A Master Switch Couples Mg2+-Assisted Catalysis to Domain Motion in B. stearothermophilus Tryptophanyl-tRNA Synthetase

We demonstrate how tryptophanyl-tRNA synthetase uses conformation-dependent Mg2+ activation to couple catalysis of tryptophan activation to specific, functional domain movements. Rate acceleration by Mg2+ requires similar to-6.0 kcal/mol in protein center dot Mg2+ interaction energy, none of which arises from the active site. A highly cooperative interaction between Mg2+ 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 Mg2+ to stabilize the transition state for k(cat) by similar to-5.O kcal/mol. Cooperative, long-range conformational effects on the metal therefore convert an inactive Mg2+ coordination into one that can stabilize the transition state if, and only if, domain motion occurs. Transient, conformation-dependent Mg2+ activation, analogous to the escapement in mechanical clocks, explains vectorial coupling.