Comparison of ligand polarization and enzyme activation in medium- and short-chain acyl-coenzyme A dehydrogenase-novel analog complexes.

Spectroelectrochemical and off-resonance Raman indicate that substrate/product binding to medium-chain acyl-coenzyme A (CoA) dehydrogenase (pMCAD) results in ligand polarization and positive flavin potential shifts, which activate the enzyme for electron transfer. Bacterial short-chain acyl-CoA dehydrogenase (bSCAD) typically exhibits smaller potential shifts upon substrate/product binding that have not been linked to ligand polarization. To further investigate the roles of ligand binding and polarization in activation, several novel aromatic carboxyloyl-CoAs were designed. These analogs allowed for the first direct comparison of pMCAD and bSCAD mechanisms. The results indicate that pMCAD activation can occur without perceptible analog polarization. bSCAD data provide the first spectral evidence of ligand polarization. The potential alterations exhibited by ligand-bound bSCAD are smaller than those of pMCAD, while their directionality and magnitude suggest differing enzyme-analog interactions. Such data provide the first indication of variations in the activation mechanism of these enzymes, which were thought to be comparable in both structure and function.

Activation of substrate/product couples by medium-chain acyl-CoA dehydrogenase.

Natural substrate/product binding activates medium-chain acyl-CoA dehydrogenase (MCAD) to accept electrons from its substrate by inducing a positive flavin midpoint potential shift. The energy source for this activation has never been fully elucidated. If ground-state alterations of the ligand, such as polarization, are entirely responsible for enzyme activation, the ligand potential should shift equally to that of the flavin but in the opposite direction. Ligand polarization is likely responsible for only a small portion of this activation. Here, thiophenepropionoyl- and furylpropionoyl-CoA analogs were used to directly measure the redox modulations of several ligand couples upon binding to MCAD. These measurements identified the thermodynamic contribution of ligand polarization to enzyme activation. Because the ligand potential alterations are significantly smaller than modulations in the flavin potential due to binding, other phenomena such as pK(a) changes, desolvation, and charge alterations are likely responsible for the thermodynamic modulations required for MCAD's activity.