PubMed1982
The magnesium chelate of the N(3)H tautomer of orotate, L3Mg, is the true substrate in the biosynthesis of orotidine 5'-monophosphate (OMP) catalyzed by yeast orotate phosphoribosyltransferase (OPRTase, E.C. 2.4.210) with a Michaelis constant KmL3Mg equal to 12(2) muM. It is postulated that Mg++ cations activate the transport of orotate to the active site by neutralizing the orotate charges; the ligand N(3)H is then exchanged between the incoming cation and the cation bound to the enzyme, thus ensuring the stabilization of the appropriate isomeric structure of orotate. This scheme, together with kinetic and thermodynamic data on orotate complexation by Mg++ and Ca++, accounts for the role of Ca++ cations that neither activate nor inhibit OMP synthesis. Cu++ and Ni++ inhibiting properties arise from the formation of inert complexes of orotate. Ni++ complexes have a poor affinity for the protein, whereas Cu++ complexes have a Michaelis constant similar to that of the L3Mg active species. The inertness of these complexes is tentatively understood in terms of low phosphoribosyl transfer rates as postulated from the kinetic study of the protonation of the complexes in water.