Flavocytochromes

Flavocytochromes are a class of redox enzymes that combine a flavin-containing subunit with a heme-containing c-type cytochrome, forming single polypeptides or multi-subunit complexes. They function as electron transfer devices in various bacteria and archaea, linking substrate oxidation to the broader respiratory chain.

The flavin domain typically houses FAD or FMN, while the cytochrome domain contains multiple c-type heme groups, enabling sequential electron transfer. The two domains can be covalently linked or associated as a tight complex. Many flavocytochromes are periplasmic or inner-membrane-associated, positioning them to receive electrons from substrates or relay them to membrane-associated partners.

Mechanism and redox properties: Upon substrate oxidation, the flavin becomes reduced and transfers electrons to the heme chain, which passes them to external electron acceptors such as cytochrome c, other components of the respiratory chain, or minerals. The flavin portion commonly acts as a two-electron carrier, while the heme chain enables one-electron transfers across multiple centers, allowing efficient bridging between different redox partners.

Biological roles and distribution: Flavocytochromes participate in anaerobic metabolism, including sulfide oxidation, formate or hydrogen oxidation, and sulfite reduction in sulfate-reducing bacteria. They also contribute to extracellular electron transfer in certain systems, aiding energy conservation under low-oxygen conditions.

Examples: Classic instances include flavocytochrome c sulfide dehydrogenase (FCCSD) and related flavocytochrome c enzymes (such as flavocytochrome c3 in some sulfate-reducing bacteria). These enzymes illustrate the modular design that integrates flavin-based catalysis with multi-heme cytochrome electron transfer.