Electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO) is a a 4Fe4S flavoprotein located in the inner mitochondrial membrane and catalyses ubiquinone (UQ) reduction. It forms a single structural domain with three functional regions (that bind FAD, the 4Fe4S cluster, and UQ) which are closely packed and share structural elements [1]. This domain includes the residues involved in UQ binding [1]. This domain is also found in FixC proteins from bacteria, which share similarities with ETF-QO [2].

1: Structure of electron transfer flavoprotein-ubiquinone oxidoreductase and electron transfer to the mitochondrial ubiquinone pool. Zhang J, Frerman FE, Kim JJ; Proc Natl Acad Sci U S A. 2006;103:16212-16217. PMID:17050691

2: The electron transfer flavoprotein fixABCX gene products from Azospirillum brasilense show a NifA-dependent promoter regulation. Sperotto RA, Gross J, Vedoy C, Passaglia LM, Schrank IS; Curr Microbiol. 2004;49:267-273. PMID:15386115