Molybdenum complexes nitrite reduction

The electrochemical transformation of a molybdenum nitrosyl complex [Mo(NO)(dttd)J [dttd = 1,2-bis(2-mercaptophenylthio)ethane] (30) is rather interesting (119). Ethylene is released from the backbone of the sulfur ligand upon electrochemical reduction. The resulting nitrosyl bis(dithiolene) complex reacts with O2 to give free nitrite and a Mo-oxo complex. Multielectron reduction of 30 in the presence of protons releases ethylene and the NO bond is cleaved, forming ammonia and a Mo-oxo complex (Scheme 15). The proposed reaction mechanism involves successive proton-coupled electron-transfer steps reminiscent of schemes proposed for Mo enzymes (120). 

The reaction of Mo(H20)63+ and nitrate in aqueous solutions results in the formation of Mo2Oi(H20)62+ and nitrite. Mo(III) coordinated to oxygen and nitrogen donor atoms of EDTA also reduces nitrate in aqueous solutions. The reduction of nitrate by a Mo(III)-EDTA complex results in the formation of nitrite and a Mo(V)-EDTA complex, as determined by chemical and spectrophotometric techniques. These reactions serve as models for biological nitrate reduction. In addition, molybdate coordinates to naturally produced phenolates. The molybdenum-coordinating phenolates also coordinate tungstate and ferric iron. Two of these phenolates contain threonine, glycine, alanine, and 2,3-dihydroxybenzoic acid. 

The enzyme contains two heme A molecules, two heme C molecules, one molybdenum atom, and five [Fe4S4] clusters in the molecule with molecular mass of 250 kDa. Molybdenum occurs as a complex with molybdopterin guanine dinucleotide (MGD) (Fukuoka et al 1987 Yoshino, 1994 Suzuki et al., 1997). The enzyme catalyzes the reduction of N. winogradskyi ferricytochrome c-550 and horse ferricytochrome c with nitrite, i.e it catalyzes the oxidation of nitrite with ferricytochromes c as the electron acceptor. The reaction is stimulated by Mn2+ and Ca2+. Although the enzyme catalyzes actively the oxidation of nitrite around pH 8, it shows also capability to catalyze the reduction of nitrate with ferrocytochrome c at pHs less than 6 the bacterium changes itself from a nitrifier to a denitrifieT at pHs less than 6. 

Nitrate reductase catalyzes the first step in conversion of nitrate to ammonia, the reduction of nitrate to nitrite (N02 ). The reaction multisubunit nitrate reductase enzyme, with Mr of about 800 kilodaltons, contains bound FAD, molybdenum, and a cytochrome called cytochrome 557 (which contains an Fe4S4 complex). Nitrate reductase carries out the overall reaction ... 

First, ferredoxin-nitrate reductase (EC 1.7.7.2) is a molybdenum-iron-sulfur protein that converts nitrate to nitrite while undergoing oxidation. Its structure is not yet well-defined. Then, ferridoxin-nitrite reductase (EC 1.7.7.1), an iron (as heme and at least one iron sulfur complex, vide infra) can continue the reduction to ammonia (NH3). 

Plants usually obtain their nitrogen by absorption of nitrate or ammonium ions from the soil (symbiotic associations between higher plants and nitrogen fixing bacteria are of course exceptions to this). Ammonium ions may be utilized directly in the synthesis of amino acids (see p. 169), but nitrate must first be reduced to ammonia. This is accomplished in two stages the reduction of nitrate to nitrite followed by the reduction of nitrite to ammonia. The first step— nitrate reduction—is catalysed by the flavo-protein enzyme complex nitrate reductase (Fig. 5.12) which contains molybdenum and FAD (flavin adenine dinucleotide) as a prosthetic group. Reduced FMN... 

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