NITROGEN METABOLISM

NITROGEN METABOLISM

8.1.    Nitrate Assimilation
Nitrate assimilation occurs mainly in leaves. While in many woody plants and legumes, it mainly occurs in the roots. Symport of nitrates into roots take place with two protons. The nitrate is stored temporarily in the vacuole of root cells. It is reduced to NH4+ in epidermal and cortical cells of the root. In the mesophyll cells, nitrate is reduced to nitrite by nitrate reductase in the cytosol and then to NH4+ in epidermal chloroplast by nitrite reductase. The reductase uses NADH as the reductant.
8.2.    Nitrate reductase
It consists of two identical subunits. Each subunit contains an electron transport chain with one FAD i.e Flavin Adenine Dinucleotide, one heme (cyt-b type) and a cofactor containing (MoCo). In this a pterin molecule having a side chain attached to Molybdenum by two sulphur bonds. The oxidation state of Mo varies between +IV to +VI. The three redox carriers: FAD, cyt b and MoCo are covalently bound to the subunits of nitrate reductase. Each redox carriers are found in 3 separate domains. Holoenzyme with these domains catalyzes the transport of electrons to acceptors. For e.g. from NADPH to Fe3+ ions by FAD domain or from reduced state of methyl-viologen to nitrate by Mo domain.

8.3.    Reduction of Nitrite to ammonia
Nitrite reductase catalyzes the reduction of nitrite. It is found exclusively in plastids. It is activated by reduced ferredoxin which is produced by PS I during photosynthetic electron transport. It is formed of a covalently bound 4Fe-4S cluster, one FAD molecule and a siroheme. These three moieties from an electron transport chain which transfers electrons from ferredoxin to nitrite. Nitrite reductase because of high affinity for nitrite completely converts it to ammonia.


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