Phenylalanyl-tRNA synthetase of bakers' yeast. Modulation of adenosine triphosphate-pyrophosphate exchange by transfer ribonucleic acid

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Abstract
Native and modified tRNAPhe can modulate phenylalanine-dependent ATP-[32P]PPi exchange activity via inhibition of adenylate synthesis. Inhibition is visualized if concentrations of L-phenylalanine, ATP and PPi are subsaturating. In the proposed mechanism, tRNAPhe is a noncompetitive inhibitor at conditions where only 1 of the 2 active sites per molecule of enzyme is occupied by L-phenylalanine, ATP and PPi At saturating concentrations of these reactants, both active sites are occupied and, according to the model, inhibition is eliminated. Occupation by these reactants is assumed to follow homotropic negative cooperativity. The type of effects depends on modification of tRNAPhe. Native tRNAPhe, .**GRAPHIC**. and .**GRAPHIC**. are inhibitors, tRNAPhepCpC has no effect, and .**GRAPHIC**. is an activator. Kinetics of activation by .**GRAPHIC**. are slow, following the time course of Schiff base formation and subsequent reduction by added cyanoborohydride. Besides showing that a putative enzyme amino group is nonessential for substrate binding and adenylate synthesis, an enzyme amino group could interact with the 3''-terminal adenyl group of cognate of tRNA. In the case of asymmetrical occupation of the enzyme active sites by all of the small reactants ATP, L-phenylalanine and PPi, the interaction with the amino group might trigger the observed noncompetitive inhibition of the PPi exchange by tRNAPhe.