EC Number   |
Natural Substrates |
|---|
   1.7.5.1 | more |
Escherichia coli expresses two different membrane-bound respiratory nitrate reductases, nitrate reductase A (NRA) and nitrate reductase Z (NRZ). The two enzymes are encoded by distinct operons located within two different loci on the Escherichia coli chromosome. The narGHJI operon, encoding nitrate reductase A, is located in the chlC locus at 27 min, along with several functionally related genes: narK, encoding a nitrate/nitrite antiporter, and the narXL operon, encoding a nitrate-activated, two component regulatory system. The narZYWV operon, encoding nitrate reductase Z, is located in the chlZ locus located at 32.5 min, a region which includes a narK homologue, narU, but no apparent homologue to the narXL operon. The two membrane-bound enzymes have similar structures and biochemical properties and are capable of reducing nitrate using normal physiological substrates. The homology of the amino acid sequences of the peptides encoded by the two operons is extremely high but the intergenic regions share no related sequences. The expression of both the narGHJI operon and the narK gene are positively regulated by two transacting factors Fnr and NarL-phosphate, activated respectively by anaerobiosis and nitrate, while the narZYWV operon and the narU gene are constitutively expressed. Nitrate reductase A, which accounts for 98% of the nitrate reductase activity when fully induced, is clearly the major respiratory nitrate reductase in Escherichia coli |
| 1.7.5.1 | more |
nitrate reductase Z expression is regulated in a manner opposite to that of nitrate reductase A. The narGHJZ operon is aerobically repressed, strongly induced by nitrate and positively regulated by the fnr gene product. The expression of narZ is anaerobically repressed, induced weakly, if at all, by nitrate and negatively regulated by the fnr gene product. The opposing regulation of these two enzymes suggests that a function of nitrate reductase Z may be to catalyse the immediate flow of electrons to nitrate during an aerobic/anaerobic transition when the bacterium is grown in the presence of nitrate |
| 1.7.5.1 | more |
nitrate reductase Z is synthesized in small amounts, the expression of its structural genes does not seem to be induced by nitrate, repressed by oxygen or activated by the product of the fnr gene. The nitrate reductase Z in mutant LCB79/pLCB14 couples formate oxidation with nitrate reduction probably via quinones and type-b cytochromes |
| 1.7.5.1 | more |
NRZ is expressed at a low level that is not influenced by anaerobiosis or nitrate. The NRZ operon is controlled mainly at the level of transcription and is induced 10fold at the onset of stationary phase in rich media. Expression of NRZ nitrate reductase is highly growth phase dependent and is controlled by the alternative vegetative sigma factor RpoS. RpoS-mediated regulation of NRZ may be an important physiological adaptation that allows the cell to use nitrate under stress-associated conditions |
| 1.7.5.1 | nitrate + quinol |
first enzyme involved in respiratory denitrification in prokaryotes |
| 1.7.5.1 | nitrate + quinol |
in order to use nitrate as an electron acceptor, Escherichia coli synthesises three distinct enzymes: a membrane-bound enzyme (nitrate reductase A, NarGHI) encoded by the narGHJI operon and a soluble periplasmic nitrate reductase (NapAB, EC 1.9.6.1) encoded by the napFDAGHBC operon. A second membrane-bound nitrate reductase (nitrate reductase Z, NarZYV) encoded by the NarZYWV operon is biochemically similar to NarGHI. Whereas NarGHI synthesis is induced by nitrate under anaerobic conditions, NarZYV is expressed at a cryptic level and may assist Escherichia coli in transition from aerobic to anaerobic respiration (physiological role of this isoenzyme at the onset of the stationary growth phase in rich media). NapAB is mainly expressed in the presence of low concentrations of nitrate under both aerobic and anaerobic conditions, and its expression is suppressed at high nitrate concentrations. Conversely, NarGHI is maximally expressed when nitrate concentration is elevated, and under these conditions becomes the predominant enzyme in Escherichia coli. Thus, NapAB (Ec 1.9.6.1) and NarGHI seem to function in different ranges of nitrate concentration in a complementary way to support anaerobic respiration on nitrate under anaerobic conditions and in the presence of nitrate |
| 1.7.5.1 | nitrate + quinol |
model of electron transfer in the nitrate reductase: electrons are provided by quinones to the NarI subunit and subsequently transferred to NarH, which eventually delivers them to the molybdenum cofactor where nitrate reduction takes place |
| 1.7.5.1 | nitrate + quinol |
nitrate reductase A reduces nitrate to nitrite and forms part of a redox loop generating a proton-motive force |
| 1.7.5.1 | nitrate + quinol |
the enzyme is essential for the fungal denitrification. The fungal formate dehydrogenase can supply electrons via quinol/quinone pool to nitrate reductase A |
| 1.7.5.1 | nitrate + quinol |
the membrane-anchored protein directs electrons from quinol oxidation at the membrane anchor, NarI, to the site of nitrate reduction in the membrane extrinsic [Fe-S] cluster and Mo-bis-MGD containing dimer, NarGH |
