Magnesium and calcium ions differentially affect human serine racemase activity and modulate its quaternary equilibrium toward a tetrameric form.

Serine racemase is the pyridoxal 5'-phosphate dependent enzyme that catalyzes both production and catabolism of d-serine, a co-agonist of the NMDA glutamate receptors. Mg, or, alternatively, Ca, activate human serine racemase by binding both at a specific site and - as ATP-metal complexes - at a distinct ATP binding site. We show that Mg and Ca bind at the metal binding site with a 4.5-fold difference in affinity, producing a similar thermal stabilization and partially shifting the dimer-tetramer equilibrium in favour of the latter. The ATP-Ca complex produces a 2-fold lower maximal activation in comparison to the ATP-Mg complex and exhibits a 3-fold higher EC. The co-presence of ATP and metals further stabilizes the tetramer. In consideration of the cellular concentrations of Mg and Ca, even taking into account the fluctuations of the latter, these results point to Mg as the sole physiologically relevant ligand both at the metal binding site and at the ATP binding site. The stabilization of the tetramer by both metals and ATP-metal complexes suggests a quaternary activation mechanism mediated by 5'-phosphonucleotides similar to that observed in the distantly related prokaryotic threonine deaminases. This allosteric mechanism has never been observed before in mammalian fold type II pyridoxal 5'-phosphate dependent enzymes.