Ferredoxins

Ferredoxins are small iron-sulfur proteins that function as one-electron carriers in a wide range of metabolic pathways. They shuttle electrons between enzymes and are defined by their iron-sulfur clusters, which undergo redox cycling between reduced and oxidized states.

The most common forms are [2Fe-2S] and [4Fe-4S] ferredoxins, though [3Fe-4S] ferredoxins are found in some anaerobic bacteria. In all cases the iron-sulfur cluster is coordinated by cysteine residues in the protein, with the ligands forming a conserved motif. The different cluster types yield distinct redox potentials and reactivity, enabling ferredoxins to couple with a variety of partner proteins.

Ferredoxins are found broadly in bacteria, archaea, plants, and some fungi and protists. In photosynthetic organisms, ferredoxin receives electrons from photosystem I and transfers them to ferredoxin-NADP+ reductase to produce NADPH required for carbon fixation. In non-photosynthetic microbes, ferredoxins participate in nitrogen fixation, hydrogen production, and central carbon metabolism, serving as electron donors or acceptors for enzymes such as nitrogenase, hydrogenases, and various oxidoreductases.

Iron-sulfur cluster biogenesis in cells supplies the clusters to apoproteins; in bacteria and organelles this involves ISC, SUF, and CIA pathways. Localization varies: chloroplast and bacterial ferredoxins are cytosolic or organellar; mitochondrial ferredoxins participate in iron-sulfur protein biogenesis and metabolism.