Study of the Position of NAD and Its Effect on the Conformation of d ‐Glyceraldehyde‐3‐phosphate Dehydrogenase by Small‐Angle X‐Ray Scattering

The structural changes of mammalian skeletal muscle d‐glyceraldehyde‐3‐phosphate dehydrogenase induced by the binding of the coenzyme, NAD, were studied by small‐angle X‐ray scattering with the double‐monitor technique. As shown by our difference method the radius of gyration of the apoenzyme decreases by 0.08 nm on the binding of four molecules of NAD. The decrease in the radius of gyration is not a monotonous function of saturation with NAD. From the analysis of our data it follows that, in the order of binding, the first and third NADs are positioned at a distance of about 2 nm from the centre of gravity of the molecule, whereas the second and fourth NADs are at a distance of about 4 nm. As indicated by the change in the radius of gyration the protomers of the tetrameric molecule are dislocated by about 0.1–0.2 nm (i.e. 1–4°) relative to each other.