W0238

Synchrotron Radiation Damage Cleaves the Thioester Bond In HMG-Coa, Mimicing the First Hydride Transfer from NADH to form the Proposed Aldehyde Reaction Intermediate. Chandra J. Duncan, John W. Burgner II, Calvin N. Steussy, Jeff Watson, Timothy J. Schmidt, Cynthia V. Stauffacher, Dept. of Biology, Purdue Univ., Lilly Hall, State St., West Lafayette, IN 47907 USA.

The (S)-HMG-CoA reductase (HMGR) from Pseudomonas mevalonii catalyzes the four electron reduction of (S)-HMG-CoA to (R)-mevalonate using NAD(P)H as the reductant. In this report, we consider the structure of a binary complex of enzyme and (S)-HMG-CoA at a resolution of 2.1Å The X-ray data for this study was obtained from two different synchrotron beam lines. In determining the structure of this this complex, we have noted progressive specific radiation damage at two locations in the structure of the complex. The S-S bond of an intermolecular cystine seems to be cleaved with increased exposure. In addition, the thioester bond of the bound HMG-CoA is also cleaved during the course of data collection. The product of this cleavage clearly involves formation of C=O and is likely to be mevaldehyde, which is an intermediate in the four electron reduction. We assume that a solvated electron reacts with the thioester forming a radical which, in turn, undergoes homolytic cleavage to form CoA and mevaldehyde. The thiohemiacetal, formed from the aldehyde and the CoA, might be expected to be a product here; however, the S-H of the CoA rotates away from the C=O of the aldehyde.