NADH nitrate reductase
| Nitrate reductase (NADH) ( Zea mays ) | ||
|---|---|---|
| Mass / length primary structure | 621 amino acids | |
| Secondary to quaternary structure | Homodimer | |
| Cofactor | FAD, heme, molybdopterin | |
| Identifier | ||
| Gene name (s) | nnr1 (MaizeGDB) | |
| External IDs | ||
| Enzyme classification | ||
| EC, category | 1.7.1.1 , Oxidoreductase | |
| Response type | Redox reaction | |
| Substrate | Nitrate + NADH + H + | |
| Products | Nitrite + NAD + + H 2 O | |
| Occurrence | ||
| Parent taxon | Plants, bacteria; few eukaryotes, archaea | |
The nitrate reductase (NADH) is a plant enzyme , the reduction of nitrate to nitrite catalyzed. It is also found in many bacteria . The reduction is necessary in order to be able to assimilate nitrate, it is continued until the ammonium , which supplies the nitrogen for amino acids .
The enzyme belongs to the group of nitrate reductases .
construction
The protein is a homodimer whose subunits (approx. 100 kDa ) consist of three domains , each with different, covalently bound cofactors . The C-terminal domain is FAD , the middle domain is heme b . In the N-terminal domain, a pterin acts as an organic chelator for molybdenum ( molybdopterin ). The electrons required for reduction are absorbed by the FAD and passed on to the molybdopterin complex via the heme. There they are then transferred to nitrate.
The active center is sunk into a funnel, the edge of which is filled with positive charges. This ensures a high selectivity of the enzyme.
Functions in the plant organism
In plants, the enzyme is part of the nitrogen metabolism and helps to provide reduced, metabolizable nitrogen in the form of ammonium for the synthesis of organic matter.
Nitrate ( oxidation level + V) absorbed from the soil water via the roots is reduced to nitrite (oxidation level + III) by the cytosolic nitrate reductase; NADH or NADPH (in non-green tissues) serves as an electron donor :
The nitrite reductase further reduces nitrite to ammonium and can be incorporated into amino acids .
regulation
Nitrate reductase is an upregulated enzyme. Translationally, nitrate reductase is regulated over the course of the day, with the rule that more enzyme is present earlier in the day than later. During the night the activity of nitrate reductase is harmful because its cytotoxic product nitrite cannot be further processed without reduction equivalents from photosynthesis and would be enriched.
The post-translational regulation of nitrate reductase takes place via a two-stage mechanism depending on the metabolic status of the cell. In the event of sudden darkening or a lack of nutrients, a serine residue in the hinge area between the heme and molybdenum subunit can be phosphorylated by protein kinases. This phosphorylated serine residue is able to bind a regulatory 14-3-3 protein in a Mg2 + -dependent manner. The complex of nitrate reductase, Mg2 + and 14-3-3 protein shows no activity. Conversely, the inactivation can be reversed by dephosphorylating the serine residue using a phosphatase; the inactivating 14-3-3 protein only binds to the phosphorylated nitrate reductase.
Other functions
An important by-product of nitrate reductase (with nitrite as a substrate) is nitrogen monoxide (NO), which serves as a signaling molecule. However, the NO synthesized in animals is provided by the NO synthase . NO is mainly known as EDRF , which is used, for example, in many drugs (e.g. Sildenafil ) (see: Organic nitrates ).
literature
- Fewson, CA and Nicholas, DJD (1961). "Nitrate reductase from Pseudomonas aeruginosa". Biochim. Biophys. Acta 49: 335-349. doi : 10.1016 / 0006-3002 (61) 90133-0 . PMID 13699254 .
- Nason, A. (1963). "Nitrate reductases". In Boyer, PD, Lardy, H. and Myrbäck, K. The Enzymes 7 (2nd ed.). New York: Academic Press. pp. 587-607.
- Nicholas, DJD and Nason, A. (1955). "Diphosphopyridine nucleotide-nitrate reductase from Escherichia coli". J. Bacteriol. 69: 580-583.
- Spencer, D. (1959). "A reduced diphosphopyridine-specific nitrate reductase from germinating wheat". Aust. J. Biol. Sci. 12: 181-189.
- Berks, BC, Ferguson, SJ, Moir, JW and Richardson, DJ (1995). "Enzymes and associated electron transport systems that catalyze the respiratory reduction of nitrogen oxides and oxyanions". Biochim. Biophys. Acta 1232: 97-173. doi : 10.1016 / 0005-2728 (95) 00092-5 . PMID 8534676 .