The NADH:ubiquinone oxidoreductase (complex I) of respiratory chains

The inner membranes of mitochondria contain three multi-subunit enzyme complexes that act successively to transfer electrons from NADH to oxygen, which is reduced to water (Fig. I). The first enzyme in the electron transfer chain, NADH:ubiquinone oxidoreductase (or complex I), is the subject of this review. It removes electrons from NADH and passes them via a series of enzyme-bound redox centres (FMN and Fe-S clusters) to the electron acceptor ubiquinone. For each pair of electrons transferred from NADH to ubiquinone it is usually considered that four protons are removed from the matrix (see section 4.1 for further discussion of this point).