TRANSITION-STATE STRUCTURE OF ARGININE KINASE - IMPLICATIONS FOR CATALYSIS OF BIMOLECULAR REACTIONS

Arginine kinase belongs to the family of enzymes, including creatine k inase, that catalyze the buffering of ATP in cells with fluctuating en ergy requirements and that has been a paradigm for classical enzymolog ical studies. The 1.86-Angstrom resolution structure of its transition -state analog complex, reported here, reveals its active site and offe rs direct evidence for the importance of precise substrate alignment i n the catalysis of bimolecular reactions, in contrast to the unimolecu lar reactions studied previously, In the transition-state analog compl ex studied here, a nitrate mimics the planar gamma-phosphoryl during a ssociative in-line transfer between ATP and arginine, The active site is unperturbed, and the reactants are not constrained covalently as in a bisubstrate complex, so it is possible to measure how precisely the y are pre-aligned by the enzyme. Alignment is exquisite, Entropic effe cts may contribute to catalysis, but the lone-pair orbitals are also a ligned close enough to their optimal trajectories for orbital steering to be a factor during nucleophilic attack. The structure suggests tha t polarization, strain toward the transition state, and acid-base cata lysis also contribute, but, in contrast to unimolecular enzyme reactio ns, their role appears to be secondary to substrate alignment in this bimolecular reaction.