Complex I (NADH:ubiquinone oxidoreductase) is the first and largest enzyme of the respiratory chain which plays a central role in cellular energy production by coupling electron transfer between NADH and ubiquinone to proton translocation and therefore contributing to the proton-motive force required for the synthesis of ATP PMID:23417064,PMID:19635800. Generally, the bacterial complex consists of 14 different subunits, while the mitochondrial complex contains homologues to these subunits in addition to approximately 31 additional proteins PMID:18394423. Structural domains comprising this superfamily share the structure of subunit 15, which is thought to be involved in complex stabilisation, and storage of iron for iron-sulphur cluster regeneration in the complex PMID:16469879.