<p>The alpha-helical ferredoxin domain contains two Fe4-S4 clusters, typical of bacterial ferredoxin. Iron-sulphur proteins play an important role in electron transfer processes and in various enzymatic reactions. In eukaryotes, the mitochondria are the major site of Fe-S cluster biosynthesis in the cell, used for the assembly of mitochondrial and non-mitochondrial Fe-S proteins. The alpha-helical ferredoxin domain is present in several proteins involved in redox reactions, including the C-terminal of the respiratory proteins succinate dehydrogenase (SQR) in bacteria/mitochondria, and fumarate reductase (QFR) in bacteria. SQR is analogous to the mitochondrial respiratory complex II, and is involved in the electron transport pathway from succinate as a donor to the acceptor ubiquinone. SQR helps prevent the formation of reactive oxygen species and is used during aerobic respiration, whereas QFR does not and, consequently, is used to catalyse the final step of anaerobic respiration using the acceptor fumarate [<cite idref="PUB00013184"/>].</p><p>The alpha-helical ferredoxin domain is also present in the N-terminal of the cytosolic protein dihydropyrimidine dehydrogenase, (DPD) which catalyses the NADPH-dependent, rate-limiting step in pyrimidine degradation, converting pyrimidines to 5,6-dihydro compounds [<cite idref="PUB00013185"/>]. DPD catalysis involves electron transfer from NADPH to the substrate via the Fe4-S4 centre and FAD. In mammals, this pathway produces the neurotransmitter beta-alanine.</p>