ferredoxin

https://doi.org/10.1351/goldbook.FT06771
A protein containing more than one iron and acid-labile sulfur, that displays electron-transfer activity but not classical enzyme function.
Source:
PAC, 1997, 69, 1251. 'Glossary of terms used in bioinorganic chemistry (IUPAC Recommendations 1997)' on page 1275 (https://doi.org/10.1351/pac199769061251)