The cytochrome b561 proteins are a family of transmembrane oxidoreductases that function as electron transporters by accepting a single electron from a cytoplasmic reducing agent - generally assumed to be ascorbate - and transferring it across a membrane to reduce a non-cytoplasmic substrate. Known non-cytoplasmic substrates (electron acceptors) include ferric (Fe3+) ions, which are reduced to ferrous (Fe2+) ions, and monodehydroascorbate, which is reduced to ascorbate. (Adapted from FBrf0258249.)

ascorbate ferrireductase (transmembrane) (7.2.1.3)