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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+ + 2eÆ 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+ + 6eÆ 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 Æ 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
