The plant aminoacyl-tRNA synthetases. 2'-DeoxyATP and ATP in reactions catalysed by yellow lupin aminoacyl-tRNA synthetases

Abstract

2'-Deoxyadenosine triphosphate (dATP) is not a substrate for valyl-tRNA, leucyl-tRNA and isoleucyl-tRNA synthetases from yellow lupin seeds. Yellow lupin seryl-tRNA, phenylalanyl-tRNA, tyrosyl-tRNA, arginyl-tRNA, lysyl-tRNA and methionyl-tRNA synthetases use dATP as a substrate both in aminoacyl deoxyadenylate formation and tRNA aminoacylation reactions. Generally, dATP is a poorer substrate, being 3 - 50% as effective as ATP. None of the other nucleoside triphosphates tested was shown to be a substrate for these enzymes. Specificity (k cat/Km) towards ATP is greater in tRNA aminoacylation than in pyrophosphate exchange reaction catalysed by seryl-tRNA synthetase. Energy of activation is the same (Ea = 18.5 kcal) for dATP- and ATP-dependent pyrophosphate exchange. Both dATP- and ATP-dependent tRNA aminoacylation reactions exhibit the same temperature dependence with Ea = 9.0 kcal. Half lives of the enzyme-bound seryl adenylate and deoxyadenylate are 2 min and 4 min, respectively (pH 8.0, 25 degrees C). Both enzyme-bound phenylalanyl adenylate and deoxyadenylate exhibit the same stability (half lives 0.3 min), and enzyme-bound tyrosyl deoxyadenylate is hydrolysed faster (half life 0.6 min) than tyrosyl adenylate (half life 2.5 min). Serine is transferred both from enzyme-bound seryl adenylate and deoxyadenylate to tRNA with the same 60% efficiency. One mol of ATP or 1.1 mol of dATP is hydrolysed per one mol of seryl-tRNA formed during aminoacylation in the complete ATP- or dATP-dependent system, respectively. AMP concentrations (up to 0.5 mM), which do not affect the equilibrium of the ATP-dependent tRNA esterification with serine, significantly change the equilibrium of the dATP-dependent reaction.