Iron cytochrome nitrite reductases

Fe Cytochrome oxidase reduction of oxygen to water Cytochrome P-450 0-insertion from O2, and detoxification Cytochromes b and c electron transport in respiration and photosynthesis Cytochrome f photosynthetic electron transport Ferredoxin electron transport in photosynthesis and nitrogen fixation Iron-sulfur proteins electron transport in respiration and photosynthesis Nitrate and nitrite reductases reduction to ammonium... 

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. 

Nitrite reductase (NAD(P)H) [EC 1.6.6.4] catalyzes the reaction of three NAD(P)H with nitrite to yield three NAD(P)+, NH4OH, and water. Cofactors for this enzyme include FAD, non-heme iron, and siroheme. (2) Nitrite reductase (cytochrome) [EC 1.7.2.1] is a copper-depen-dent system that catalyzes the reaction of nitric oxide with two ferricytochrome c and water to produce nitrite and two ferrocytochrome c. (3) Ferredoxin-nitrite reductase [EC 1.7.7.1], a heme- and iron-dependent enzyme, catalyzes the reaction of ammonia with three oxidized ferredoxin to produce nitrite and three reduced ferredoxin. (4) Nitrite reductase [EC 1.7.99.3] is a copper- and FAD-dependent enzyme that catalyzes the reaction of two nitric oxide with an acceptor substrate and two water to produce two nitrite and the reduced acceptor. 

Based on crystallographic observations it was suggested that the HA intermediate is bound to the cytochrome reductase via the iron atom, Fe(II)—NH2OH, and undergoes subsequent reduction to produce the NH3 that then dissociates from the protein ". It is of interest that the specific activity of cytochrome c-nitrite reductase from S. deleyianum in the conversion of N02 to NH3 is only 2-fold greater than that recorded for the conversion of HA to ammonia by the same enzyme, an observation that strongly supports the involvement of HA as an intermediate in the catalytic reduction of nitrite to NH3 . 

In all photoautotrophs, reduction of NOj" to NH4 is achieved in two distinct enzymatic steps (Campbell, 2001). First, assimilatory nitrate reductase (NR) catalyzes the two electron reduction from NOj" to NO2. NR is a large soluble cytoplasmic enzyme with FAD (flavin adinine dinucleotide), an iron-containing cytochrome and molybdopterin prosthetic groups, and requires NADH and/or NADPH as an electron donor (Guerrero et al, 1981). Functional NR is in the form of a homodimer and therefore requires two atoms of iron per enzyme. Following transport into the chloroplast, NO2 undergoes a 6 e reduction to NH4 via assimilatory nitrite reductase (NiR). NiR, a soluble chloroplastic enzyme, contains five iron atoms per active enzyme molecule, and requires photosynthetically reduced ferredoxin as an electron donor (Guerrero et al., 1981). 

Although there is another nitrite reductase (EC 1.9.3.2), which contains iron and utilizes the redox-active cytochromes c and dx, the following will only treat the copper enzyme [90]. 

Copper occurs in almost all life forms and it plays a role at the active site of a large number of enzymes. Copper is the third most abundant transition metal in the human body after iron and zinc. Enzymes of copper include superoxide dismutase, tyrosinase, nitrite reductase and cytochrome c oxidase. Most copper proteins and enzymes have roles as electron transfer agents and in redox reactions, as Cu(II) and Cu(I) are accessible. 

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... 

Heme derivatives. Name for derivatives of heme, i.e., for iron complexes of the porphyrins and hy-droporphyrins, in the strictest sense of protoporphy-rins. These H. d. are the prosthetic groups of hemo-proteins such as the cytochromes, cytochrome oxidase, peroxidases, catalases, leghemoglobin, nitrite reductases, sulfite reductase see also heme d, heme di, and siroheme. In spite of their similar names heme-rythrin, hemocyanin, hemocuprein, and hemovana-dins do not belong to the H. d. 

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