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P8. CLASSICAL ENZYMOLOGY AT ATOMIC RESOLUTION - SUBSTRATE ALIGNMENT & INDUCED FIT

Arginine Kinase, like its homologue Creatine Kinase, buffers cellular ATP levels and is a member of the most thoroughly investigated family of enzymes, it has been claimed. After 30 years of attempts, several structures of creatine kinase in its inactive substrate-free configuration have emerged. Our structures include the 42 kD arginine kinase as a transition state analog complex (the first for a bimolecular enzyme) now refined at 1.2 Å resolution with R = 12%. Substrates are within a few degrees of their optimal reaction trajectories, suggesting that pre-alignment is a more important factor in catalysis than in the single-substrate enzymes visualized in prior transition state complexes. The structure is consistent only with molecular mechanics calculations that have a protonation state compatible with concerted proton/phosphoryl transfer. Mutants, alternative substrates, classical kinetics and structure determination have been used to probe the role of substrate alignment and other catalytic effects. Recently, we have obtained the substrate-free structure, revealing the full extent of the induced-fit conformational changes. There is a 17° domain rotation near the guanidine substrate, a 10 Å closure around the nucleotide, and a 15 Å loop movement capping the active site. Unlike hexokinase and other small molecule kinases, both substrates induce change and configure the active site. Such complicated movements may have evolved because both ATP and phosphoarginine are “high energy compounds” whose hydrolysis would be wasteful of the cell’s energy. Thus, detailed structure analysis, combined with kinetics is beginning to cast new light on some classical enzymological questions.