Question

What are the functional differences between nitrate reductase and nitrite reductase?

Answer 1

Answer:

15.2.2.1 Nitrate reduction

Nitrate reductase (NR) catalyzes the first reaction in nitrate assimilation, the reduction of nitrate to nitrite. Nitrate reductase requires molybdenum (Mo) as cofactor. Nitrate reductase in higher plants is proposed to be a homodimer, with two identical subunits joined and held together by the Mo cofactor (Kleinhofs et al., 1989).

Two cDNAs representing two alleles of the NR from potato have been described (Harris et al., 2000), which closely resemble the respective Arabidopsis encoding sequences and the ones from other plant species.

Ferredoxin-nitrite reductase (Fd-NiR) subsequently catalyzes the six-electron reduction of nitrite to ammonia, using reduced Fd as the electron donor. The Fd-dependent NiRs of plants, algae and cyanobacteria are monomeric proteins with molecular masses near 63 kDa which contain a single [4Fe-4S] cluster and a single siroheme (which serves as the binding site for nitrite) as prosthetic groups (Dose et al., 1997). Ferredoxin-NiR has been cloned from several higher plant species; however, no corresponding sequence so far has been isolated from potato.

In higher plants, nitrate reduction is highly regulated. A range of environmental factors influence the expression of the corresponding genes as well as the enzyme activity levels. NR activity expression and activity is controlled by light, temperature, pH, CO2, O2, water potential and N source. Drought for instance causes increased NR protein turnover and accelerated mRNA turnover (Ferrario-Mery et al., 1998; Foyer et al., 1998).

Spinach leaf NR undergoes a reversible phosphorylation in response to light/dark transitions, which leads to an inactivation of the enzyme (Huber et al., 1992). The low-activity, phosphorylated form of NR from darkened leaves of spinach is activated during purification, because it is separated from a approximately 110-kDa NR inhibitory protein (NIP). Re-addition of NIP inactivated the purified phosphorylated NR, but not the active dephosphorylated form of NR, indicating that the inactivation of NR requires both, phosphorylation and interaction with NIP. In addition, NR that had been inactivated in vitro through phosphorylation, and interaction with NIP could be reactivated either by dephosphorylation or by dissociation of NIP from NR (MacKintosh et al., 1995).

NIP has been shown to be a member of the family of 14-3-3 proteins. 14-3-3 proteins are chaperone proteins that modulate interactions between components of signal transduction cascades and enzymes, which finally results in the activation or inactivation of the interacting proteins (Aitken, 1996; Wu et al., 1997). 14-3-3-protein-binding proteins in plants amongst others also include NR and sucrose-phosphate synthase (SPS). Extensive work has been performed in potato to modulate the expression level of different isoforms of the 14-3-3 proteins (Zuk et al., 2003, 2005). Transgenic potato plants with lowered levels of five isoforms of the 14-3-3 proteins show increased NR as well as SPS activities, which ultimately leads to an increased productivity of the plants as they accumulated more starch in their tubers in greenhouse experiments. These experiments were done using a constitutive promoter. The authors therefore speculate that the increases in starch accumulation are a result of enhanced sucrose supply from the source leaves.

These results are in agreement with observations made with transgenic tobacco plants carrying an NR with an N-terminal deletion and hence an enzyme that is not subjected to phosphorylation, which results in the abolishment of post-transcriptional regulation of NR by light and higher NR activities (Nussaume et al., 1995). This could be due to a difference in dissociation of the NR–NIP complex (Lillo et al., 1997) or a different way of binding for 14-3-3 in the truncated NR (Provan et al., 2000). The truncated and deregulated form of NR also was over-expressed in transgenic potato plants. In agreement with the experiments where the 14-3-3 protein expression was reduced, under some environmental conditions the plants showed an increased productivity (Djennane et al., 2002, 2004). The only change that was seen, however, was a reduction of the nitrate contents of the transgenic potato tubers, which might support the speculation that rather the higher activation status of SPS is the reason for the enhanced productivity of plants with reduced 14-3-3 protein levels.

Answer 2

Functional differences between nitrate reductase and nitrite reductase:

Nitrate reductase:

• Nitrate reductases help to catalyze the rate of nitrate assimilation. By this reducing process nitrate changes to nitrite.
• The nitrate reductase changed the inorganic nitrogen to the organic form of the nitrogen.

Nitrite reductase:

• Nitrite reductases are the type of the enzymes which reduce the nitrite. There are two types of the Nitrite reductases can be found.
• A multi haem enzyme helps to reduce the $NO{2}$ to the various products of the nitrogen.

To know more

Which element is required for nitrate reductase activity?

https://brainly.in/question/10661393

Match the following:

AB

(i) Nitrate reductase (a) nitrogen fixation

(ii) Nitrite reductase b) nitrate reduction

(iii) Nitrogenase c) nitrite reduction

https://brainly.in/question/12874668