Tyrosyl-tRNA synthetase from Escherichia coli. Stoichiometry of ligand binding and half-of-the-sites reactivity in aminoacylation.

The tyrosyl-tRNA synthetase from Escherichia coli binds only 1 mol of tRNA, tyrosine, and tyrosyl adenylate per mol of enzyme dimer. However, like the enzyme from Bacillus stearothermophilus, once one active site is occupied by tyrosyl adenylate the other becomes accessible to bind a further molecule each of tyrosine and ATP. Both bacterial enzymes show biphasic kinetics with respect to tyrosine in the aminoacylation of tRNA. Equilibrium dialysis experiments show that this is due to 2 mol of tyrosine binding in the presence of ATP and tRNA. A method is given for a correction for the effects of hydrolysis of the charged tRNA on the aminoacylation kinetics.