rubredoxin

An iron-sulfur protein without acid-labile sulfur, in which an iron centre is coordinated by four sulfur-containing ligands, usually cysteine. The function, where known, is an electron carrier.
Source:
PAC, 1997, 69, 1251 (Glossary of terms used in bioinorganic chemistry (IUPAC Recommendations 1997)) on page 1296