Determination of a Histidine Residue at the Yeast Orotate Phosphoribosyltransferase Active-Site
A purified preparation of orotate phosphoribosyltransferase(0-PRTase) from yeast has been shown to catalyze the formation of orotidine monophosphate (OMP) from orotate and 5-phosphoribosyl α-1-pyrophosphate (PRibPP) through the use of a ping pong kinetic mechanism (1) in the presence of an optimum concentration of Mgll(2). The present investigation was initiated to search for nucleo-philic active-site residues of 0-PRTase which would stabilize (in an SN1 elimination) or react with (in a triple SN2 displacement) the enzyme-phosphoribosyl intermediate, formed as a consequence of this mechanism. Analysis of the amino acid composition, pH dependency of the 0-PRTase activity in the forward and reverse directions, and chemical modification of 0-PRTase using histidine-specific reagents have been accomplished.
KeywordsAmino Acid Composition Histidine Residue Enzyme Active Site Orotate Phosphoribosyltransferase Active Site Histidine
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