a midpoint potential of -337mV and is also reduced by NADPH. Fe2S2 ferredoxins Members of the 2Fe–2S ferredoxin superfamily () have a general core structure consisting of beta(2)-alpha-beta(2), which includes putidaredoxin, terpredoxin, and adrenodoxin. They are proteins of around one hundred amino acids with four conserved cysteine residues to which the 2Fe–2S cluster is ligated. This conserved region is also found as a domain in various metabolic enzymes and in multidomain proteins, such as aldehyde oxidoreductase (N-terminal), xanthine oxidase (N-terminal), phthalate dioxygenase reductase (C-terminal), succinate dehydrogenase iron–sulphur protein (N-terminal), and methane monooxygenase reductase (N-terminal). Plant-type ferredoxins One group of ferredoxins, originally found in chloroplast membranes, has been termed "chloroplast-type" or "plant-type" (). Its active center is a [Fe2S2] cluster, where the iron atoms are tetrahedrally coordinated both by inorganic sulfur atoms and by sulfurs of four conserved cysteine (Cys) residues. In chloroplasts, Fe2S2 ferredoxins function as electron carriers in the photosynthetic electron transport chain and as electron donors to various cellular proteins, such as glutamate synthase, nitrite reductase, sulfite reductase, and the cyclase of chlorophyll biosynthesis. Since the cyclase is a ferredoxin dependent enzyme this may provide a mechanism for coordination between photosynthesis and the chloroplasts need for chlorophyll by linking chlorophyll biosynthesis to the photosynthetic electron transport chain. In hydroxylating bacterial dioxygenase systems, they serve as intermediate electron-transfer carriers between reductase flavoproteins and oxygenase. Thioredoxin-like ferredoxins The Fe2S2 ferredoxin from Clostridium pasteurianum (Cp2FeFd; ) has been recognized as distinct protein family on the basis of its amino acid sequence, spectroscopic properties of its iron–sulfur cluster and the unique ligand swapping ability of two cysteine ligands to the [Fe2S2] cluster. Although the physiological role of this ferredoxin remains unclear, a strong and specific interaction of Cp2FeFd with the molybdenum-iron protein of nitrogenase has been revealed. Homologous ferredoxins from Azotobacter vinelandii (Av2FeFdI; ) and Aquifex aeolicus (AaFd; ) have been characterized. The crystal structure of AaFd has been solved. AaFd exists as a dimer. The structure of AaFd monomer is different from other Fe2S2 ferredoxins. The fold belongs to the α+β class, with first four β-strands and two α-helices adopting a variant of the thioredoxin fold. UniProt categorizes these as the "2Fe2S Shethna-type ferredoxin" family. Adrenodoxin-type ferredoxins Adrenodoxin (adrenal ferredoxin; ), putidaredoxin, and terpredoxin make up a family of soluble Fe2S2 proteins that act as single electron carriers, mainly found in eukaryotic mitochondria and Pseudomonadota. The human variant of adrenodoxin is