Ultracentrifuge Studies on Binding of Aliphatic Amino Acids to Isoleucyl‐tRNA Synthetase from Escherichia coli MRE 600

An ultracentrifuge technique has been used to investigate the binding of aliphatic C 1 to C 6 α‐amino acids to isoleucyl‐tRNA synthetase from Escherichia coli MRE 600. The synthetase was found to have an absolute requirement for l ‐isomers, but would accept a range of side‐chain structures. l ‐Isoleucine was bound best, followed by the lower homologues l ‐valine and l ‐nor‐valine; the isomers of l ‐isoleucine are poorly bound, as is α‐amino‐ n ‐butyric acid, glycine, l ‐alanine, and d,l ‐ tert. ‐leucine are inert. The method is applicable for investigating interactions of small ligands with macromolecules generally.