Nitrite reductase localization

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). 

Urata, K., and Satoh, T. (1991). Enzyme localization and orientation of the active site of dissimilatory nitrite reductase from Bacillus firmus. Arch. Microbiol. 156, 24-27. 

Nitrite reductase is a homotrimer in its native state [272]. Depending on the origin the chain lengths of the respective subunits vary between 340 and 379 amino acid residues. The trimer contains three type 1 and three type 2 copper centers. The type 1 copper centers are localized within the subunits, whereas the type 2 copper centers are coordinated by residues from two different subunits [272,273] (Fig. 33). 

How the mitochondria can enhance nitrite reduction in leaves in the dark is not known as the bulk of the experimental evidence indicates that nitrite reductase is localized in the chloroplast. The reductant is ferredoxin generated by light or by the NADPH-ferredoxin reductase and an unknown NADPH-generating system in the dark. The inhibition of nitrite reduction by DNP and arsenate (Kessler and Bucker, 1960 Kessler, 1964 Hattori and Myers, 1966) has been interpreted to implicate a high energy phosphate requirement in nitrite reduction. One can speculate that the role of ATP could be to form an active nitrite required for reduction or to facilitate the entry of nitrite into the chloroplast. 

In C4 plants, nitrate and nitrite reductases, especially the former, were localized primarily in the mesophyll leaf cells, although some nitrite reductase was found in the bundle sheath cells (Mellor and Treguna, 1971 Rathnam and Edwards, 1976 Hard et al., 1977 Neyra and Hageman, 1978). 

The organization of the enzymes of denitrification in gram-negative bacteria, as determined by antibody labelling and electron microscopy studies, is shown in Figure 5. The first enzyme, nitrate reductase (NaR), resides in the cytoplasmic membrane with its active site accessed from the cytoplasmic side, necessitating transport of nitrate across both the periplasmic and cytoplasmic membranes. The product nitrite is transported back into the periplasmic space, where it is reduced by the nitrite reductase (NiR). Most NiR s appear to be soluble enzymes, although there have been reports of preparations in which the activity was associated with membrane fractions. The nitric oxide reductase (NoR) is also localized in the cytoplasmic membrane, and releases its product N2O back into the cytoplasmic space, where the soluble enzyme nitrous oxide reductase (NoS) converts it to N2. 

---