Thermodynamics of Assembly of Escherichia Coli Aspartate Transcarbamoylase

Reaction microcalorimetry and potentiometry have been used to define the thermodynamics of assembly of Escherichia coli aspartate transcarbamoylase (aspartate carbamoyltransferase, carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) from its catalytic and regulatory subunits and the linkage between assembly and proton binding. Over the pH range 7-9.5 and the temperature range 15-30 degrees C, assembly is characterized by negative enthalpy and heat capacity changes and positive entropy changes. The dependence of the enthalpy and entropy changes on pH is complex; however, the negative heat capacity change results in both quantities becoming more negative with increasing temperature. Assembly is linked to the binding of protons; the effects observed can be fit to models involving six or more ionizable groups with pK values of 7.3-7.4, 8.5-8.8, and 9.2-9.5, which ionize cooperatively. Contributions from additional groups cannot be ruled out and are in fact expected. The overall pattern of thermodynamic effects implies a complex set of intersubunit interactions. Protonation reactions and increased hydrogen bonding are likely to be the major sources of the negative enthalpy change; however, the negative heat capacity change results primarily from changes in solvent structure associated with hydrophobic and electrostatic bond formation with changes in low-frequency vibrational modes making a secondary contribution. Similarly, the relatively small entropy change observed within the temperature range examined probably reflects the balance between positive contributions from increased hydrophobic and electrostatic bonding and negative contributions from increased hydrogen bonding and damping of low-frequency vibrational modes.