The mechanism of inhibition of valyl-tRNA synthetase by S-adenosylhomocysteine.

S-Adenosylhomocysteine inhibits ATP-PPi exchange and tRNA aminoacylation reactions catalysed by pure lupin valyl-tRNA synthetase. The reactions catalysed by phenylalanyl-tRNA, tyrosyl-tRNA, seryl-tRNA, arginyl-tRNA and tryptophanyl-tRNA synthetases are not inhibited by S-adenosylhomocysteine. The inhibition is proportional to first power of S-adenosylhomocysteine concentration. S-Adenosylhomocysteine is a competitive inhibitor of the lupin valyl-tRNA synthetase with respect to ATP and valine in the ATP-PPi exchange reaction, noncompetitive with respect to ATP and valine and uncompetitive with respect to tRNA in the tRNA aminoacylation reaction. Inhibition constant for S-adenosylhomocysteine determined from the reciprocal of Km for tRNA vs. the inhibitor concentration (at saturating ATP and not saturating valine concentration) is 0.35 mM. Inhibition constants for S-adenosylhomocysteine determined from the ATP-PPi exchange are 0.21 mM at saturating valine concentration with variable ATP and 0.33 mM at saturating ATP concentration with variable valine. The data are consistent with the binding of S-adenosylhomocysteine to the valyl adenylate site on the enzyme.