Assimilatory nitrate reductase

There are four different classes of nitrate reductases (234). The nitrate reductases from D. desulfuricans show a strong homology to the a-subunit of the class of periplasmic respiratory nitrate reductases, and also to some of the enzymes that are included on the class of cytoplasmic assimilatory nitrate reductases. Because of this fact, a proposal was made for a new class of monomeric NAP, which contains the minimal arrangement of metal centers to perform nitrate reduction one [4Fe-4S] cluster and a Mo bound to two MGD. 

Assembly-induced GTP hydrolysis, MICROTUBULE ASSEMBLY KINETICS Assimilatory nitrate reductase,... 

The assimilatory nitrate reductase (Eq. 16-61) of fungi and green plants (Chapter 24) also belongs to the sulfite oxidase family. 

Bacterial assimilatory nitrate reductases have similar properties.86/86a In addition, many bacteria, including E. coli, are able to use nitrate ions as an oxidant for nitrate respiration under anaerobic conditions (Chapter 18). Tire dissimilatory nitrate reductases involved also contain molybdenum as well as Fe-S centers.85 Tire E. coli enzyme receives electrons from reduced quinones in the plasma membrane, passing them through cytochrome b, Fe-S centers, and molybdopterin to nitrate. The three-subunit aPy enzyme contains cytochrome b in one subunit, an Fe3S4 center as well as three Fe4S4 clusters in another, and the molybdenum cofactor in the third.87 Nitrate reduction to nitrite is also on the pathway of denitrification, which can lead to release of nitrogen as NO, NzO, and N2 by the action of dissimi-latory nitrite reductases. These enzymes873 have been discussed in Chapters 16 and 18. 

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... 

Nitrate reductase from Chlorella, an assimilatory enzyme, is a homotetramer of molecular weight 360 000 and contains one each of Mo, heme and FAD per subunit. The nitrate reductase from E. coli is a dissimilatory enzyme. EXAFS data are available on the molybdenum sites in both enzymes (Table 24).1050 The environment of the molybdenum in the assimilatory enzyme is similar to that found for sulfite oxidase, with at least two sulfur ligands near the molybdenum and a shuttle between monoxo and dioxo forms with redox change in the enzyme. This allows a similar mechanism to be put forward for the assimilatory nitrate reductase,1051 shown in equation (57), where an oxo group is transferred from nitrate to MoIV with production of nitrite and MoVI. 

The assimilatory nitrate reductase from Chlorella contains the molybdenum cofactor, as evidenced by the ability of the enzyme to donate the cofactor to the nitrate reductase of the mutant nit-1 of N. crassa. Reduction of the enzyme with NADH gives the Mov ESR signal, which is abolished on reoxidation with nitrate. Line shape and g values of the signal show a pH dependence similar to those observed previously for sulfite oxidase. The signal observed at pH 7.0 shows evidence for interaction with a single exchangeable proton.1053... 

Both assimilatory and dissimilatory nitrate reductases are molybdoenzymes, which bind nitrate at the molybdenum. EXAFS studies1050 have shown that there are structural differences between the assimilatory nitrate reductase from Chlorella vulgaris and the dissimilatory enzyme from E. coli. The Chlorella enzyme strongly resembles sulfite oxidase1050,1053 and shuttles between mon-and di-oxo forms, suggesting an oxo-transfer mechanism for reduction of nitrate. This does not appear to be the case for the E. coli enzyme, for which an oxo-transfer mechanism seems to be unlikely. The E. coli enzyme probably involves an electron transfer and protonation mechanism for the reduction of nitrate.1056 It is noteworthy that the EXAFS study on the E. coli nitrate reductase showed a long-distance interaction with what could be an electron-transfer subunit. 

Campbell, W.H. Kinghom, J.R. (1990). Functional domains of assimilatory nitrate reductases and nitrite reductases. Trends in Biochemistry 15, 315-19. 

LS, K.H.D. Lederer, F. (1983). On the presence of a heme-binding domain homologous to cytochrome b5 in Neurospora crassa assimilatory nitrate reductase. The EMBO Journal 2, 1909-14. 

Solomonson, L.P. Barber, M.J. (1990). Assimilatory nitrate reductase functional properties and regulation. Annual Review of Plant Physiology and Plant Molecular Biology 41, 225-53. 

