title: Two substrate binding sites on tryptophanyl transfer ribonucleic acid synthetase of Escherichia coli.
abstract: Tryptophanyl-tRNA synthetase of Escherichia coli has 1.8 binding sites for L-tryptophan with Kdiss of 12 x 10(-5) M as shown by equilibrium dialysis. The results are in accord with the known structure of the enzyme, and alpha2 dimer of 74,000 molecular weight, and with 2 binding sites for tryptophanyl-ATP ester. Ordinary sucrose density gradient centrifugation reveals a complex composed of one tRNATrp bound per enzyme dimer. When tRNATrp is mixed throughout the gradient at concentrations from 5.4 x 10(-6) M to 2.0 x 10(-5) M, a new peak appears in the position expected for a complex with two tRNATrp molecules bound per enzyme dimer. Sedimentation through gradients lacking tRNATrp favors dissociation of the 1:2 complex but not the 1:1 complex. The data indicate 2 binding sites for tRNATrp on tryptophanyl-tRNA synthetase.
mesh_terms: D000604:Amino Acyl-tRNA Synthetases / Q000378:metabolism; D001665:Binding Sites; D002499:Centrifugation, Density Gradient; D004926:Escherichia coli / Q000201:enzymology*; D007700:Kinetics; D046911:Macromolecular Substances; D008970:Molecular Weight; D011485:Protein Binding; D012343:RNA, Transfer; D014364:Tryptophan; D014369:Tryptophan-tRNA Ligase / Q000378:metabolism
chemical_list: D046911:Macromolecular Substances; D014364:Tryptophan; D012343:RNA, Transfer; D000604:Amino Acyl-tRNA Synthetases; D014369:Tryptophan-tRNA Ligase