FUNCTIONALLY LINKED HYDRATION CHANGES IN ESCHERICHIA-COLI ASPARTATE-TRANSCARBAMYLASE AND ITS CATALYTIC SUBUNIT

Aspartate transcarbamylase (ATCase) is a highly regulated, dodecameric enzyme that catalyzes the first committed step in pyrimidine biosynth esis. Upon ligation, ATCase undergoes a conformational transition from a low-activity T-state to a high-activity R-state. This transition in volves major changes in the molecular architecture, including structur al rearrangements of several intersubunit interfaces and a 12 Angstrom expansion of the molecule along its 3-fold axis. Solute-induced osmot ic stress experiments report that similar to 208 solvent waters are ta ken up by ATCase as it binds substrate. Solvent-accessible surface are a calculations conducted on the T and R conformers of ATCase agree ver y well with this result, predicting that similar to 189 waters are tak en up during this conformational change. Both osmotic stress measureme nts and surface area calculations on the catalytic trimer of ATCase pr edict water release upon ligation of the trimer. Specific aspects of t he application of osmotic stress to ATCase are also discussed, includi ng solute size effects, and an assessment of potential alternative exp lanations for these results.