Electron transfer cytochrome nitrite reductases

Fig. 6.9 The catalysts for denitrification. Nitrate is reduced by a molybdenum enzyme while nitrite and oxides of nitrogen are reduced today mainly by copper enzymes. However, there are alternatives, probably earlier iron enzymes. The electron transfer bct complex is common to that in oxidative phosphorylation and similar to the bf complex of photosynthesis, while cytochrome c2 is to be compared with cytochrome c of oxidative phosphorylation. These four processes are linked in energy capture via proton (H+) gradients see Figure 6.8(a) and (b) and the lower parts of Fig. 6.9 which show separately the active site of the all iron NO-reductase, and the active site of cytochrome oxidase (02 reductase).

Cytochromes, catalases, and peroxidases all contain iron-heme centers. Nitrite and sulfite reductases, involved in N-O and S-O reductive cleavage reactions to NH3 and HS-, contain iron-heme centers coupled to [Fe ] iron-sulfur clusters. Photosynthetic reaction center complexes contain porphyrins that are implicated in the photoinitiated electron transfers carried out by the complexes. 

Fe atoms. It had been anticipated that the c-type cytochrome center would have His/Met coordination, but His/His is observed. The former is the more usual coordination, especially at the high potential end E° > +200 mV) ofthe typical bacterial electron transfer chain to which the nitrite reductase is connected (Fig. 2) (7). The second curious feature is that the di heme iron is also six-coordinate thus, the enzyme does not offer a substrate-binding site at either heme. In addition to an expected axial histidine ligand there was an axial tyrosine (residue 25) ligand to the d heme (Fig. 4a). Each monomer is organized into two domains. 

Kobayashi, K., Koppenh fer, A., Ferguson, S. J., and Tagawa, S., 1997, Pulse radiolysis studies on cytochrome cd nitrite reductase from Thiosphaera pantotropha Evidence for a fast intramolecular electron transfer from c heme to d, heme, RiocAemtstry 36 1361 In 13616. 

Williams, P A., FJ"p, V., Leung, Y.-C., Chan, C., Moir, J. W. B., Hewlett, G., Ferguson, S. J., Radford, S. E., and Hajdu, J., 1995, Pseudospecific docking surfaces on electron transfer proteins as illustrated by pseudoazurin, cytochrome C550 and cytochrome cd nitrite reductase, Nat. Struct. Biol. 2 975n982. 

Nitrite reductases catalyze both of the reactions below the physiological electron donors are either c-type cytochromes or small blue-copper proteins (eqnations 1 and 2). h28 xhe Type 1 center acts as an electron-accepting site, which then transfers the electron to the Type 2 copper where snbstrate binding and rednction occur. 

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