Ferredoxin

Ferredoxins (from Latin ferrum: iron + redox, often abbreviated "fd") are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied to the "iron protein" first purified in 1962 by Mortenson, Valentine, and Carnahan from the anaerobic bacterium Clostridium pasteurianum (Valentine, 1964). Another redox protein, isolated from spinach chloroplasts by Tagawa and Arnon in 1962, was termed "chloroplast ferredoxin." The chloroplast ferredoxin is involved in both cyclic and non-cyclic photophosphorylation reactions of photosynthesis. In non-cyclic photophosphorylation, ferredoxin is the last electron acceptor and reduces the enzyme NADP⁺ reductase. It accepts electrons produced from sunlight-excited chlorophyll and transfers them to the enzyme ferredoxin:NADP⁺ oxidoreductase EC 1.18.1.2. Ferredoxins are small proteins containing iron and sulfur atoms organized as iron-sulfur clusters. These biological "capacitors" can accept or discharge electrons, the effect being change in the oxidation states (+2 or +3) of the iron atoms. This way, ferredoxin acts as electron transfer agents in biological redox reactions. Other bioinorganic electron transport systems include rubredoxins, cytochromes, blue copper proteins, and the structurally related Rieske proteins.

Ferredoxins can be classified according to the nature of their iron-sulfur clusters and by sequence similarity.

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Fe₂S₂ ferredoxins

Plant-type ferredoxins

One group of ferredoxins, originally found in chloroplast membranes, has been termed "chloroplast-type" or "plant-type". The active center is a [Fe₂S₂] cluster, where the iron atoms are tetrahedrally coordinated both by inorganic sulfur atoms and by sulfurs provided by 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, nitrate reductase and sulfite reductase. In hydroxylating bacterial dioxygenase systems, they serve as intermediate electron-transfer carriers between reductase flavoproteins and oxygenase.

Adrenodoxin-type ferredoxins

Adrenodoxin, putidaredoxin and terpredoxin are soluble Fe₂S₂ proteins that act as single electron carriers. In mitochondrial monooxygenase systems, adrenodoxin transfers an electron from NADPH:adrenodoxin reductase to membrane-bound cytochrome P450. In bacteria, putidaredoxin and terpredoxin serve as electron carriers between corresponding NADH-dependent ferredoxin reductases and soluble P450s. The exact functions of other members of this family are not known, although Escherichia coli Fdx is shown to be involved in biogenesis of Fe-S clusters. Despite low sequence similarity between adrenodoxin-type and plant-type ferredoxins, the two classes have a similar folding topology.

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 intraction 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.

Fe₄S₄ and Fe₃S₄ ferredoxins

The [Fe₄S₄] ferredoxins may be further subdivided into low-potential (bacterial-type) and high-potential (HiPIP) ferredoxins.

Low- and high-potential ferredoxins are related by the following redox scheme:

The formal oxidation numbers of the iron ions can be [2Fe³⁺, 2Fe²⁺] or [1Fe³⁺, 3Fe²⁺] in low-potential ferredoxins. The oxidation numbers of the iron ions in high-potential ferredoxins can be [3Fe³⁺, 1Fe²⁺] or [2Fe³⁺, 2Fe²⁺].

Bacterial-type ferredoxins

A group of Fe₄S₄ ferredoxins, originally found in bacteria, has been termed "bacterial-type". Bacterial-type ferredoxins may in turn be subdivided into further groups, based on their sequence properties. Most contain at least one conserved domain, including four cysteine residues that bind to a [Fe₄S₄] cluster. In Pyrococcus furiosus Fe₄S₄ ferredoxin, one of conserved Cys residues is substituted with aspartic acid.

During the evolution of bacterial-type ferredoxins, intrasequence gene duplication, transposition and fusion events occurred, resulting in the appearance of proteins with multiple iron-sulfur centers. In some bacterial ferredoxins, one of the duplicated domains has lost one or more of the four conserved Cys residues. These domains have either lost their iron-sulfur binding property, or bind to a [Fe₃S₄] cluster instead of a [Fe₄S₄] cluster.

3-D structures are known for a number of monocluster and dicluster bacterial-type ferredoxins. The fold belongs to the α+β class, with 2-7 α-helices and four β-strands forming a barrel-like structure, and an extruded loop containing three "proximal" Cys ligands of the iron-sulfur cluster.

High potential iron-sulfur proteins

High potential iron-sulfur proteins (HiPIPs) form a unique family of Fe₄S₄ ferredoxins that function in anaerobic electron transport chains. Some HiPIPs have a redox potential higher than any other known iron-sulfur protein (e.g., HiPIP from Rhodopila globiformis has a redox potential of ca. 450 mV). Several HiPIPs have so far been characterized structurally, their folds belonging to the α+β class. As in other bacterial ferredoxins, the [Fe₄S₄] cluster adopts a cubane-like conformation and is ligated to the protein via four Cys residues.

References

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• Tagawa, K. and Arnon, D.I. (1962). "Ferredoxins as electron carriers in photosynthesis and in the biological production and consumption of hydrogen gas". Nature 195: 537–543. PMID 14039612.
• Valentine, R.C. (1964). "Bacterial ferredoxin". Bacteriol Rev. 28: 497–517. PMID 14244728.