InterPro domain: IPR042106

This entry represents chain 6 from NADH:ubiquinone oxidoreductase and NADH-plastoquinone oxidoreductase. Bacterial proton-translocating NADH-quinone oxidoreductase (NDH-1) is composed of 14 different subunits. The chain belonging to this family is a subunit that constitutes the membrane sector of the complex. It reduces ubiquinone to ubiquinol utilising NADH. Plant chloroplastic NADH-plastoquinone oxidoreductase reduces plastoquinone to plastoquinol. Mitochondrial NADH-ubiquinone oxidoreductase from a variety of sources reduces ubiquinone to ubiquinol.

NADH:ubiquinone oxidoreductase (complex I) ( 7.1.1.2 ) is a respiratory-chain enzyme that catalyses the transfer of two electrons from NADH to ubiquinone in a reaction that is associated with proton translocation across the membrane (NADH + ubiquinone = NAD+ + ubiquinol) [ 1 ]. Complex I is a major source of reactive oxygen species (ROS) that are predominantly formed by electron transfer from FMNH(2). Complex I is found in bacteria, cyanobacteria (as a NADH-plastoquinone oxidoreductase), archaea [ 2 ], mitochondria, and in the hydrogenosome, a mitochondria-derived organelle. In general, the bacterial complex consists of 14 different subunits, while the mitochondrial complex contains homologues to these subunits in addition to approximately 31 additional proteins [ 3 ].

Subunit NuoJ has an unusual non-globular fold: three linearly arranged amino-terminal TM helices border NuoK, and TMs 4 and 5 are separated at the opposite sides of the domain. Thus, NuoJ interweaves between NuoK, A and N, stabilising the complex. TM3 of NuoJ contains in its middle a pi-bulge/kink, so this helix is probably flexible and mechanistically important. TM3 is the most conserved helix in NuoJ and is a hotspot for human mitochondrial disease mutations [ 4 ].

1. The NADH:ubiquinone oxidoreductase (complex I) of respiratory chains. Q. Rev. Biophys. 25, 253-324
2. The respiratory complex I of bacteria, archaea and eukarya and its module common with membrane-bound multisubunit hydrogenases. FEBS Lett. 479, 1-5
3. Assembly of the Escherichia coli NADH:ubiquinone oxidoreductase (complex I). Biochim. Biophys. Acta 1777, 735-9
4. Structure of the membrane domain of respiratory complex I. Nature 476, 414-20