The long term goal of this research is the development of thermodynamic approaches towards defining the catalytic and regulatory mechanisms of E. coli aspartate transcarbamylase. From analyses of the energetics of ligand binding and of the subunit interactions, thermodynamic criteria will be developed which will be used to test and develop models of both mechanisms. Calorimetric measurements will be emphasized. The principal goals for the coming year are the following: 1. The completion of a calorimetric analysis of the binding of CTP and ATP, over a range of pH, and in the presence and absence of PALA, to c6r6, c3 and r2. 2. The application of hydrogen exchange techniques to characterize the magnitude of the conformational transitions induced in 3c, r2, and c6r6, in the presence and absence of PALA, CTP, and ATP. 3. A preliminary examination of mutant and chemically modified enzymes, particularly those produced by treatment with exopeptidases.