[Properties of 5-phosphoryl-1-pyrophosphate amidotransferase from the yeast Saccharomyces cerevisiae wild type and mutant with altered purine biosynthesis regulation].

The properties of partially purified 5-phosphoribosyl-1-pyrophosphate amidotransferase (EC 2.4.2.14) from S. cerevisiae 15V-P4 wild type and mutant aza 165 were studied. The latter is characterized by a higher sensitively of de novo purine synthesis to the inhibitory effect of exogenous guanine. Both enzymes were stable to short-term heating at 60 degrees. The rate of the enzyme-catalyzed reaction was dependent on the enzyme and substrate concentrations. Both enzymes had identical affinities for the substrates (Km for phosphoribosyl pyrophosphate was 0.44 mM for the wild type and 0.5 mM for mutant amidotransferase; Km for glutamine was 2.6 mM for both strains). No differences in the enzyme sensitivity to AMP and IMP inhibition were observed. There were essential differences in the sensitivity to inhibition by GMP: the level of inhibition was more than 80% for mutant amidotransferase and only 35% for the wild type enzyme. The inhibition with respect to phosphoribosylpyrophosphate for the former enzyme was of a mixed type, that for the wild type enzyme was of the "non-competitive" type. This feature of amidotransferase regulation in S. cerevisiae is the cause of the decreased sensitivity of de novo purine nucleotide biosynthesis to inhibition by exogenous guanine.