| Abstract: | The binding of oxidized and reduced coenzyme (NAD+ and NADH) to 3-phosphoglyceroyl-glyceraldehyde-3-phosphate dehydrogenase has been studied spectrophotometrically and fluorimetrically. The binding of NAD+ to the acylated sturgeon enzyme is characterized by a significant quenching of the enzyme fluorescence (about 25%) and the induction of a difference spectrum in the ultraviolet absorbance region of the enzyme. Both of these spectroscopic properties are quantitatively distinguishable from those of the corresponding binary enzyme-NAD+ complex. Binding isotherms estimated by gel filtration of the acylated enzyme are in close agreement to those obtained by spectrophotometric and fluorimetric titrations. Up to four NAD+ molecules are bound to the enzyme tetramer. No anticooperativity can be detected in the binding of oxidized coenzyme, which is well described on the basis of a single class of four binding sites with a dissociation constant of 25 muM at 10 degrees C, pH 7.0. The binding of NADH to the acylenzyme has been characterized spectrophotometrically. The absorption band of the dihydronicotinamide moiety of the coenzyme is blue-shifted to 335 nm with respect to free NADH. In addition, a large hypochromicity (23%) is observed together with a significant increase of the bandwidth at half height of this absorption band. This last property is specific to the acylenzyme-DADH complex, since it disappears upon arsenolysis of the acylenzyme. The binding affinity of NADH to the acylated enzyme has been estimated by performing simultaneous spectrophotometric and fluorimetric titrations of the NADH appearance upon addition of NAD+ to a mixture of enzyme and excess glyceraldehyde 3-phosphate. In contrast to NAD+, the reduced coenzyme NADH appears to be relatively strongly bound to the acylated enzyme, the dissociation constant of the acylenzyme-NADH complex being estimated as 2.0 muM at 25 degrees C. In addition a large quenching of the NADH fluorescence (about 83%) is observed. The comparison of the dissociation constants of the coenzyme-acylenzyme complexes and the corresponding Michaelis constants suggests a reaction mechanism of the enzyme in which significant formation and dissociation of NAD+-acylenzyme and NADH-acylenzyme complexes occur. Under physiological conditions the activity of the enzyme can be regulated by the ratio of oxidized and reduced coenzymes. Possible reasons for the lack of anticooperativity in coenzyme binding to the acylated form of the enzyme are discussed. |
