This is domain is found in Ferredoxin I (FdI), an Iron-sulfur ([Fe-S]) cluster-containing protein, present in species such as Azotobacter vinelandii. [Fe-S] proteins participate in electron transfer, catalytic, regulatory, and structural function. The FdI cluster exhibits a pH-dependent reduction potential and reversible protonation in the reduced state [1].

1: Oxidized and reduced Azotobacter vinelandii ferredoxin I at 1.4 A resolution: conformational change of surface residues without significant change in the [3Fe-4S]+/0 cluster. Schipke CG, Goodin DB, McRee DE, Stout CD; Biochemistry. 1999;38:8228-8239. PMID:10387068