14
Fritz Haber 1868 - 1934 • 1918 Nobel prize “synthesized ammonia from its elements”
Fritz Haber 1868 - 1934
• 1918 Nobel prize “synthesized ammonia from its elements”
Annual Global N-fixation
Type of fixation N2 fixed ( x 106 metric tons year-1)
Non-biological Industrial 50 Combustion 20 Lightning 10Total non-biological 80
Biological Agricultural land 90 Forest and non-agricultural land 50 Sea 35Total biological 175
Data from various sources, compiled by DF Bezdicek & AC Kennedy, in Microorganisms in Action (eds. JM Lynch & JE Hobbie). Blackwell Scientific 1998.
Possess homologs to NifH and Nif D but not known to fix N2
Include taxa that fix N2
Do not include taxa that fix N2
Raymond et al 2004 Mol. Biol. Evol. 21:541-554
Nitrogenase enzyme complex
MoFe protein• Dinitrogenase• Tetramer of two different dimers
2 2 heterodimer• = NifD, = NifK
• Contains– Two MoFeS centers (active site for N2
reduction)
– Four Fe4S4 centers
• Combines N2 and H+ to reduce N2 to NH3
Nitrogenase enzyme complex
From: Catalina Achimhttp://www.chem.cmu.edu/groups/achim/research/magneto.html
Fe protein
Fe protein
MoFe protein NH3
N2
190 Å
65 Å
e-
Nitrogenase enzyme complex
Fe protein
• Dinitrogenase reductase
• Dimer of two identical subunits2 dimer
= NifH
• Contains single Fe4S4 center
• Accepts reducing power (electrons) and reduces MoFe protein (dinitrogenase)
Nitrogenase enzyme complex
From: Catalina Achimhttp://www.chem.cmu.edu/groups/achim/research/magneto.html
Fe protein
Fe protein
MoFe protein NH3
N2
190 Å
65 Å
e-
Nitrogenase enzyme complex
Name Formula Major products
Proton H+ H2
Dinitrogen NN NH3 + H2
Nitrous oxide NN+-O- N2 + H2O
Azide [NN+-N]- N2 + NH3 + N2H4
Acetylene HCCH H2C=CH2
Cyanide [CN]- CH4 + NH3
Carbon monoxide CO CO (binds to N-binding site but is notreduced. Blocks rxnwith other substrates)
Versatility of nitrogenase complex
Vesicles are the site of N2-fixation in Frankia. Nitrogenase reduces nitrogen using electrons and ATP from compounds donated by the plant (succinate is only one possibility). The product ammonia is most likely excreted from the vesicle where plant enzymes (GS) convert it to organic nitrogen (glutamine). (D. Benson)
Ferredoxinreduced
Ferredoxinoxidized
e-
e-
Feox.
Fered.
e-
2 ADP
2 ATP
MoFered.
MoFeox.
N2, 8H+
2 NH3,H2
e- e-
e-
2 Pi
Nitrogenase enzyme complex
From: Catalina Achimhttp://www.chem.cmu.edu/groups/achim/research/magneto.html
Fe protein
Fe protein
MoFe protein NH3
N2
190 Å
65 Å
e-
Nitrogenase enzyme complex
Groups I - IIIpossess functional nitrogenases
Raymond et al 2004 Mol. Biol. Evol. 21:541-554
Tree of NifH and NifD homologs
Two proposed nif gene vs. spp. trees
(a) Last common ancestor (LCA) hypothesis
(b) Methanogen origin hypothesis
