Phosphoglycerate kinase--glyceraldehyde-3-phosphate dehydrogenase interaction: reaction rate studies

Rate studies using phosphoglycerate kinase (PGK)--glyceraldehyde-3-phosphate dehydrogenase (GPDH) enzyme pair have been carried out to distinguish between the two mechanisms of intermediate metabolite transfer, namely diffusion through the solvent versus "substrate channelling" within an enzyme-enzyme complex. A procedure has been described for the assay of the rates of PGK-catalysed and the PGK-GPDH coupled reactions at high (saturating) GPDH concentration. With PGKs of rabbit muscle and yeast, the coupled reaction proceeded faster than the PGK-catalysed reaction. At a high salt concentration (0.5 M KCl), where a PGK-GPDH complex is known to dissociate, the two reactions proceeded at almost equal rates. At fixed PGK concentration, the rate of the coupled reaction at high (saturating) GPDH concentration varied with the nature (biological origin) of the latter enzyme. In the presence of 0.5 M KCl, the saturating rate values with different GPDHs were almost equal. The PGK-catalysed reaction exhibited typical Michaelian behaviour on varying the substrate concentrations (linear double reciprocal plots). The Km values for 3-PGA (0.51 mM) and ATP (0.40 mM) were independent of the concentration of the second substrate. The double reciprocal plots for the coupled reaction showed downward curvature, i.e. activation at higher substrate concentrations. The ratio of the rate of the coupled reaction: the rate of the PGK catalysed reaction was found to be a function of the nature of PGK, nature of GPDH, nature of buffer, pH, salt concentration and substrate concentrations. The ratio varied between close to unity at low substrate concentrations, to three when the Vmax values of the two reactions were compared. At low substrate concentrations, the rate of the coupled reaction became independent of the nature of GPDH. It has been suggested that in the PGK-GPDH pair, the intermediate metabolite (BPG) is transferred directly from one enzyme to the other within an enzyme-enzyme complex, except at high salt or low substrate concentrations. Under the latter conditions, data were consistent with metabolite transfer by diffusion. Implications of these results for coupled enzyme assays have been discussed.