1. Fritz Haber
1918 Nobel prize “synthesized ammonia from its elements”

2. Annual Global N-fixation
Data from various sources, compiled by DF Bezdicek & AC Kennedy, in Microorganisms in Action (eds. JM Lynch & JE Hobbie). Blackwell Scientific 1998.

3. Do not include taxa that fix N2
Raymond et al 2004 Mol. Biol. Evol. 21:
Possess homologs to NifH and Nif D but not known to fix N2

4. Nitrogenase enzyme complex

5. 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

6. Nitrogenase enzyme complex
NH3
MoFe protein N2
Fe protein
Fe protein e-
65 Å
190 Å
From: Catalina Achim

7. Nitrogenase enzyme complex
Fe protein
Dinitrogenase reductase
Dimer of two identical subunits
2 dimer
 = NifH
Contains single Fe4S4 center
Accepts reducing power (electrons) and reduces MoFe protein (dinitrogenase)

8. Nitrogenase enzyme complex
NH3
MoFe protein N2
Fe protein
Fe protein e-
65 Å
190 Å
From: Catalina Achim

9. Versatility of nitrogenase complex

10. Vesicles are the site of N2-fixation in Frankia
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)

11. Nitrogenase enzyme complex
2 ATP
Ferredoxinoxidized
Fered.
MoFeox.
2 NH3,H2 e- e- e- e- e- e-
Ferredoxinreduced
Feox.
MoFered.
N2, 8H+
2 ADP
2 Pi

12. Nitrogenase enzyme complex
NH3
MoFe protein N2
Fe protein
Fe protein e-
65 Å
190 Å
From: Catalina Achim

13. Tree of NifH and NifD homologs
Groups I - III possess functional nitrogenases
Raymond et al Mol. Biol. Evol. 21:

14. Two proposed nif gene vs. spp. trees
(b) Methanogen origin hypothesis
(a) Last common ancestor (LCA) hypothesis