Thermodynamics of ligand binding and catalysis in human liver medium-chain acyl-CoA dehydrogenase: comparative studies involving normal and 3'-dephosphorylated C8-CoAs and wild-type and Asn191 --> Ala (N191A) mutant enzymes.

Following our demonstration that the terminal 3'-phosphate group of acyl-CoA substrates (which is confined to the exterior of the protein structure, and is fully exposed to the outside solvent environment) exhibits a functional role in the recombinant human liver medium-chain acyl-CoA dehydrogenase (MCAD)-catalyzed reaction [Peterson, K. L., and Srivastava, D. K. (1997) Biochem. J. 325, 751-760], we became interested in delineating its thermodynamic contribution in stabilizing the "ground" and "transition" state structures during enzyme catalysis. Since the 3'-phosphate group of the coenzyme A thiolester has the potential to form a hydrogen bond with the side chain group of Asn-191, these studies were performed utilizing both normal and 3'-dephosphorylated forms of octanoyl-CoA and octenoyl-CoA (cumulatively referred to as C8-CoA) as the physiological substrate and product of the enzyme, respectively, as well as utilizing wild-type and Asn191 --> Ala (N191A) site-specific mutant enzymes. The experimental data revealed that the enthalpic contribution of the 3'-phosphate group was similar in both ground and transition states, and was primarily derived from the London-van der Waals interactions (between the 3'-phosphate group of C8-CoA and the surrounding protein moiety), rather than from the potential hydrogen bonding. The temperature dependence of DeltaH degrees for the binding of octenoyl-CoA and 3'-dephosphooctenoyl-CoA revealed that the deletion of the 3'-phosphate group from octenoyl-CoA increased the magnitude of the heat capacity changes (DeltaCp degrees) from -0.53 to -0.59 kcal mol-1 K-1. Although the latter effect could be attributed to an increase in the relative hydrophobicity of the ligand, the experimentally observed DeltaCp degrees's (for either of the ligands) could not be predicted on the basis of the changes in the solvent-accessible surface areas of the enzyme and ligand species. These coupled with the fact that the DeltaCp degrees for the binding of octenoyl-CoA to pig kidney MCAD (which is believed to be structurally identical to human liver MCAD) is only -0.37 kcal mol-1 K-1 [Srivastava, D. K., Wang, S., and Peterson, K. L. (1997) Biochemistry 36, 6359-6366] prompt us to question the reliability of predicting the DeltaCp degrees values of the enzyme-ligand complexes from their X-ray crystallographic data. Arguments are presented that certain intrinisic limitations of the crystallographic data preclude kinetic and thermodynamic predictions about the enzyme-ligand complexes and enzyme catalysis.