Kinetics, control, and mechanism of ubiquinone reduction by the mammalian respiratory chain-linked NADH-ubiquinone reductase.

In mammalian cells the membrane-bound NADH-quinone oxidoreductase serves as the entry point for oxidation of NADH in the respiratory chain and as the proton-translocating unit which conserves the free energy of the enzyme intramolecular redox reactions as the free energy of the electrochemical proton gradient across the coupling membrane. This review summarizes the kinetic properties of the mammalian enzyme. Emphasis is placed on the hysteretic properties of the enzyme as related to the possible control of intramitochondrial NADH oxidation and to the mechanism of the enzyme interaction with ubiquinone. Recent evidence for participation of flavin and the protein-bound ubisemiquinone pair in the enzyme-catalyzed proton translocation mechanism are discussed.