Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
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