BETA-THIO AND GAMMA-THIO ANALOGS OF ADENOSINE-TRIPHOSPHATE AS PROBES OF THE ESCHERICHIA-COLI VALYL TRANSFER RIBONUCLEIC-ACID SYNTHETASE REACTION PATHWAY - NOVEL STEREOSPECIFIC INTERCHANGE OF ADENOSINE 5'-O-(2-THIOTRIPHOSPHATE) TO ADENOSINE 5'-O-(3-THIOTRIPHOSPHATE)

Atpσs and the diastereomers of ATPΒS were compared to ATP as substrates for the pyrophosphate (PPi) exchange and aminoacylation reactions catalyzed by the va-lyl-tRNA synthetase from Escherichia coli. The rates with ATP'S were similar to those with ATP, whereas the rates with ATP'S were reduced at least by 1-2 orders of magnitude. While the overall aminoacylation reaction was stereoselective for the B diastereomer by a factor of about 3.5, the A dia-stereomer was the preferred substrate for the exchange reaction by a factor of about 90. The anomalous difference in ste-reospeciflcity is explicable in terms of a novel reaction catalyzed by the enzyme, the formation of ATP'S from ATP'S in the absence of PPi designated the interchange reaction. This reaction is characterized by a high stereospecificity (>500) for the A diastereomer, opposite to the stereoselectivity of aminoacylation. Further, the equilibrium for the interchange reaction lies far toward ATP'S. The rate of aminoacylation from ATP'SA is similar to the rate obtained for formation of ATP'S from ATP'SA, suggesting that the aminoacylation observed with this analogue resulted from the intermediate formation, by interchange, of ATP'S which then serves as the substrate for aminoacylation. Comparable rates of interchange and of exchange of ATP'SA with PPj imply that thiopyrophosphate (SPPi) is not released during the interchange reaction and that the process of interchange occurs while the SPPi remains bound to the enzyme. Possible mechanisms as well as a possible physiological role for the interchange reaction are discussed. © 1979, American Chemical Society. All rights reserved.