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4) Symposium Posters | Main^

P4. W0120: Orbital Steering and Entropy: A Bimolecular Phosphoryl Transferase Transition State Analog at 1.8 Å Resolution.

1999 American Crystallographi Association Meeting, May 22-27, Buffalo, NY.
James L. Gattis, Genfa Zhou, Pamela S. Pruett, Thayumanasamy Somasundaram, Ross Ellington, and Michael Chapman.
Institute of Molecular Biophysics, Florida State Univ., Tallahassee, FL 32306, USA

The structure of Arginine Kinase, a homolog of the ATP-buffering creatine kinase, has been determined as a complex with MgADP, nitrate, and arginine at 1.86 Å resolution (Zhou et al 1998). The current refinement at approximately 1.2 Å resolution will be presented as will the kinetic analysis of active site mutants. In this transition state analog complex, the gamma phosphate of ATP (which is transferred through a planer transition state) is replaced by a nitrate ion forming a stable complex without constraining the substrates covalently. To our knowledge this is the first high resolution structure of a bimolecular enzyme as a transition state analog complex. It offers a unique opportunity to address questions concerning the mechanism of bimolecular reactions in comparison to the unimolecular reactions (hydrolases, isomerases, etc.) predominately studied in the past. As with unimolecular systems there is evidence of general acid-base catalysis, strain, and charge bias contributions to catalysis. The structure also shows very precise alignment of substrates with angles of approach within a few degrees of ideal for in-line transfer of the gamma phosphate of ATP. These observations are consistent with the ideas of Jenks & Koshland (ca 1970) that entropy reduction and/or the steering of orbitals along trajectories for nucleophilic attack may be the predominant means of catalysis in multimolecular reactions. Genfa Zhou, Thayumanasamy Somasundaram, Eric Blanc, Gopalakrishnan Parthasarathy, W. Ross Ellington, Michael S. Chapman. 'Transition state structure of arginine kinase: Implications for catalysis of bimolecular reactions.' PNAS. Vol. 95 pp. 8449-8454. July 1998.

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