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Biochim Biophys Acta Proteins Proteom. 2017 Apr;1865(4):381-387. doi: 10.1016/j.bbapap.2017.01.001. Epub 2017 Jan 9.

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

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Department of Pharmacy, University of Parma, Italy. Electronic address:
Department of Pharmacy, University of Parma, Italy.
Interdepartment Center SITEIA.PARMA, University of Parma, Italy.
Department of Neurosciences, University of Parma, Italy.
Department of Pharmacy, University of Parma, Italy; Institute of Biophysics, CNR, Pisa, Italy; National Institute of Biostructures and Biomolecules, Rome, Italy.


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.


Allosteric regulation; Enzyme regulation; Metal-protein interaction; Metalloproteins; Serine racemase

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