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

Abstract

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. Mg2+, or, alternatively, Ca2+, 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 Mg2+ and Ca2+ 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-Ca2+ complex produces a 2-fold lower maximal activation in comparison to the ATP-Mg2+ complex and exhibits a 3-fold higher EC50. The co-presence of ATP and metals further stabilizes the tetramer. In consideration of the cellular concentrations of Mg2+ and Ca2+, even taking into account the fluctuations of the latter, these results point to Mg2+ 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.

DOI: 10.1016/j.bbapap.2017.01.001

Cite this paper

@article{Bruno2017MagnesiumAC, title={Magnesium and calcium ions differentially affect human serine racemase activity and modulate its quaternary equilibrium toward a tetrameric form.}, author={Stefano Bruno and Marilena Margiotta and Francesco Marchesani and Gianluca Paredi and Valentina Orlandi and Serena Faggiano and Luca Ronda and Barbara Campanini and Andrea Mozzarelli}, journal={Biochimica et biophysica acta}, year={2017}, volume={1865 4}, pages={381-387} }