Nitrite reductase cytochrome structure

STRUCTURE AND ENZYMOLOGY OF TWO BACTERIAL DIHEME ENZYMES CYTOCHROME cd NITRITE REDUCTASE AND CYTOCHROME c PEROXIDASE... 

Since HA is unstable in vivo , and is known to rapidly associate with the heme part of heme proteins , and possibly also with a variety of biological oxidants, such as the superoxide anion that is produced by many mammalian cells, it is difficult to demonstrate its accumulation in vivo. Already in 1932 Lindsey and Rhines discussed some analytical difficulties in the detection of HA, since when added externally, it disappeared rapidly from bacterial cultures this led to the conclusion that even if it is produced as an intermediate, its consumption is too fast to allow the accumulation of sufficient quantities for analytical demonstration. Compelling indirect evidence for the presence of HA as an intermediate in the enzymatically catalyzed reduction of nitrite (N02 ) to NH3 was provided by Einsle and colleagues , who characterized the crystal structure of the complex obtained by soaking cytochrome c-nitrite reductase with NH20H. ... 

Some metalloflavoproteins contain heme groups. The previously mentioned flavocytochrome b2 of yeast is a 230-kDa tetramer, one domain of which carries riboflavin phosphate and another heme. A flavocytochrome from the photosynthetic sulfur bacterium Chromatium (cytochrome c-552)279 is a complex of a 21-kDa cytochrome c and a 46-kDa flavoprotein containing 8a-(S-cysteinyl)-FAD. The 670-kDa sulfite reductase of E. coli has an a8P4 subunit structure. The eight a chains bind four molecules of FAD and four of riboflavin phosphate, while the P chains bind three or four molecules of siroheme (Fig. 16-6) and also contain Fe4S4 clusters.280 281 Many nitrate and some nitrite reductases are flavoproteins which also contain Mo or... 

Cytochrome cd (nitrite reductase) is the only other protein, whose structure has been determined, that has an eight bladed p-propeller... 

Cytochrome cdi Nitrite Reductase Structure Raises Interesting Mechanistic Questions... 

There are several types of nitrite reduetion reaetion in biology and the use of the eommon name initrite reductasei for the enzymes eatalysing these reactions causes endless confusion. Thus we begin this article by outlining the different types and functions of the nitrite reductases before focusing on the structure/function relationships for one type of enzyme, cytochrome cdi nitrite reductase. 

FIGURE 3. The X-ray crystal structure of the oxidised state of cytochrome ctf, nitrite reductase from P. pantotrophus. (Drawn from PDB entry Iqks)... 

Cytochrome c-552 belongs to cytochrome c6 or c8 group. As the cytochromes of this group react rapidly with Pseudomonas aeruginosa nitrite reductase but does not react with cow cytochrome c oxidase (Yamanaka, 1992), their structures have been supposed to be similar to each other. Thus, their amino acid sequences resemble each other though the sequence of Chlorobium limicola f. thiosulfatophi-lum cytochrome c-555 shows smaller similarity to other cytochromes c6 or c8 than the similarities that other cytochromes c6 or c8 show each other (Table 3.2). 

The bacterial, iron-containing cd nitrite reductases constitute another family of enzymes catalyzing the one-electron reduction of nitrite to nitric oxide (74). These enzymes are homodimers of 60-kDa subunits, each containing one heme-c and one heme-rii. Extensive studies have established heme-c as the electron entry site, whereas heme-dj is the catalytic center where nitrite is reduced (95). Three-dimensional structures of two different cytochromes cd have been determined in oxidized and reduced states P. pantotrophus, Pp-NiR (96, 97) and P. aeruginosa, Pa-NiR (98, 99). In both enzymes, heme-c is covalently linked to the A-terminal a-helical domain and heme-di is bound noncovalently to the C-terminal B-propeller domain (Fig. 17). Intramolecular ET between c and di hemes is an essential step in the catalytic cycle this reaction has been studied by several groups using different methods (95, 100-106). The rate constants for Pa-NiR are on the order of 1 s (95, 100-102, 104), while intramolecular ET in Pp-NiR is significantly faster (rate constant of... 

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