Mechanism of Metabolite transfer between Enzymes in Glycolysis and Connected Pathways

Abstract: The metabolite produced by one enzyme in a metabolic pathway can be transferred to the next enzyme in the pathway by free diffusion via solution or by channelling in an enzyme-enzyme complex without prior release of the metabolite to solution. While metabolite channelling is known to occur in systems where stable multienzyme complexes form, conflicting views have been expressed as to its existence in pathways involving "soluble" enzymes. This thesis examines the kinetic evidence that has been presented in support of a direct (channelled) metabolite transfer between soluble enzymes in glycolysis and connected pathways. More specifically, transient-state kinetic studies have been performed to test the proposal that there is a channelled transfer of glyceraldehyde-3-phosphate from aldolase to glyceraldehyde-3-phosphate dehydrogenase. We found that this proposal is based on a misinterpretation of kinetic data that are actually fully consistent with a free-diffusion mechanism of metabolite transfer. Furthermore, the much-discussed hypothesis that a channelled transfer of NADH occurs between dehydrogenases of distinct coenzyme specificity has been tested by crucial steady-state and transient-state kinetic experiments. Our results lend no support to this hypothesis, but are fully consistent with a free-diffusion mechanism of NADH transfer. It is concluded that no tenable kinetic evidence presently is available for a channelled transfer of metabolites between enzymes that do not form stable multienzyme complexes.