Allosteric kinetics of pyruvate kinase of Saccharomyces carlsbergensis

Abstract

The allosteric model of Monod et al. (1965) has been used to analyse the steadystate kinetics of pyruvate kinase from Saccharomyces carlsbergensis. The dissociation constants for the substrate phosphoenolpyruvate, the inhibitor ATP as well as the activator fructose-1, 6-diphosphate from the R and T state were calculated using a series of computer programs. On the basis of a crucial relation (derived in the Appendix), which correlates the Hill coefficient and the half-saturating concentration of substrate saturation curves with the parameters of the model of Monod et al., it is possible to differentiate between exclusive and non-exclusive ligand binding. On the other hand, this relation makes it possible to fit the experimental data to an extended model assuming only partially concerted transitions in each enzyme molecule. The physical data of yeast pyruvate kinase point to a tetrameric structure, whereas the steady-state kinetics favour a trimeric one. This discrepancy in the number of protomers can be overcome by the use of an extended model, which permits the occurrence of hybrid states R tT n−t. The introduction of one symmetrical hybrid state R 2T 2 into the model explains the kinetic data of yeast pyruvate kinase on the basis of four, probably identical, protomers. The equilibrium constants between the states are given. In the Appendix the derivation of the equation describing the occurrence of hybrid states is reported.