Effect of pH on the mitochondrial energy-linked and non-energy-linked transhydrogenation reactions

The effect of pH on the kinetics of the following transhydrogenation reactions catalyzed by energized and nonenergized submitochondrial particles has been studied: NADH leads to 3-acetylpyridine adenine dinucleotide phosphate (AcPyADP), NADH leads to thionicotinamide adenine dinucleotide phosphate (thioNADP), NADPH leads to AcPyADP, and NADPH leads to thioNADP. The effect of membrane energization on reaction rates can be approximated in the case of NADH leads to AcPyADP and thioNADP transhydrogenations, or equaled in the case of NADPH leads to AcPyADP and thioNADP transhydrogenations by lowering the assay pH to less than or equal to 6.0. For the reactions NADH leads to AcPyADP and thioNADP under energy-linked conditions, substrate Km values are lowest and Vmax values are highest at pH 7 to 7.5, the optimum pH for mitochondrial energy transduction processes. Under non-energy-linked conditions, however, Km values were lowest and Vmax values were highest at the most acid conditions (pH = 5.5) examined. Plots of ln (Vmax/Km) (an index of enzyme-substrate affinity to form a complex) versus pH showed the highest affinity at pH 7 to 7.5 for energy-linked conditions. Similar plots for non-energy-linked conditions. Similar plots for non-energy-linked conditions. Similar plots for non-energy-linked conditions showed a sharp and linear increase in the value of ln (Vmax/Km) as the assay pH was lowered from 8.5 to 6 to 6.5. This was followed by a less steep line down to pH 5.5, with a clear break at pH 6 to 6.5. These results suggested the involvement of ionizable group(s) with pK value(s) at pH 6 to 6.5 affecting enzyme-substrate binding under non-energy-linked conditions. An analogous mechanism, possibly by way of proton-induced conformation change of the transhydrogenase enzyme (EC 1.6.1.1), might be involved in increasing enzyme-substrate affinity and consequently the rate of transhydrogenation under energy-linked conditions.roton-induced conformation change of the transhydrogenase enzyme (EC 1.6.1.1), might be involved in increasing enzyme-substrate affinity and consequently the rate of transhydrogenation under energy-linked conditions.