Relation between fluorescence and conformation of iεNAD+ bound to dehydrogenases

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Abstract
This work reports on the interaction of the fluorescent nicotinamide 1,N6-ethenoadenine dinucleotide (epsilonNAD+) with horse liver alcohol dehydrogenase, octopine dehydrogenase, and glyceraldehyde-3-phosphate dehydrogenase from different sources (yeast, lobster muscle, and rabbit muscle). The coenzyme fluorescence is enhanced by a factor of 10-13 in all systems investigated. It is shown that this enhancement cannot be due to changes in the polarity of the environment upon binding, and that it must be rather ascribed to structural properties of the bound coenzyme. Although dynamic factors could also be important for inducing changes in the quantum yield of epsilonNAD+ fluorescence, the close similarity of the fluorescence enhancement factor in all cases investigated indicates that the conformation of bound coenzyme is rather invariant in the different enzyme systems and overwhelmingly shifted toward an open form. Dissociation constants for epsilonNAD+-dehydrogenases complexes can be determined by monitoring the coenzyme fluorescence enhancement or the protein fluorescence quenching. In the case of yeast glyceraldehyde-3-phosphate dehydrogenase at pH 7.0 and t = 20 degrees the binding plots obtained by the two methods are coincident, and show no cooperativity. The affinity of epsilonNAD+ is generally lower than that of NAD+, although epsilonNAD+ maintains most of the binding characteristics of NAD+. For example, it forms a tight complex with horse liver alcohol dehydrogenase and pyrazole, and with octopine dehydrogenase saturated by L-arginine and pyruvate. One major difference in the binding behavior of NAD+ and epsilonNAD+ seems to be present in the muscle glyceraldehyde-3-phosphate dehydrogenase. In fact, no difference was found for epsilon NAD+ between the affinities of the third and fourth binding sites. The results and implications of this work are compared with those obtained recently by other authors.

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