Ferredoxins

Ferredoxins

	Ferredoxins - Ferredoxins are iron and sulphide containing proteins which function as electron carriers, but have no enzymic function of their own.Ferredoxin from various nitrogen fixing organisms differ in molecular weight, Fe and sulphide content, redox properties and biological activity. All ferredoxins isolated from nitrogen fixing organisms react with their homologous nitrogenase. Azotobacter vinelandii ferredoxin has a MW of 14,500 and contains 8 atoms of Fe and 8 sulphide residues. It is distinct from other iron-sulphur proteins purified previously from the organism. Rhizobium ferredoxin has a MW of about 9,400. It is difficult to purify because of its oxygen sensitivity.
Cyanobacteria ferredoxin has a MW of about 10,000 and contains 2 atoms of Fe and 2 of sulphide. It thus resembles the ferredoxin of higher plants. The ferredoxins of Chlorobium (green photosynthetic bacteria) and Chromatium (purple photosynthetic bacteria) resemble those of clostridia. Chromatium ferredoxin has a MW of about 9,000 and contains 8 Fe atoms and 8 sulphide groups.
The red bacterium Rhodospirillum rubrum is unique in having two types of ferredoxin. Type I ferredoxin (MW 8,700) has 6 Fe and 6 sulphides, while Type II (MW 7,500) has 2 Fe and sulphides. The prosthetic group of ferredoxin may be a dimeric iron sulphur cluster (Fe₂S₂) or a tetrameric iron sulphur cluster (Fe₄S₄). The sulphur is inorganic or "acid-labile" sulphur, so called because acid treatment liberates it as H2S during denaturation of the protein. It has a valence of 2. An individual protein may contain one or more copies of the basic Fe-S structures. According to one model of the tetrameric structure, 4 Fe and 4 S atoms occupy alternate corners of a cube. Projecting in a tetrahedral fashion from the iron atoms arc 4 S atoms of cysteine (Cys) residues which anchor the iron sulphur cluster to the protein

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