扬州大学：《生物化学 Biochemistry》课程教学课件（讲稿）chapter 18 amino acid metabolism

Chapter 18Nitrogen Metabolism

Chapter 18 Nitrogen Metabolism

OxidationNitrogen CycleNitrogenstate-N-containing biological molecules-3NH,NH,:Ammonia,ammonium·Amino acidsion-2H,N-NH,HydrazineNucleotides-1NH,OHHydroxylamine: Cofactors (biotin)。N,NitrogenMany oxidation statesN,O+1NitrousoxideNO+2Nitricoxide. N2, N,O, NO, HNO2,NH3+3HNO,NO,Nitrous acid,? More than any other major elementnitriteion+4NO,NitrogenWhere can we get it?dioxide+5HNO, NO,Nitric acid,: about 78% of air is N2nitrateion+6Dependence on prokaryotes [prau'keript]· Cycle would not exist without bacteria and archaea

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 CycleFixing nitrogen (N2)·Essential for convertingN intousable chemicalform>Nonbiologicalprocesses(Lighting, Industry, combustion)

Nitrogen Cycle Fixing nitrogen (N2) • Essential for converting N into usable chemical form Nonbiological processes (Lighting, Industry, combustion)

Molybdenum/ma'libdanam/Nitrogen CycleFixing nitrogen (N2)Mo>Biologically/.dal.a'zau.trpf/. by microbes: diazotrophs (azote = nitrogen).CatalyzedbynitrogenaseVMetalloprotein:Fe-Sclusters,Fe-MocofactorsVSensitivetoO2:oxidizesFe-ScofactorsN,+8H++8e-+16ATP+16H0←→2NH,+H,+16ADP+16PLReadily becomes NH: (oK-9)

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 CyclenitrogendenitrificationfixationnitrogenaseWhere else can we get NH +?>Nitrates and nitritesNONHNitrateOccur naturally in water and soils>Reduction of NO3nitritenitratereductasereductase(nitrate) to NO2- (nitrite)nitrificationnitrificationNO;thentoammoniaNitrite>Carried out by plants, fungi, bacteria

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 CycleNitrification>Oxidation of NH + to NO, (nitrite) then to NO, (nitrate)>Some microbes can fully oxidation to nitrate - some to only nitrite>Replenishes nitrites and nitrates in soilN2nitrogendenitrificationDenitrificationfixationnitrogenase>Conversion of NO3toN,>CompletesthecycleNOjNHNitratenitritenitratereductasereductasenitrificationnitrificationNONitrite

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 fundamentaltransformation, consists of two proteins: a reductase, which provideselectrons with high reducing power, and a nitrogenase, which uses theseelectrons to reduce N, to NH3The transfer of electrons from the reductase to the nitrogenase componentis coupled to the hydrolysis of ATP by the reductase.ElectronsfromreducedADPATPferredoxin+PReductaseNitrogenase(Fe protein)(MoFeprotein)

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 AssimilationNow that we have useful form of N (NH)>How does it get incorporated into biological molecules?> NH + toxic to cells (pH)Initial assimilation>Glutamine synthetase (synthetase uses an NTP)>Found inall organisms

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 AssimilationGlutamine synthetase (synthetase uses an NTP)> Glu first activated by phosphoryl transfer>NH,exchangedforP;>Activity is tightly regulated>Glnisacarrierofaminegroups>Glu + Gln → higher concentrations than other amino acidscOOADPcOOcooTPNH:H2HOPOTNH*NH,+NH,*NHyGlutamateGlutamine

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

NitrogenAssimilationInitial assimilation>In bacteria and plants> glutamate synthase (-ase not -tase, so no NTP needed)NADPHCOO-COO-COOCOOH.NHN-HH+NADP+HA-HHCH2CH2CH2H2glutamateCH2CHzCH2H2synthaseCOOCooCooNH22Glutamatea-KetoglutarateGlutamine>Gln synthetase (Glu→Gln)+Glu synthase (α-KG +Gln → 2 Glu)α-ketoglutarate+NH,++NADPH+ATP→Glu+NADP++ADP+P,>Mammals don't have Glu synthase (synthesized in other ways)

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)