Molecular basis for the transfer of nicotinamide adenine dinucleotide among dehydrogenases

Abstract

NADH is transferred directly from one dehydrogenase enzyme site to another without intervention of the aqueous solvent whenever the 2 dehydrogenases are of opposite chiral specificity as regards the C4 H of NADH which is transferred in the catalyzed reduction reaction. When both enzymes catalyze the transfer of H from the same face of the nicotinamide ring, direct enzyme-enzyme transfer of NADH is not possible. Utilizing an advanced computer graphics facility, and the known 3-dimensional coordinates for 3 dehydrogenases, the feasibility of various aspects of the direct transfer of dinucleotide from the site of one enzyme to the site of the other was investigated. The facile passage of the coenzyme through the 1st enzyme site requires an open protein conformation, characteristic of the apoenzyme rather than the holoenzyme structure. Since 2 dehydrogenases of the same chirality bind coenzyme in the same conformation, the direct transfer of coenzyme from one site to the other is impossible due to the restriction in molecular rotation of the coenzyme in the path of transfer from one binding site to the other; therefore, coenzyme can only be transferred from one dehydrogenase site to another site via the intermediate dissociation of coenzyme into the aqueous milieu. When an A dehydrogenase and a B dehydrogenase are juxtaposed, it is stereochemically feasible to transfer the nicotinamide ring from its specific binding site in one enzyme to the site in the other. Dissociation of the 1st enzyme from the rest of the coenzyme molecule permits an internal rotation of the nicotinamide about the glycosidic bond by 180.degree.. This permits the entry of the rest of the coenzyme molecule into the 2nd enzyme binding site in proper conformation for specific interaction with the 2nd enzyme protein. A calculation of the electrostatic potential molecular surface of the enzyme face containing entry to the coenzyme binding site of the 3 dehydrogenases of known structure reveals that the 2 A dehydrogenases have large areas of negative potential, whereas the B dehydrogenase has large areas of positive potential, on the surface around the opening to the nicotinamide end of the binding site. The structural information available is consistent with the observed exclusive direct transfer of coenzyme between dehydrogenases of opposite chirality.