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 biosynthesis. Upon ligation, ATCase undergoes a conformational transition from a low-activity T-state to a high-activity R-state. This transition involves major changes in the molecular architecture, including structural rearrangements of several intersubunit interfaces and a 12 Å expansion of the molecule along its 3-fold axis. Solute-induced osmotic stress experiments report that ?208 solvent waters are taken up by ATCase as it binds substrate. Solvent-accessible surface area calculations conducted on the T and R conformers of ATCase agree very well with this result, predicting that ?189 waters are taken up during this conformational change. Both osmotic stress measurements and surface area calculations on the catalytic trimer of ATCase predict water release upon ligation of the trimer. Specific aspects of the application of osmotic stress to ATCase are also discussed, including solute size effects, and an assessment of potential alternative explanations for these results.