They include a number of transmembrane cytochrome b-like proteins including coenzyme Q - cytochrome c reductase (cytochrome bc1 ); cytochrome b6f complex; formate dehydrogenase, respiratory nitrate reductase; succinate - coenzyme Q reductase (fumarate reductase); and succinate dehydrogenase.  See electron transport chain.
*3.D.1 The H or Na-translocating NADH Dehydrogenase ("complex I") family 
*3.D.2 The Proton-translocating Transhydrogenase (PTH) Family
*3.D.3 The Proton-translocating Quinol:Cytochrome c Reductase) Superfamily
*3.D.4 Proton-translocating Cytochrome Oxidase (COX) Superfamily
*3.D.5 The Na-translocating NADH:Quinone Dehydrogenase (Na-NDH or NQR) Family
*3.D.6 The Putative Ion (H or Na)-translocating NADH:Ferredoxin Oxidoreductase (NFO or RNF) Family
*3.D.7 The H:Heterodisulfide Oxidoreductase (HHO) Family
*3.D.8 The Na- or H-Pumping Formyl Methanofuran Dehydrogenase (FMF-DH) Family
*3.D.9 The H-translocating F420H2 Dehydrogenase (F420H2DH) Family
*3.D.10 The Prokaryotic Succinate Dehydrogenase (SDH) Family