Alternative complex III (ACIII) is a multisubunit quinol:electron acceptor oxidoreductase that couples quinol oxidation with transmembrane proton translocation similar to bc1 complex (or complex III). It is a key component of the respiratory and photosynthetic electron transport chains in many bacteria. It has six subunits (ActA-F), ActD subunit represented in this entry. The transmembrane regions of this subunit form part of the TM arm of the complex [1,2]. Although the function of this subunit is not clear, it may play a role in stabilizing the TM region of ACIII which contributes to a stable menaquinol binding pocket and proton translocation passage [2].

1: Structure of the alternative complex III in a supercomplex with cytochrome oxidase. Sun C, Benlekbir S, Venkatakrishnan P, Wang Y, Hong S, Hosler J, Tajkhorshid E, Rubinstein JL, Gennis RB; Nature. 2018;557:123-126. PMID:29695868

2: Cryo-EM structures of the air-oxidized and dithionite-reduced photosynthetic alternative complex III from Roseiflexus castenholzii. Shi Y, Xin Y, Wang C, Blankenship RE, Sun F, Xu X; Sci Adv. 2020;6:eaba2739. PMID:32832681