Nitrite reductase denitrifying bacteria

Copper enzymes participate in two important reactions catalyzed by denitrifying bacteria. Nitrite reductases from species of Achromohactei and Alcaligene are trimeric proteins made up of 37-kDa subunits, each of which contains one type 1 (blue) copper and one type 2 (nonblue) copper. The first copper serves as an electron acceptor from a small blue pseudoazurin. LS44a second copper,... 

Heme d,6 another isobacteriochlorin, occurs as one of two cofactors in the reductase cytochrome cdj which mediates the nitrite reduction to nitrogen monoxide (NO) and from there to dinitrogen oxide (N20) in denitrifying bacteria.7... 

Nitrite reductases (NiRs)—enzymes found in several strains of denitrifying bacteria— catalyze the one-electron reduction of nitrite anion to nitric oxide (Equation 1). - In addition to the importance of this process in the global nitrogen cycle (Figure 1), further incentive for the study of the denitrification process is provided by its environmental impact, ranging from the production of NO as a pollutant and NjO as a potent greenhouse gas, to lake eutrophication due to farm runoff that contains high concentrations of nitrates and nitrites. 

Nitrite reductase Denitrification Denitrifying bacteria Liu etal. (1986)... 

Although the pathway of Eq. (1) is now based on much evidence (Section 111) and is unambiguous in the case of at least one bacterium [Pseudomonas stutzeri strain Zobell (f. sp. P. perfectomarina)], there have been alternative hypothesis. One hypothesis, advanced by the Hollocher group (Garber and Hollocher, 1981 St. John and Hollocher, 1977), considered NO as a likely intermediate, but one that remained at least partly enzyme-bound and was not entirely free to diffuse. This view was based on the outcome of certain kinetic and isotope experiments which can be summarized as follows. When denitrifying bacteria were challenged simultaneously with [ N]nitrite and ordinary NO, the cells reduced both compounds concomitantly to N2 (or to N2O in the presence of acetylene which is a specific inhibitor (Balderston et al., 1976 Yoshinari and Knowles, 1976) of nitrous oxide reductase). In the process, little NO was generally detected in the gas phase pool of NO and there was relatively little isotopically mixed N2O formed. That is, most of the N and N reduced to NjO appeared as N2O... 

O-exchange studies of Ye et al. (1991) support, we believe, the catalysis by nitrite reductase of redox reversibility between nitrite and NO as depicted in the first line of Eq. (3). They observed by analyzing the 0 content of product N2O that all eight strains of denitrifying bacteria studied could catalyze the exchange of 0 between water and nitrite or NO by way of an electrophilic (nitrosyl donor) species of NO. The rates and extent of these exchange reactions depended on whether the bacterium made use of a heme- or Cu-type nitrite reductase. Contrary to the conclusions of Ye et al. (1991), we do not believe that this study otherwise informs about the pathway of denitrification or whether NO is an intermediate. 

Although the nitrite reductases of denitrifying bacteria seem to be NO-pro-ducing reductases in vitro, we cannot so easily say that they similarly are NO-producing in vivo. It is not so rare for an enzyme to catalyze more than one reaction or be served by more than one substrate, but it is virtually without precedence for an enzyme to change its chemistry with the same substrate between in vivo and in vitro situations. The reduction of nitrite in vivo has been probed by several means which target the activity of either nitrite or nitric oxide reductase. 

V. DISSIMILATORY NITRITE REDUCTASES, ENZYMES THAT GENERATE NITRIC OXIDE IN DENITRIFYING BACTERIA... 

Denitrifying bacteria produce one or the other of two usually soluble but very dissimilar nitrite reductases, heme- and Cu-containing types (Brittain et al., 1992 Coyne et al., 1989), both of which are found in the periplasmic space of gram-negative bacteria (Coyne et al., 1990). 

Although the Cu-type family of nitrite reductase is comprised of soluble enzyme and localized in the periplasmic space in gram-negative bacteria, it has proved to be a membrane-bound enzyme in denitrifying Bacillus, which is gram positive and lacks an outer membrane and periplasmic space (Denariaz et ai, 1991 Urata and Satoh, 1991 Ho etal., 1993). 

It was long believed that bacteria were unique in their ability to denitrify. However, Shoun and Tanimoto (1991) and Shoun et al., (1989) demonstrated that the fungus, Fusarium oxysporum, could be induced to synthesize an enzyme system capable of the anaerobic reduction of nitrite to N2O. Induction occurred under conditions of low oxygen concentrations in the presence of nitrate or nitrite. One and pethaps the only component of this nitrite reductase system is a unique, soluble cytochrome P-450 (P-450dNIR), which is more similar in its cDNA-inferred amino acid sequence to soluble, bacterial P-450 enzymes (espe-... 

Calmels, S., Ohshima, H., and Bartsch, H. (1988). Nitrosamine formation by denitrifying and non-denitryfying bacteria Implications of nitrite reductase and nitrate reductase in nitrosation catalysis. . Gen. Microbiol. 134, 221-226. 

Coyne, M. S., Arunakumari, A., Averill, B. A., and Tiedje, J. M. (1989). Immunological identification and distribution of dissimilatory heme cd, and nonheme copper nitrite reductases in denitrifying bacteria. Appl. Environ. Microbiol. 55, 2924-2931. 

Heme d is a chlorin,85 as is acrylochlorin heme from certain bacterial nitrite reductases (Fig. 16-6).86 87 Siroheme (Fig. 16-6), which is found in both nitrite and sulfite reductases of bacteria (Chapter 24) 38/89 is an isobacteriochlorin in which both the A and B rings are reduced. It apparently occurs as an amide siroamide (Fig. 16-6) in Desulfovibrio.90 Heme of nitrite reductases of denitrifying bacteria is a dioxo-bacteriochlorin derivative (Fig. 16-6).91 92... 

Surprisingly, its biological redox partners remain largely unknown. It has been implicated in anaerobic nitrite respiration and it has been shown that azurin can donate electrons to nitrite reductase, a function that is proposed to be carried out by another cupredoxin, pseudoazurin (see Section IV, E). On the other hand, azurin is not an inducible protein and denitrifying bacteria express azurin constitutively under aerobic conditions. 

Braker, G., Zhou, J., Wu, L., Devol, A. H., and Tiedje, J. M. (2000). Nitrite reductase genes (mVlCand nirS) as functional markers to investigate diversity of denitrifying bacteria in Pacific Northwest marine sediment communities. Appl. Environ. Microbiol. 66, 2096—2104. 

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