A Master Switch Couples Mg~(2+)-Assisted Catalysis to Domain Motion in B. stearothermophilus Tryptophanyl-tRNA Synthetase

摘要

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.O 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.