Nitrogen Metabolism

• Nitrogen cycle• Nitrate Assimilation• Biological Nitrogen

Fixation

Red Alder(Alnus rubra)

Nitrogen cycle

Biogeochemical nitrogen cycle

• Lightning• Photochemical rxns• Biological nitrogen

fixation• Industrial fixation• NH3 is toxic

Nitrate Assimilation• Plants primarily assimilate nitrate (NO3-)• Nitrate reductase composed of FAD, Mo, and Heme ring• Found in root, shoot and leaf• Influenced by light, Nitrate and carbohydrate at

transcriptional and translational level

• NO3- + NAD(P)H + H+ + 2e-

NO2- + NAD(P) + H2O

Induction of Nitrate reduction in Barley

Nitrite reductase

• Nitrite (NO2-) is toxic• Transport from

cytosol to chloroplast (plastid in roots)

• NO2- + 6FDred + 8H+ + 6e-

NH4+ + 6FDox + 2H2O

Ammonium Assimilation

• Convert nitrate to ammonia• Incorporated into amino acids

Biological Nitrogen Fixation

• Free living & Symbiotic bacteria

• Prokaryotes only• N2 converted to

ammonia• Enzyme: Nitrogenase

Nitrogen fixation

• Low oxygen conditions required

• Need lots of energy

• N2 + 8e- + 8H+ + 16 ATP

2NH3 + H2 + 16ADP + 16Pi

Symbiotic Nitrogen fixation

• Host specificity• Root nodules• Cost and benefit to

the plant

Infection• Chemotactic signals

secreted from roots (isoflavinoids/betaine)

• Signal from bacteria: Nod factors

Nitrogenase

• Two components: Fe protein (2 subunits), inactivated by oxygen in 30-45 sec

• MoFe (4 subunits), inactivated by oxygen in 10 minutes

• Reacts with N2 and H+

Nitrogenase complex

• Leghemogolobin = oxygen binding protein in cytoplasm of infected cell [700 um], pink color

• Host plant produces globin, bacteria heme• Km = 0.01 um, 10x higher than B-globin in human• Stores some oxygen, transports Oxygen to respiring

bacterial cells

Transport forms of Nitrogen

•Amide exporters•Uriede exporters•Xylem sap