Recent Studies on the Allosteric Glyceraldehyde-3-Phosphate Dehydrogenase from Yeast

Abstract

Independent physical and chemical investigations have shown, that glyceraldehyde-3-phosphate dehydrogenase is a tetramer consisting of subunits with identical primary structure (1, 2, 3). At neutral pH and 20°C NAD is bound to four identical and independent sites on the enzyme. The sites of Y-GAPDH are characterized by a relatively low affinity (K_D = 10^-6 –10^-4M) compared to GAPDH from other sources (K_D for the enzyme from rabbit muscle and lobster = 10^-7 – 10^-9M (4)). The enzyme-NAD complex exhibits a weak absorption band between 320 and 450mμ (5). Utilizing this band the binding of the coenzyme can be followed with equilibrium and kinetic methods (6, 7, 8). Under certain conditions (e.g. pH 8, 5 and 40°C) the binding of NAD is positively cooperative. In this paper the previous studies on the concerted mechanism of NAD-binding shall be reviewed only shortly. Recent observations about the anisotropy of the gross enzyme structure and its change as a function of the degree of saturation will be presented. However, the main interest will be devoted to the validity of the model at other pH-values and temperatures. Furthermore some recent investigations about the reversible dissociation of the enzyme into subunits will be reported.