Iron-sulfur proteins sulfite reductase

There is some evidence that the iron-sulfur protein, FhuF, participates in the mobilization of iron from hydroxamate siderophores in E. coli (Muller et ah, 1998 Hantke, K. unpublished observations). However, a reductase activity of FhuF has not been demonstrated. Many siderophore-iron reductases have been shown to be active in vitro and some have been purified. The characterization of these reductases has revealed them to be flavin reductases which obtain the electrons for flavin reduction from NAD(P)H, and whose main functions are in areas other than reduction of ferric iron (e.g. flavin reductase Fre, sulfite reductase). To date, no specialized siderophore-iron reductases have been identified. It has been suggested that the reduced flavins from flavin oxidoreductases are the electron donors for ferric iron reduction (Fontecave et ah, 1994). Recently it has been shown, after a fruitless search for a reducing enzyme, that reduction of Co3+ in cobalamin is achieved by reduced flavin. Also in this case it was suggested that cobalamins and corrinoids are reduced in vivo by flavins which may be generated by the flavin... 

APS is reduced (Eq. 18-32, step b) by APS reductase, a 220-kDa iron-sulfur protein containing FAD and several Fe-S clusters. An intermediate in the reaction may be the adduct of sulfite with FAD, which may be formed as in Eq. 18-32. The initial step in this hypothetical mechanism is displacement on sulfur by a strong nucleophile generated by transfer of electrons from reduced ferredoxin to cytochrome c3 to the flavin.364... 

The six-electron reduction of sulfite to sulfide is catalyzed by sulfite reductase, a multisubunit complex composed of a flavoprotein and a heme iron-sulfur protein. 

Figure 16-18 Mossbauer X-ray absorption spectra of iron-sulfur clusters. (See Chapter 23 for a brief description of the method.) Quadrupole doublets are indicated by brackets and isomer shifts are marked by triangles. (A) [Fe2S2]1+ cluster of the Rieske protein from Pseudomonas mendocina, at temperature T = 200 K. (B) [Fe3S4]1+ state of D. gigas ferre-doxin II, T = 90 K. (C) [Fe3S4]° state of D. gigas ferredoxin II, T = 15 K. (D) [Fe4S4]2+ cluster of E. coli FNR protein, T = 4.2 K. (E) [Fe4S4]1+ cluster of E. coli sulfite reductase, T = 110 K. From Beinert et al.260...

Another promising MS approach is the investigation of noncovalent complexes. A beautiful example of this was provided by Leary in a study of 5 -adenylylsulfate (APS) reductase, a 4F-4S iron—sulfur cluster-containing protein.131 APS reductase catalyzes the reduction of APS to sulfite. Leary and coworkers were able to observe... 

In bacteria PAPS is a substrate for sulfate reduction. In plants, adenosine-5 -phosphosulfate is the substrate. Thioredoxin, a small thiol-containing protein, reduces the sulfate in PAPS to sulfite (SOS "). Sulfite is reduced by sulfite reductase in a six electron transfer through the intermediates NADPH, FAD, FMN, an iron-sulfur center, and the porphyrin siroheme. The end product is H2S. 

Paramagnetic centers are found in many specific enzymes and proteins that function in animal tissues and bacteria (oxygenases, sulfite or nitrite reductases, xanthine oxidase, nitrogenase, etc.) besides mitochondria and microsomes. The catalytic site can include Mo, Cu, Co, Ni, Mn, and other metal ions. Elowever heme- and iron-sulfur centers constitute the majority of the paramagnetic centers found outside the mitochondria. 

Molybdoenzymns At present, 6 oligomeric oxi-doreductases are known, which contain Mo as an essential constituent 1. Nitrogenase (see) 2. Nitrate reductase, EC 1.6.6.3 (see) 3. Xanthine oxidase, EC 1.2.3.2 (see), from animals and bacteria 4. Aldehyde oxidase, EC 1.2.3.1 from animal liver, which catalyses the reaction R-CHO + HjO R-COOH + 2H + 2e 5. Sulfite oxidase, EC 1.18.3.1 from mammalian and bird liver (Af, 114,000 2 subunits), which catalyses the reaction S03 + HjO-> SO/ + 2H + 2e this enzyme also contains a b5-like cytochrome and passes electrons directly to cytochrome c in the respiratory chain and 6. Formate dehydrogenase, EC 1.2.1.2, a membrane-bound protein from E. coli, containing one atom each of molybdenum and selenium, one heme group and nonheme iron-sulfur centers. It is NAD -dependent and catalyses the reaction HCOO + NAD CO2 -I- NADH. 

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