The below mentioned article provides a short note on reductase synthesis.
Ammonia produced after nitrogen fixation is either immediately converted to organic nitrogen or to nitrate by nitrifying bacteria like Nitrosomonas in soil. The nitrates are then taken up by the plants and reduced to ammonia by a series of enzymes involving eight electrons transfer per molecule of NH3 formed.
The first enzyme which converts nitrate to nitrite is nitrate reductase (NR), a metalloflavoprotein. The heme component of oligomeric protein of the NAD (P)-nitrate reductase of eukaryotic plants has been identified cytochrome b-557. Nitrate reductase from Neurospora consists of two equal cytochrome containing subunits. Prokaryotic bacteria and blue-green algae contain a ferredoxin nitrate reductase.
Nitrate and Mo are required for the synthesis of nitrate reductase. Induction by nitrate involves the mRNA-dependent synthesis of the apoprotein whereas the activation by Mo of the enzyme is independent of protein synthesis (see above). The carrier-mediated uptake of nitrate is an ATP- dependent process.
Ammonia inhibits nitrate uptake in some organisms but was found to be ineffective in others. A release into the medium of OH– with a ratio 1: 1 is coupled with the uptake of NO3–. Butz and Jackson (1977) described a tetrameric structure of nitrate reductase which is loosely attached to membrane. One monomer acts as a nitrate permease and other three monomers for reduction.
Each monomer contains, NADH binding site and Mo containing site. Each monomer also has 12 non-heme Fe, atoms of acid labile sulfide and 24 cysteine residue. Since nitrate reductase is known to be substrate and light inducible, the role of light is still controversial.
There are certain evidences that light supplies energy for maintaining active polyribosome levels for protein synthesis. Under aerobic conditions, glucose can substitute for the light requirements for induction, which otherwise would have been supplied by light reaction through photosynthesis.
Although nitrate reduction is ultimately dependent on photosynthesis for reductant supply yet this dependence has been argued to be indirect. NADH is supplied to the cytoplasm by dehydrogenation of triose phosphate exported from the chloroplast.
Arsenate, which uncouples ATP production without affecting NADH generation, clearly shows that the primary role of light on nitrate reduction is via ATP and not through generation of NADH from the oxidation of the photosynthates.
An interrelationship has been established between nitrate assimilation and carbohydrate metabolism, particularly in relation to the supply of NADPH by pentose phosphate pathway for nitrate reductase. The reduction of nitrite in green plants takes place in the chloroplasts and is closely coupled to the photophosphorylation.
This enzyme from spinach contains 3 Fe; one of them is in the siroheme and the other two constitute with two atoms of acid labile sulphide, an iron sulphur centre.