Cytochrome cd, nitrite reductase

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

Such an involvement of an amino acid side-chain ligand switch within each catalytic cycle was a novel proposal and as such needs to be scrutinized by a variety of experimental procedures as well as analysis in the context of information known for cytochrome cd nitrite reductase from another source (see later discussion). However, it is interesting to note that something similar has been proposed for the protocate-chuate 3,4-dioxygenase enzyme from Pseudomonas putida (15). On the other hand, bacterial cytochrome c peroxidase offers an example where ligand switching seemingly relates only to an activation phenomenon. 

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

FIGURE S. The X-ray crystal structure of the reduced state of cytochrome cd nitrite reductase from P. pantoTrophiis. (Drawn from PDB entry laof)... 

In concluding this section we note that cytochrome cd nitrite reductase also has an oxidase activity (F,Fp et al., 1995). This four electron reaction, which contrasts with the one electron reduction of nitrite to nitric oxide, is outside the scope of this article. 

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. 

Cytochrome cd nitrite reductase from Paracoceus pantotrophus has a different mechanism, with two identical subunits, each with domains containing a c-type cytochrome heme and a dj-type cytochrome heme. Electrons from external donors enter through the c heme the d heme is the site of nitrite reduction to NO and oxygen reduction to water. One of the puzzles of the mechanism is how the NO can escape from... 

Lojou, E., F. Cutruzzola, M. Tegoni, and P. Bianco (2003). Electrochemical study of the intermolecular electron transfer to Pseudomonas aeruginosa cytochrome cd nitrite reductase. Electrochim. Acta 48, 1055-1064. 

Two major pathways have been shown to exist in nitrite reduction [274]. In the first pathway, nitrite is reduced to NO, while in the second there is a direct conversion of nitrite to NH3 or NH4" ". Two classes of nitrite reductase (NIR), namely the cytochromes cd [274], and the copper nitrite reductase [274], have been identified for the first pathway and two classes of enzyme, namely the siroheme nitrite reductase and cytochrome c nitrite reductase, have been proposed to follow the second pathway. The mechanism of these four enzymes has been recently reviewed [274], and only a brief summary of the electron-transfer reactions of cytochrome cd nitrite reductase will be given here. The initial step in the conversion of NO2 to NO involves a binding of the nitrite ion to the metal of the reduced heme. This first step is followed by the uptake of two protons and the loss of one water molecule to yield an electrophilic ferrous pe +-NO+ species, also formulated as a pe +-NO" complex. The dissociation of NO from this species produces the ferric heme d, which is in turn reduced back to its original state by heme c. Why the eri2yme does not reduce the nitrosyl species, Fe -]s[0 or Fe -NO to its Fe -NO form, prior to dissociation of NO in the heme, has been discussed in the literature [274], and may... 

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