title: A master switch couples Mg²⁺-assisted catalysis to domain motion in B. stearothermophilus tryptophanyl-tRNA Synthetase.
abstract: We demonstrate how tryptophanyl-tRNA synthetase uses conformation-dependent Mg(2+) activation to couple catalysis of tryptophan activation to specific, functional domain movements. Rate acceleration by Mg(2+) requires ∼-6.0 kcal/mol in protein⋅Mg(2+) interaction energy, none of which arises from the active site. A highly cooperative interaction between Mg(2+) and four residues from a remote, conserved motif that mediates the shear of domain movement (1) destabilizes the pretransition state conformation, thereby (2) inducing the Mg(2+) to stabilize the transition state for k(cat) by ∼-5.0 kcal/mol. Cooperative, long-range conformational effects on the metal therefore convert an inactive Mg(2+) coordination into one that can stabilize the transition state if, and only if, domain motion occurs. Transient, conformation-dependent Mg(2+) activation, analogous to the escapement in mechanical clocks, explains vectorial coupling.
authors.affiliation: Department of Biochemistry and Biophysics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7260, USA.
authors.forename: Violetta
authors.initials: V
authors.lastname: Weinreb
authors.forename: Li
authors.initials: L
authors.lastname: Li
authors.forename: Charles W
authors.initials: CW
authors.lastname: Carter
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D002384:Catalysis; D001411:Geobacillus stearothermophilus / Q000201:enzymology* / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism*; D008958:Models, Molecular; D016296:Mutagenesis; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D014369:Tryptophan-tRNA Ligase / Q000737:chemistry* / Q000235:genetics
chemical_list: D000255:Adenosine Triphosphate; D014369:Tryptophan-tRNA Ligase; D008274:Magnesium