Coenzyme Q can control the efficiency of oxidative phosphorylation

Energy of metabolic oxidations is conserved in the form of ATP by the process of oxidative phosphorylation in mitochondria. The possibility to recognize alterations in the efficiency of oxidative phosphorylation in pathological states and to improve this efficiency in order to correct diseases requires knowledge of the mechanisms controlling the rate of ATP synthesis. This task is hampered by uncertainties still existing on the organization and mechanism of the enzymes carrying out oxidative phosphorylation. The authors have collected experimental evidence that coenzyme Q concentration in the mitochondrial membrane phospholipids in physiologically not saturating for maximal electron-transfer rate: in fact the Km of the redox enzymic complexes, using the oxidized and reduced form of coenzyme Q, for these substrates, are in the range of their concentrations in the membrane. The addition of exogenous coenzyme Q enhances the respiratory turnover above the physiological rate but without reaching theoretical Vmax, owing to the limited miscibility of ubiquinones with the membrane phospholipids. On the contrary, a decrease of ubiquinone content in the membrane lowers electron-transfer activity in a reversible fashion. Taking account that the rate of lateral coenzyme Q diffusion in the membrane does not appear to control electron transfer, it is suggested that only ubiquinone concentration affects the efficiency of oxidative phosphorylation, with interesting pathological and pharmacological implications.