Nitrite reductase fungi

Assimilatory nitrite reductases of plants, fungi, and bacteria carry out the six-electron reduction of nitrite to ammonium ions (Eq. 24-13) using electron donors such as reduced ferredoxins or NADPH. 

Green plants, algae, fungi, cyanobacteria and bacteria that assimilate nitrate also produce assimilatory nitrite reductases, which catalyze the six-electron reduction of nitrite to ammonia (equation 89). The formation of heme-nitrosyl intermediates has been detected in several cases,1515 while hydroxylamine is commonly thought to be an intermediate. Added hydroxylamine is rapidly reduced to ammonia. However, no intermediates are released, and ammonia is the only product... 

Apart from the report by Sims et al. (1%8) that enzymes involved in nitrate assimilation in yeast may be aggregated into an oligomeric complex termed a nitrosome. The majority of other investigations with fungi characterize nitrate and nitrite reductases as separate soluble enzymes. 

In the global environment, nitrogen in the form nitrate or nitrite is assimilated (immobilized) into biomass in the form of NH3. This reduction is catalyzed by two assimilatory enzymes (nitrite reductase and nitrate reductase) and can be carried out by plants, fungi, and prokaryotes. This process is likely to dominate when reduced nitrogen is in low supply (e.g., during aerobic conditions). 

Dissimilatory nitrite reductases (NiR) play a pivotal role in the anaerobic respiration cascade of denitrifying bacteria, archaea and fungi by catalysing the first committed step of the pathway.The net reaction of NiR yields the conversion of nitrite (N02 ) into gaseous nitric oxide (NO) and water (H2O) (eqn (3.12) ref. 207) ... 

The opposite sequence, reduction of nitrate and nitrite ions, provides a major route of acquisition of ammonia for incorporation into cells by bacteria, fungi, and green plants (Fig. 24-1). Assimilatory (biosynthetic) nitrate reductases catalyze the two-electron reduction of nitrate to nitrite (Eq. 16-61). This is thought to occur at the molybdenum atom of the large 900-residue highly regulated793 molybdopterin-dependent enzyme. In green plants the reductant is... 

The enzymes from green plants and fungi are large multifunctional proteins,80 which may resemble assimilatory sulfite reductases (Fig. 16-19). These contain siroheme (Fig. 16-6), which accepts electrons from either reduced ferredoxin (in photosynthetic organisms) or from NADH or NADPH. FAD acts as an intermediate carrier. It seems likely that the nitrite N binds to Fe of the siroheme and remains there during the entire six-electron reduction to NH3. Nitroxyl (NOH) and hydroxylamine (NH2OH) may be bound intermediates as is suggested in steps a-c of Eq. 24-14. 

Nitrate reductases have been isolated from bacteria, plants and fungi and always contain molybdenum. Two types may be distinguished (a) the assimilatory nitrate reductases which catalyze the reduction of nitrate to nitrite, which ultimately is reduced to ammonia and used by... 

The overall pathway of denitrification is shown in Figure 4. As shown, a total of four enzymatic steps are involved in most organisms. (A few organisms appear to lack the last enzyme, and consequently produce N2O as the major product.) Certain fungi also appear to reduce nitrite to N2O with the involvement of a cytochrome P-450-like enzyme (29). In contrast to nitrification, the reactions of denitrification are typically all carried out by a single organism. In addition, with the exception of NO reductase, the enzymes of the denitrification pathway are generally soluble, reasonably easy to obtain in pure form, and exhibit less complexity in terms of their metal cofactor contents (14). Consequently, the mechanisms of the enzymes of denitrification are in several cases well-understood. 

Nitrate reductase, the first enzyme in the nitrate assimilation pathway, catalyzes the reduction of nitrate to nitrite. This requires two electrons which are donated by either NADH or NADPH in the eukaryotic NRs. The NADH-spe-cific NRs (EC 1.6.6.1) are found in most higher plants and numerous eukaryotic algae while the NADPH-specific NRs (EC 1.6.6.3) are found in the fungi. The NAD(P)H-bispecific NRs (EC 1.6.6.2) occur in some higher plant and algae species. The prokaryotic NRs which utilize a variety of electron donors, including ferredoxin, reduced pyridine nucleotides, and respiratory intermediates, will not be considered in this article (see Stewart, 1988). 

A number of different enzymes can carry out the reduction of nitrite to either ammonium or nitric oxide and/or nitrous oxide. The latter types are involved with the denitrification process (Payne, 1973) and will not be considered here. Among the enzymes that catalyze the six-electron reduction of nitrite to ammonia, several different types are recognized. These are (I) assimilatory NiRs that function in biosynthetic nitrate assimilation of higher plants, algae, and fungi, (2) ammonia-forming dissimilatory NiRs involved in anaerobic nitrate respiration of diverse bacteria, and (3) assimilatory and dissimilatory sulfite reductases... 

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