Transacylation rates of aminoacyladenosine moiety at the 3'-terminus of aminoacyl-transfer ribonucleic acid

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Abstract
The rates of migration of the aminoacyl group (transacylation) between 2''-O-(aminoacyl)-tRNA and 3''-O-(aminoacyl)-tRNA were studied by the NMR analyses of 3''-terminal fragment models, with regard to the significance of transacylation in the process of protein biosynthesis. 2''(3'')-O-L-Alanyladenosine, -valyladenosine, -isoleucyladenosine, -phenylalanyladenosine and -methionyladenosine, and 2''(3'')-O-L-phenylalanyladenosine 5''-phosphate and methionyladenosine 5''-phosphate were chemically synthesized, and the rates of transacylation in deuterated buffer were directly measured by the NMR saturation transfer method. The dependences of transacylation rates on p2H and temperature were analyzed. The transacylation rates are significantly affected by the ionization states of the .alpha.-amino group of the amino acid moiety but not by the presence of the 5''-phosphate group of the adenylate moiety. The second-order rate constants for the base-catalyzed transacylation reactions were also determined for the ionized form (with .alpha.-N2H3+ group) of (aminoacyl)adenosines. The transacylation rates of (aminoacyl)adenosines in 1H2O solution at p1H 7.3 and 37.degree. C (intracellular environment) were evaluated as 3-11 s-1 for the 2'' .fwdarw. 3'' transacylation and 1-4 s-1 for the 3'' .fwdarw. 2'' transacylation, indicating that the transacylation rate of free aminoacyl-tRNA is slower than the overall rate of polypeptide chain elongation per ribosome. This suggests the presence of some enzymatic factor for enhancing the transacylation rates of aminoacyl-tRNA in the polypeptide chain elongation process in vivo.