Ferredoxins are acidic, low molecular weight, soluble iron-sulfur proteins found in various organisms, and act as multifuncitonal electron carriers in diverse redox systems.
Iron-sulfur proteins are defined as proteins carrying iron-sulfur cluster(s) in which the iron is at least partially coordinated by sulfur. Iron-sulfur clusters are prosthetic groups commonly found in various proteins that participate in oxidation-reduction reactions and catalysis.
In most instances, the iron is bound to sulfur(s) from cysteine residues in polypeptide backbone and also to inorganic sulfurs in the iron-sulfur cluster. The chief role of the iron-sulfur cluster is to facillitate electron transfer, while in others, the clusters contribute to catalytic function.
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The major forms of iron-sulfur cluster are as follows: |
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(a) |
the [2Fe-2S] cluster the two iron atoms are bridged to one another by two inorganic sulfur atoms and ligated to four cysteines from peptide backbone. |
(b) |
the [4Fe-4S] cluster the four iron atoms are ligated to four cysteines and form a cubic structure with the four inorganic sulfur atoms. |
(c) |
the [3Fe-4S] cluster a single iron of the [4Fe-4S] cluster is absent. |