Ferredoxin

• UniProt: P0A9R4

Ferredoxins are iron-and sulfur -containing proteins involved as an electron carrier in metabolic reactions and occur in eukaryotes and anaerobic bacteria. The human ferredoxin is called adrenodoxin. Another redox protein, which was isolated in 1962 from chloroplasts in spinach by Tagawa and Arnon is called " chloroplast ferredoxin ". This protein plays a role in both cyclic and non-cyclic photophosphorylation in photosynthesis. In the non-cyclic photophosphorylation ferredoxin is the last electron acceptor and reduces the coenzyme NADP to NADPH / H . It accepts electrons from the excited by sunlight chlorophyll and transmits them to the enzyme ferredoxin NADP ( ) reductase.

Ferredoxins are small proteins containing iron and sulfur atoms which are arranged in an iron -sulfur cluster. Ferredoxins act as "biological capacitors " by the iron atom can change its oxidation state ( 2 or 3). Thus they act in biological redox reactions as electron carriers.

• 2.1 Bacterial ferredoxins
• 2.2 High - potential iron - sulfur proteins

Fe2S2 ferredoxins

Herbal ferredoxins

The species originally found in chloroplast ferredoxins are called " chloroplast ferredoxin ". The active site is here a [ Fe2S2 ] cluster in which the iron atoms are arranged tetrahedrally by inorganic sulfur atoms and sulfur of the cysteine ​​residues. In chloroplasts, the Fe2S2 ferredoxins acting as electron carriers in the electron transport chain of photosystem I and as electron donors for various proteins, such as glutamate synthase, nitrate reductase and Schwefelreduktase. In bacterial Dioxygenasesystemen they serve as electron carriers between Flavoproteinreduktase and oxygenase.

Adrenodoxin - ferredoxins

Adrenodoxin, putidaredoxin and Terpredoxin are soluble Fe2S2 ferredoxins, which act as electron carriers. In the mitochondrial Monooxygenasesystemen adrenodoxin transferring an electron from NADPH - adrenodoxin reductase to the membrane-bound cytochrome P450 cholesterol monooxygenase ( CYP11A1 ) or steroid 11-beta- hydroxylase ( CYP11B1 or CYP11B2 ). The system acts seitenkettenabspaltend and is found in the mitochondria of the adrenal cortex, where it is involved in the catalysis of steroid hormones. In bacteria putidaredoxin and Terpedoxin serve as electron carriers between NADH-dependent cytochrome P450 Ferredoxinreduktasen and soluble. Other functions of other ferredoxins of this variety are not yet clarified. Although there is no great similarities between the amino acid sequence of plant ferredoxin and adrenodoxin, yet both molecules have a similar fold structure.

Thioredoxin - ferredoxins

Fe2S2 ferredoxin from Clostridium pasteurianum ( Cp2FeFd ) will be able to replace the cluster, due to its different type of amino acid sequence spectroscopic properties of its iron - sulfur cluster and its unique ability to ligands between cysteine ​​and the [ Fe2S2 ] recognized as a distinct family of proteins. Although the physiological role of this ferredoxin is still unclear, it was a specific interaction between Cp2FeFd and the molybdenum-iron nitrogenase group be established. Homologous ferredoxins from Azotobacter vinelandii ( Av2FeFdI ) and Aquifex aeolicus ( AAFD ) have also been described. The crystal structure of AAFD has been elucidated, AAFD is present as a dimer. The structure of the AAFD monomer differs from other Fe2S2 ferredoxins. The folding of the secondary structure comprising α - and β -turns, the first four β - chains and two α chains assume a variant of Thioredoxinfaltung.

Fe4S4 and Fe3S4 ferredoxins

Fe4S4 ferredoxins are further subdivided into "low- potential" ferredoxins (LPF ) (for bacteria) and "high- potential" ( HiPIP ) ferredoxins. Both categories are in the scheme of redox reactions similar to:

In LPF, the oxidation numbers of the iron can [ 2Fe3 , 2Fe2 ] or [ 1Fe3 , 3Fe2 ] be in HiPIP [ 3Fe3 , 1Fe2 ] or [ 2Fe3 , 2Fe2 ].

Bacterial ferredoxins

One type of Fe4S4 ferredoxins, which was originally found in bacteria, " Bacterial - type " called. Bacterial ferredoxins may in turn be broken down into further sub- groups, depending on the existing amino acid sequence. Most contain at least one preserved domain, which contains four cysteine ​​residues that bind the cluster of the [ Fe4S4 ]. In the ferredoxin from Pyrococcus furiosus a cysteine ​​domain obtained is replaced by aspartic acid.

During the evolution of bacterial ferredoxins it came through gene duplications and gene replacement in the occurrence of proteins with multiple iron-sulfur centers. In some bacterial ferredoxins one of the duplicated domains has lost one or more cysteine ​​residues obtained. These domains have either their iron-sulfur binding property lost or bind to a [ Fe3S4 ] cluster, rather than to a [ Fe4S4 ] cluster. Meanwhile, the 3-D spatial structures for some bacterial ferredoxin mono-and Dicluster are known. The folding is α and β, wherein 2-7 - α - windings, and four β - strands form a barrel-like structure and a pressed through loop includes three proximal cysteine ​​ligands of the iron - sulfur cluster.

High - potential iron - sulfur proteins

High - potential iron-sulfur proteins ( HiPIPs ) represent a separate family of Fe4S4 ferredoxins, which act in the anaerobic electron transport chain. Some HiPIPs have redox potentials higher than any other iron-sulfur protein ( such that HiPIP of Rhodopila globiformis has a redox potential of about 450 mV). The structure of some HiPIPs have now been clarified, their folds are attributed to α and β folds. As in other bacterial ferredoxins, also here the [ Fe4S4 ] cluster adopts a cubane- like structure and is connected by four cysteine ​​residues with the protein.