Crystallographic analysis revealed that the nicotinamide ring of NAD can bind with multiconformations to aldehyde dehydrogenase (ALDH) (Ni, L., Zhou, J., Hurley, T. D., and Weiner, H. (1999) Protein Sci. 8, 2784-2790). Electron densities can be defined for two conformations, neither of which appears to be compatible with the catalytic reaction. In one conformation, it would prevent glutamate 268 from functioning as a general base needed to activate the catalytic nucleophile, cysteine 302. In the other confromation, the nicotinamide is too far from the enzyme- substrate adduct for efficient hydride transfer. In this study, NMR and fluorescence spectroscopies were used to demonstrate that NAD and NADH bind to human liver cytosol and mitochondrial ALDH such that the nicotinamide samples a population of conformations while the adenosine region remains relatively immobile. Although the nicotinamide possesses extensive conformational heterogeneity, the catalyzed reaction leads to the stereospecific transfer of hydride to the coenzyme. Mobility allows the nicotinamide to move into position to be reduced by the enzyme-substrate adduct. Although the reduced nicotinamide ring retains mobility after NADH formation, the extent of the motion is less than that of NAD. It appears that after reduction the population of favored nicotinamide conformations shifts toward those that do not interfere with the ability of the enzyme to release the reaction product. In the case of the mitochondrial, but not the cytosolic, enzyme this change in conformational preference is promoted by the presence of Mg2+ ions. Coenzyme conformational mobility appears to be beneficial to catalysis by ALDH throughout the catalytic cycle.
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