The potentials found for nitrate reductases [96] vary with the role of the particular enzyme. Assimilatory nitrate reductase, found in plants, algae, and fungi, is involved in the first step in nitrogen assimilation and has a molybdenum center that operates at around 0 mV. Respiratory (dissimilatory) nitrate reductase, utilized by bacteria in energy yielding processes, has a molybdenum center that operates at around +200 mV [97,98],... 

Figure 7 The modus operandi of nitrate reductase (a) assimilatory nitrate reductase (plants, fungi, algae) (b) respiratory (dissimilatory) nitrate reductase (.Escherichia coli, Pseudomonas).

IV.D. The LMovl(0)2(S-cys) Site The Cofactor of Sulfite Oxidase and Assimilatory Nitrate Reductase... 

Protein sequence homology suggests that sulfite oxidase and assimilatory nitrate reductase are members of the same molybdenum enzyme subfamily [31]. Consistent with this classification, the cofactors of sulfite oxidase and assimilatory nitrate reductase differ significantly from those in dmso reductase, aldehyde oxido-reductase, xanthine oxidase (see Section IV.E.), and even respiratory nitrate reductase (Section IV.D). The EXAFS of both sulfite oxidase [132-136] and assimilatory nitrate reductase [131,137,138] and x-ray studies of sulfite oxidase (chicken liver) [116] confirm that the molybdenum center is coordinated by two sulfur atoms from a single MPT ligand and by the sulfur atom of a cysteine side chain. The Movl state is bis(oxido) coordinated (Figure 14). 

Nitrogenase MoFe protein NADH dehydrogenase Respiratory nitrate reductase Assimilatory nitrate reductase Sulfite oxidase Aldehyde oxidase Xanthine oxidase Xanthine dehydrogenase... 

Nitrate reductases (14) are found in a wide range of eukaryotes and prokaryotes and have a crucial role in nitrogen assimilation (33, 34) and dissimilation (35). These reductases catalyze the reduction of NO3 to N02. For the assimilatory nitrate reductases this reaction is followed by... 

Many species of bacteria also have an assimilatory nitrite reductase which is located in the cytoplasm. There is relatively little known about such enzymes but the electron donor is throught to be NADPH and the active site again has siroheme (Cole, 1988). The assimilatory nitrite reductases of both plants and bacteria use nitrite that is provided as the product of the assimilatory nitrate reductases. Nitrate is a very common natural N source for plant and bacterial growth. 

This family includes the sulfite oxidases and dehydrogenases of prokaryotes Thiobacilli sp.), plants, birds, and animals, and the assimilatory nitrate reductases from bacteria, algae, fungi, and plants. The sulfite oxidases of higher eukaryotes are 100-110kDa homodimers (Table 1) they are located in the mitochrondrial intermembrane space and catalyze the oxidation of toxic sulfite to innocuous sulfate (equation 7). Human sulfite oxidase deficiency leads to major neurological abnormalities, mental retardation, dislocation of the ocular lenses, and early death. ... 

Song, B., and Ward, B. B. (2004). Molecular characterization ofthe assimilatory nitrate reductase gene and its expression in the marine green alga Dunaliella tertiolecta (Chlorophyceae). J. Phycol. 40, 721-731. 

Jenkins, B. D., Zehr, J. P., Gibson, A., and Campbell, L. (2006). Cyanobacteria assimilatory nitrate reductase gene diversity in coastal and oligotrophic marine environments. Environ. Microbiol. 8,... 

In all photoautotrophs, reduction of NOj" to NH4 is achieved in two distinct enzymatic steps (Campbell, 2001). First, assimilatory nitrate reductase (NR) catalyzes the two electron reduction from NOj" to NO2. NR is a large soluble cytoplasmic enzyme with FAD (flavin adinine dinucleotide), an iron-containing cytochrome and molybdopterin prosthetic groups, and requires NADH and/or NADPH as an electron donor (Guerrero et al, 1981). Functional NR is in the form of a homodimer and therefore requires two atoms of iron per enzyme. Following transport into the chloroplast, NO2 undergoes a 6 e reduction to NH4 via assimilatory nitrite reductase (NiR). NiR, a soluble chloroplastic enzyme, contains five iron atoms per active enzyme molecule, and requires photosynthetically reduced ferredoxin as an electron donor (Guerrero et al., 1981). 

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