Interacting binding sites of isoleucyl-tRNA synthetase from Escherichia coli studied by equilibrium partition

The binding of tRNAIIe to isoleucyl-tRNA synthetase in the presence of isoleucine or ATP was investigated using the equilibrium partition method. Isoleucine decreased the affinity of tRNAIIe for the enzyme by a factor of about 5. For the free standard energy of interaction a value of about 1 kcal/mol (4.2 kJ/mol) was calculated. ATP exhibits qualitatively the same effect as isoleucine. A binding of two molecules isoleucine per molecule of enzyme could not be demonstrated even in the presence of ATP and pyrophosphatase.