Bacteria That Reduce Nitrate to Nitrogen Gas

In the heterotrophic nitrate respiration, in most cases hydrogen atoms derived from organic compounds ([H], mostly in the form of NADH in the case of heterotrophic denitriliers) are first oxidized with nitrate. The reaction is catalyzed by nitrate reductase. The enzyme contains Mo, [Fe4S4] and [Fe2S2] clusters (Fe/S), and cytochrome b (Chaudhry and MacGregor, 1983). Mo is present in the enzyme as molybdenum cofactors combining with molybdopterin or molybdopterin guanine dinucleotide. The enzyme catalyzes the reduction of nitrate to nitrite with ubiquinol or menaquinol (QH2) as the electron donor. 

nitrite is reduced to nitric oxide by the catalysis of nitrite reductase. Two kinds of nitrite reductases are known in the denitrifying bacteria (N2 forming) cytochrome cdx-type enzyme (Yamanaka et al., 1960, 1961, 1963 Yamanaka and Okunuki, 1963a,b,c Yamanaka, 1964) and copper protein-type enzyme (Iwasaki and Matsubara, 1972). However, no case has been found in which one species of the denitrifying bacterium has both types of the enzymes simultaneously (Coyne et al., 1989). 

Cytochrome cd, is the first cytochrome that has been found to have two kinds of hemes, hemes C and I ) in one molecule. After the discovery of cytochrome cdu many cytochromes have been found which possess two or three kinds of hemes in the molecule cytochromes bd, ba3, baa3, bo (or bo3), caa3, cbb3, and aco (or cao or cao3) (see Yamanaka 1992). 

The copper-protein type nitrite reductases have been purified from several denitrifying bacteria. They have 2 1 copper atoms in the molecule one copper per subunit (30 10 kDa). The enzymes catalyze the reduction of nitrite to nitric oxide, but the physiological electron donors for the enzymes have not been clarified, though they are said to be cytochrome c in some cases and a copper-protein in some cases (see Otsuka and Yamanaka, 1988). 

Nitric oxide formed from nitrite by the catalysis of nitrite reductase is next reduced to nitrous oxide (N20). The reduction of nitric oxide is catalyzed by nitric oxide reductase (NO reductase) which is a cytochrome ebb (Carr and Ferguson, 

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