Chapter 18 Nitrogen Metabolism
Nitrogen Cycle N-containing biological molecules • Amino acids • Nucleotides • Cofactors (biotin) Many oxidation states • N2, N2O, NO, HNO2,NH3 • More than any other major element Where can we get it? • about 78% of air is N2 Dependence on prokaryotes • Cycle would not exist without bacteria and archaea [prəʊˈkærɪɒt]
Nitrogen Cycle Fixing nitrogen (N2) • Essential for converting N into usable chemical form Nonbiological processes (Lighting, Industry, combustion)
Nitrogen Cycle Fixing nitrogen (N2) Biologically /ˌdaɪ.əˈzəʊ.trɒf/ • by microbes: diazotrophs (azote = nitrogen) • Catalyzed by nitrogenase Metalloprotein: Fe-S clusters, Fe-Mo cofactors Sensitive to O2: oxidizes Fe-S cofactors • Molybdenum /məˈlɪbdənəm/
Nitrogen Cycle Where else can we get NH4 +? Nitrates and nitrites Occur naturally in water and soils Reduction of NO3 _ (nitrate) to NO2 _ (nitrite) then to ammonia Carried out by plants, fungi, bacteria
Nitrogen Cycle Nitrification Oxidation of NH4 + to NO2 _ (nitrite) then to NO3 _ (nitrate) Some microbes can fully oxidation to nitrate – some to only nitrite Replenishes nitrites and nitrates in soil Denitrification Conversion of NO3 _ to N2 Completes the cycle
Nitrogen fixation • The nitrogenase complex, which carries out this fundamental transformation, consists of two proteins: a reductase, which provides electrons with high reducing power, and a nitrogenase, which uses these electrons to reduce N2 to NH3. • The transfer of electrons from the reductase to the nitrogenase component is coupled to the hydrolysis of ATP by the reductase
Nitrogen Assimilation Now that we have useful form of N (NH3) How does it get incorporated into biological molecules? NH4 + toxic to cells (pH） Initial assimilation Glutamine synthetase (synthetase uses an NTP) Found in all organisms
Nitrogen Assimilation Glutamine synthetase (synthetase uses an NTP) Glu first activated by phosphoryl transfer NH2 exchanged for Pi Activity is tightly regulated Gln is a carrier of amine groups Glu + Gln → higher concentrations than other amino acids
Nitrogen Assimilation Initial assimilation In bacteria and plants glutamate synthase (-ase not –tase, so no NTP needed) Gln synthetase (Glu → Gln) + Glu synthase (α-KG + Gln → 2 Glu) Mammals don’t have Glu synthase (synthesized in other ways)