Rubredoxin is a small nonheme, iron protein lacking acid-labile sulfide. Its single Fe, chelated to 4 Cys, functions as an electron acceptor and may also stabilize the conformation of the molecule. Binds 1 Fe(3+) ion per subunit. Observed in four forms, with and without iron, and with and without formylation at Met-1. Without formylation at Met-1, and without iron. With formylation at Met-1, and without iron. Without formylation at Met-1, and with iron. With formylation at Met-1, and with iron. Belongs to the rubredoxin family.