Substrate channeling in glycolysis: a phantom phenomenon

It has been proposed that glycolytic enzymes form multienzyme complexes for direct transfer of metabolites from the producing enzyme to the utilizing one. Reexamination of the supporting evidence, which involves the transfer of NADH between its complexes with glycerol-3-phosphate dehydrogenase (alpha-glycerol phosphate dehydrogenase, GPDH; EC 1.1.1.8) and with L-lactate dehydrogenase (LDH; EC 1.1.1.27), has shown that the supporting evidence is based on misinterpretation of the kinetics of ligand exchange. Srivastava et al. have responded with a revision of their own and criticism of our data. To clarify this problem, we have carried out detailed kinetic studies on NADH binding to GPHD and LDH and on the displacement of enzyme-bound NADH by LDH or GPDH. The experiments were conducted at 10 degrees C in 50 mM Hepes, pH 7.5/100 mM KCl/1 mM EDTA/1 mM 2-mercaptoethanol, using rabbit muscle GPDH and LDH. The results show that the kinetic patterns exhibited by the displacement of NADH-bound enzyme by either GPDH or LDH are consistent with a dissociative mechanism but not with a direct transfer mechanism. Theoretical analysis shows that a combined dissociative and direct transfer mechanism can explain the transient kinetic data reported by Srivastava et al. if, and only if, a majority (approximately 90%) of the enzyme present in lower concentration exists as a complex with the second enzyme. However, data from tracer and traditional sedimentation equilibrium and from gel filtration experiments show that LDH and GPDH do not form complexes in the presence of saturating NADH concentration when the enzyme concentrations are ranged between 4 and 50 microM, a concentration equal to or greater than that used by Srivastava et al. Our results demonstrate that GPDH and LDH do not form multienzyme complex and the transfer of NADH between these enzymes proceeds via a dissociative mechanism.