NITROGEN METABOLISM IN PLANTS

Importance of Nitrogen It is the polymeric nitrogen containing compounds proteins and nucleic acids that define the major attributes of organism such as function and structure. It is a common element-Proteins, NAs, PGRs, chlorophyll. N,P,K is the essential element for plant growth and devlopment.

Metabolism of Nitrogen The topic of nitrogen metabolism includes: - Nitrogen Fixation - Biosynthesis and Breakdown of amino acids. Nitrogen Fixation. The process by which inorganic molecular nitrogen(N 2 )from the atmosphere is incorporated first into ammonia and then into organic compounds that are of use to organism. N 2 + 8e - + 8H ATP → 2NH 3 +H ADP+16P i The nitrogenase enzyme complex catalyzes this reaction (Dixon and Khan 2004;Seefeldt et al.2009)

Ammonia thus formed as a result of nitrogen fixation is used for the synthesis of amino acids. Amino acids are the building blocks for the synthesis of proteins. The organic nitrogen again returns to soil in animal waste and decomposition after death. So N cycle processes include Ammonification : organic N  ammonia (NH 4 + ) Nitrification : ammonia → nitrite→ nitrate. Denitrification : NO 3 - → N 2

Key metabolic step in conversion of fixed ammonia to organic N The ammonium (NH 4+ ) and nitrate (NO 3 − ) ion in the soil solution that are generated through fixation or released through decomposition of soil organic matter are now available for plant. Nitrate is usually the predominant form of available nitrogen that is absorbed by the plant. Ammonium can predominate in grasslands, and in flooded, anaerobic soils.

Nitrate assimilation Plant roots absorbs nitrate from the soil solution. The reduction of nitrate to nitrite occurs in the cytosol and it involves the transfer of two electrons(e-). NO NAD (P)H +H + →NO NAD(P)+ + H 2 O. The reaction is catalyzed by the enzyme nitrate reductase. Nitrate reductase is composed of two identical subunits each containing 3 prosthetic groups- Flavin adenine dinucliotide (FAD),heme and Mo-atom complex.

Nitrite reductase converts the nitrite to ammonium The nitrite reduction occurs in the chloroplast (leaves) and plastids(Roots). The reaction involves the transfer of 6e- NO Fd red /NADHPH + 8 H + → NH Fd ox + 2H 2 O. Nitrite reductase consists of a single polypeptide containing two prosthetic groups-an iron-sulfur cluster (Fe4S4) & a specialized heme. Ammonium assimilation Ammonium is converted to amino acids. This amino acid conversion occurs by the sequational action of Glutamine synthatase & Glutamate synthase.(Lea et al.1992)

Glutamine synthatase Glutamine synthase combines ammonium with glutamate to form glutamine. Glutamate + NH ATP →Glutamine + ADP +P i The reaction requires the hydrolysis of one ATP & a divalent cation such as Mg 2+, Mn 2+ or Co 2+. Glutamate synthase Glutamine stimulates the activity of glutamate synthase.(also known as glutamine-2-oxoglutarate aminotransferase or GOGAT) The enzyme transfers the amino group from glutamine to 2-oxoglutarate yielding two molecules of glutamate. Plant contains two types of GOGAT- NADH-GOGAT (located in plastids ) Fd-GOGAT (located in Chloroplast)

Transamination reaction transfer nitrogen After the formation of glutamine or glutamate, other amino acids are formed via a reaction transamination. The enzyme aminotransferase catalyzes the reaction. E.g-Asp-AT, enzyme convert the glutamate to Asparatate. Glutamate + oxaloacetate →Aspartate + 2-oxo-glutarate. All transamination reaction requires pyridoxal phosphate (Vit.B6) as a co-facto.