<p>Electron transfer flavoproteins (ETFs) serve as specific electron acceptors for primary dehydrogenases, transferring the electrons to terminal respiratory systems. They can be functionally classified into constitutive, "housekeeping" ETFs, mainly involved in the oxidation of fatty acids (Group I), and ETFs produced by some prokaryotes under specific growth conditions, receiving electrons only from the oxidation of specific substrates (Group II) [<cite idref="PUB00033231"/>]. </p><p>ETFs are heterodimeric proteins composed of an alpha and beta subunit, and contain an FAD cofactor and AMP [<cite idref="PUB00004879"/>, <cite idref="PUB00024527"/>, <cite idref="PUB00033232"/>]. ETF consists of three domains: domains I and II are formed by the N- and C-terminal portions of the alpha subunit, respectively, while domain III is formed by the beta subunit. Domains I and III share an almost identical alpha-beta-alpha sandwich fold, while domain II forms an alpha-beta-alpha sandwich similar to that of bacterial flavodoxins. FAD is bound in a cleft between domains II and III, while domain III binds the AMP molecule. Interactions between domains I and III stabilise the protein, forming a shallow bowl where domain II resides.</p><p>This entry represents the beta subunit (domain III) of both Group I and Group II ETFs.</p>