 2, and causes a 30-fold increase in enzyme efficiency by reducing Kmand increasingkcat.- Ligands of the active site, such as glycine or malonate, allos-terically increase ATP affinity by 100 fold, and, conversely, ATPbinding increases malonate and glycine affinity by 9 and 15 fold,respectively.- Binding of ATP to the external aldimine intermediate,formed by hSR with glycine, stabilizes a closed conformation ofthe active site, as probed by the coenzyme fluorescence quenchingby iodide.- Chloride and fluoride anions increase 1.5- and 3-fold, respec-tively, the beta-elimination activity of hSR, whereas iodide abol-ishes it. Unlike with other PLP-dependent enzymes, monovalentcations do not have significant effects on catalysis.- Metal chelators (such as EDTA), phosphate, and, to a lesserextent, imidazole, strongly stabilize the enzyme.These findings were interpreted on the basis of a model inwhich different ligands bring about the selective stabilization ofalternative enzyme conformations, characterized by high and lowaffinity and reactivity.