Iron sulfur clusters oxygenates

A substantial fraction of the named enzymes are oxido-reductases, responsible for shuttling electrons along metabolic pathways that reduce carbon dioxide to sugar (in the case of plants), or reduce oxygen to water (in the case of mammals). The oxido-reductases that drive these processes involve a small set of redox active cofactors , that is, small chemical groups that gain or lose electrons. These cofactors include iron porjDhyrins, iron-sulfur clusters and copper complexes as well as organic species that are ET active. 

Chapter 6). Other iron-sulfur proteins, so named because they contain iron sulfur clusters of various sizes, include the rubredoxins and ferredoxins. Rubredoxins are found in anaerobic bacteria and contain iron ligated to four cysteine sulfurs. Ferredoxins are found in plant chloroplasts and mammalian tissue and contain spin-coupled [2Fe-2S] clusters. Cytochromes comprise several large classes of electron transfer metalloproteins widespread in nature. At least four cytochromes are involved in the mitrochondrial electron transfer chain, which reduces oxygen to water according to equation 1.29. Further discussion of these proteins can be found in Chapters 6 and 7 of reference 13. 

Figure 8.7 Simplified model of nicotiniamine (NA) function in plant cells. Iron is transported across the plasma membrane by the Strategy I or Strategy II uptake systems. Once inside the cell, NA is the default chelator of iron to avoid precipitation and catalysis of radical oxygen species. The iron is then donated to proteins, iron-sulfur clusters and haem, while ferritin and iron precipitation are only present during iron excess. (From Hell and Stephan, 2003. With kind permission of Springer Science and Business Media.)...

Figure 10.3 Z-scheme of oxygenic photosynthesis in green algae and cyanobacteria, showing links to hydrogenase. Q (plastoquinone) and X (an iron-sulfur cluster) are electron acceptors from photosystems II and I, respectively.The two hydrogenases shown are the NADP-dependent bidirectional hydrogenase and a ferredoxin-dependent enzyme.

Fridovich recently summarized important aspects concerning the accurate detection and measurement of superoxide. He indicates that univalent reduction of O2 to superoxide is a facile process, but the instability of superoxide in aqueous solutions hinders its detection and measurement. To measure intracellular superoxide, he favors use of the rapid inactivation of [4Fe-4S]-containing dehydratases (such as aconitase) by oxidation of their iron-sulfur clusters. See Oxygen, Oxides Oxygen Radicals... 

One oxygen of 02 is incorporated into the hydroxyl group of the product, whereas the other is incorporated into water. The enzymes usually include a cytochrome P450, an iron-sulfur cluster-containing protein, such as adrenodoxin or putidaredoxin, and a flavoprotein reductase. 

E. coli uses nitrate as a terminal electron acceptor through a respiratory, dissimilatory nitrate reductase whose synthesis is induced when nitrate is provided, and which is repressed by oxygen. Nitrate reductase is discussed with other molybdoenzymes in Section 62.1.9, and catalyzes the reduction of nitrate to nitrite. The enzyme is isolated from the cytoplasmic membrane of E. coli, and contains three subunits (a, j8 and y) although the y-subunit may be absent in some preparations. The -y-subunit is a b-type cytochrome, and the a-subunit is reported to be the catalytic subunit. The enzyme contains a number of iron-sulfur clusters, including a HiPIP and at least two ferredoxins.1054,1437... 

Tachezy J, Sdnchez LB, Muller M (2001) Mitochondrial type iron-sulfur cluster assembly in the amitochondriate eukaryotes Trichomonas vaginalis and Giardia intestinalis, as indicated by the phylogeny of IscS. Mol Biol Evol 18 1919-1928 Takahashi Y, Tokumoto U (2002) A third bacterial system for the assembly of iron-sulfur clusters with homologs in archaea and plastids. J Biol Chem 277 28380-28383 Thong KW, Coombs GH (1987) Comparative study of ferredoxin-linked and oxygen-metabolizing enzymes of trichomonads. Comp Biochem Physiol B 87 637-641 Tielens AGM, Rotte C, van Hellemond JJ, Martin W (2002) Mitochondria as we don t know them. Trends Biochem Sci 27 564-572... 

Goris T, Wait AF, Saggu M, et al. A unique iron-sulfur cluster is crucial for oxygen tolerance of a [NiFe]-hydrogenase. Nat Chem Biol. 2011 7(5) 310—8. 

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