Influence of alpha-CH-->NH substitution in C8-CoA on the kinetics of association and dissociation of ligands with medium chain acyl-CoA dehydrogenase.

We previously reported that the kinetic profiles for the association and dissociation of functionally diverse C(8)-CoA-ligands, viz., octanoyl-CoA (substrate), octenoyl-CoA (product), and octynoyl-CoA (inactivator) with medium chain acyl-CoA dehydrogenase (MCAD), were essentially identical, suggesting that the protein conformational changes played an essential role during ligand binding and/or catalysis [Peterson, K. L., Sergienko, E. E., Wu, Y., Kumar, N. R., Strauss, A. W., Oleson, A. E., Muhonen, W. W., Shabb, J. B., and Srivastava, D. K. (1995) Biochemisry 34, 14942-14953]. To ascertain the structural basis of the above similarity, we investigated the kinetics of association and dissociation of alpha-CH-->NH-substituted C(8)-CoA, namely, 2-azaoctanoyl-CoA, with the recombinant form of human liver MCAD. The rapid-scanning and single wavelength stopped-flow data for the binding of 2-azaoctanoyl-CoA to MCAD revealed that the overall interaction proceeds via two steps. The first (fast) step involves the formation of an enzyme-ligand collision complex (with a dissociation constant of K(c)), followed by a slow isomerization step (with forward and reverse rate constants of k(f) and k(r), respectively) with concomitant changes in the electronic structure of the enzyme-bound FAD. Since the latter step involves a concurrent change in the enzyme's tryptophan fluorescence, it is suggested that the isomerization step is coupled to the changes in the protein conformation. Although the overall binding affinity (K(d)) of the enzyme-2-azaoctanoyl-CoA complex is similar to that of the enzyme-octenoyl-CoA complex, their microscopic equilibria within the collision and isomerized complexes show an opposite relationship. These results coupled with the isothermal titration microcalorimetric studies lead to the suggestion that the electrostatic interaction within the enzyme site phase modulates the microscopic steps, as well as their corresponding ground and transition states, during the course of the enzyme-ligand interaction.