The Mechanism of Proton Pumping by Respiratory Complex I

Abstract

Complex I (NADH:ubiquinone oxidoreductase) is the largest component of the respiratory chain. In mitochondria it contains one FMN and eight iron-sulfur clusters as prosthetic groups and is composed of some 40 subunits with a total mass of ∼1,000 kDa. Human complex I deficiencies are the most frequent cause of inherited mitochondrial disorders and have been implicated in different neurodegenerative disorders and biological ageing.Complex I uses the energy released by the transfer of two electrons from NADH to ubiquinone to pump four protons across the bacterial plasma membrane or the inner mitochondrial membrane. The structures of complex I obtained by us and others from different species, now finally allows deep insight into the still obscure molecular mechanism of this process and its regulation by the so-called active/deactive transition. The ubiquinone chemistry taking place in the peripheral domain plays a pivotal role in this mechanism of energy conversion, by providing the energy that drives vectorial proton transport at the four putative pump sites in the membrane domain complex I.The two-state stabilization change mechanism of energy conversion by complex I is consistent with the functional and structural evidence now available. According to this comprehensive hypothetical model of energy conversion that will be presented in detail, the stepwise reduction and pronation of ubiquinone drives a concerted structural rearrangement, which exerts strokes passing into the membrane domain over a distance of ∼200 A to drive the proton pump modules. The proposed mechanism is the first to inherently include thermodynamically feasible rationales for the reverse mode of the enzyme and its regulation by the active/deactive transition. Thereby, it has immediate implications for our understanding of different pathophysiological conditions involving mitochondrial function that will be discussed.