Mechanism of activation of phenylalanine and synthesis of P1,P4-bis(5'-adenosyl)tetraphosphate by yeast phenylalanyl-tRNA synthetase

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Abstract
The activation of L-phenylalanine by yeast phenylalanyl-tRNA synthetase using adenosine 5''-[(S)-.alpha.-17O,.alpha.,.alpha.-18O2]triphosphate is shown to proceed with inversion of configuration at P.alpha. of ATP. This observation taken together with the lack of positional isotope exchange when adenosine 5''-[.beta.,.beta.-18O2]triphosphate is incubated with the enzyme in the absence of phenylalanine and in the presence of the competitive inhibitor phenylalaninol indicates that activation of phenylalanine occurs by a direct in-line adenylyl-transfer reaction. In the presence of Zn2+, yeast phenylalanyl-tRNA synthetase also catalyzes the phenylalanine-dependent hydrolysis of ATP to AMP and the synthesis of P1,P4-bis(5''-adenosyl) tetraphosphate (Ap4A). With adenosine 5''-[(S)-.alpha.-17O,.alpha.,.alpha.-18O2]triphosphate, the formation of AMP and Ap4A is shown to occur with inversion and retention of configuration, respectively. Thus, phenylalanyl adenylate is an intermediate in both processes, Zn2+ promoting AMP formation by hydrolytic cleavage of the C.sbd.O bond and Ap4A formation by displacement at P of phenylalanine by ATP.

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