title: Subunit interactions in the methionyl-tRNA synthetase of Bacillus stearothermophilus.
abstract: The methionyl-tRNA synthetase from Bacillus stearothermophilus is shown to be a dimer of 2 x 82,000 with identical subunits. It exhibits negative cooperativity in substrate binding and "virtual" halt-of-the-sites reactivity. The enzyme binds only 1 mol of methionine in the absence of other ligands, but several methods show that 2 mol of methionyl adenylate are bound per enzyme dimer. However, one of these adenylates is formed 480 times faster than the other (k1 = 29 sec-1 and k2 = 0.06 sec-1). While the rapid phase of the reaction follows normal saturation kinetics with respect to substrate concentration, the rate of the slow phase is independent of substrate concentrations down to 1 muM. It is suggested that the very slow rate of formation of the second adenylate reflects a rate limiting conformational change which precedes a more rapid chemical step on the second subunit.
mesh_terms: D000595:Amino Acid Sequence; D000604:Amino Acyl-tRNA Synthetases / Q000378:metabolism*; D001665:Binding Sites; D001411:Geobacillus stearothermophilus / Q000201:enzymology*; D007700:Kinetics; D046911:Macromolecular Substances; D008433:Mathematics; D008718:Methionine-tRNA Ligase / Q000378:metabolism*; D008970:Molecular Weight; D011485:Protein Binding; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence
chemical_list: D046911:Macromolecular Substances; D000604:Amino Acyl-tRNA Synthetases; D008718:Methionine-tRNA Ligase