Comparison of van 't Hoff and calorimetrically determined enthalpies of binding of N-phosphonacetyl-L-aspartate to E. coli aspartate transcarbamylase.

A comparison has been made of the values obtained by direct calorimetric measurements and van 't Hoff analysis, under similar conditions, for the enthalpy of binding of the bisubstrate analog N-phosphonacetyl-L-aspartate (PALA) to E. coli aspartate transcarbamylase and its catalytic subunit. In the case of the catalytic subunit, data were obtained at both saturating and non-saturating concentrations of L-Asp, and at two ionic strengths. Despite a 1000-fold difference in protein concentrations, and the obligatory omission of carbamyl phosphate in the calorimetric experiments, the values obtained by the two methods are shown to agree to within 15% when appropriate corrections are made. These results suggest that subunit dissociation is not a significant factor at the low protein concentrations used in the van 't Hoff analysis, and, conversely, that aggregation of the protein is negligible at the high protein concentrations used in the calorimetric experiments. They also imply that, at pH 8.3, the enthalpic difference between the two conformational states of the enzyme which exist in the presence and absence of substrates is less than 2.5 kcal/mol. In addition, the trends in the three sets of data for the catalytic subunit indicate that ionic bonds are involved in binding PALA to the active site, and that non-productive binding by L-Asp is negligible under these experimental conditions.