SUBSTRATE BINDING SPECIFICITY AND PROPERTIES OF INOSINE MONOPHOSPHATE - PYROPHOSPHATE PHOSPHORIBOSYLTRANSFERASE (EC2.4.2.8) FROM BREWERS YEAST

The substrate binding specificity of inosine monophosphate: pyrophosphate phosphoribosyltransferasefrom brewers yeast has been studied with 46 purine analogs. It was found that the following structural features are required for purine binding: (1) an intact purine ring, (2) an exocyclic double bond in the 6 position of the purine ring, (3) a single bond between ring carbon 2 and substituents on this site, and (4) N-7 must be the most basic nitrogen in the purine ring. Guanine, hypoxanthine,6-mercaptopurine, 8-azaguanine, and 2-amino-6-mercaptopurine were the only compounds foundto be either substrates or inhibitors of guanosine monophosphate or inosine monophosphate synthesis. All substrates except 8-azaguanine were shown to be competitive inhibitors of guanosine monophosphate andinosine monophosphate synthesis. Michaelis constants and inhibition constants with respect to both guanosine monophosphate and inosine monophosphate synthesis were determined for these compounds. Arrhenius plots exhibit a biphasic character for guanine, 2-amino-6-mercaptopurine, and 8-azaguanine with transition temperatures of 19, 30, and 38°, respectively. Hypoxanthine does notshow this biphasic character. In all cases except that of hypoxanthine the higher activation energies werefound at the higher temperatures. Evidence is presented which supports the concept of a single enzyme catalyzing both inosine monophosphate and guanosine monophosphate synthesis and a temperature-dependent conformational change in the enzyme. © 1969, American Chemical Society. All rights reserved.