Specific protein dynamics near the solvent glass transition assayed by radiation-induced structural changes

The nature of the dynamical coupling between a protein and its surrounding solvent is an important, yet open issue. Here we used temperature-dependent protein crystallography to study structural alterations that arise in the enzyme acetylcholinesterase upon X-ray irradiation at two temperatures: bel ow and above the glass transition of the crystal solvent. A buried disulfid e bond, a buried cysteine, and solvent exposed methionine residues show dra stically increased radiation damage at 155 K, in comparison to 100 K. Addit ionally, the irradiation-induced unit cell volume increase is linear at 100 K, but not at 155 K, which is attributed to the increased solvent mobility at 155 K. Most importantly, we observed conformational changes in the cata lytic triad at the active site at 155 K but not at 100 K. These changes lea d to an inactive catalytic triad conformation and represent, therefore, the observation of radiation-inactivation of an enzyme at the atomic level. Ou r results show that at 155 K, the protein has acquired-at least locally-suf ficient conformational flexibility to adapt to irradiation-induced alterati ons in the conformational energy landscape. The increased protein flexibili ty may be a direct consequence of the solvent glass transition, which expre sses as dynamical changes in the enzyme's environment. Our results reveal t he importance of protein and solvent dynamics in specific radiation damage to biological macromolecules, which in turn can serve as a tool to study pr otein flexibility and its relation to changes in a protein's environment.