Nitrogen Fixation

Nitrogen fixation converts N₂ →

NH₃ (ammonia), the form living things can actually build into amino acids, nucleotides, and other biomolecules

Only certain prokaryotes (bacteria and archaea)

xan fix nitrogen

N₂ + 8H⁺ + 8e⁻ + 16 ATP →

 2 NH₃ + H₂ + 16 ADP + 16 Pi

nitrogen ficxation is highly

endergonuc and energy expensive

nitrogen fixation requires a stong

source of electrons for reductive processes

Dinitrogenase reductase (Fe protein)

smaller; binds and hydrolyzes ATP; delivers electrons one at a time.

Dinitrogenase (MoFe protein)

larger; contains the FeMo-cofactor, the actual active site where N₂ is bound and reduced.

Nitrogenase is irreversibly

inactivated by O₂!!!!! 

Heterocysts

thick-walled cells lacking photosystem II, physically separating fixation from O₂-producing photosynthesis

Leghemoglobin

binds O₂ tightly, keeps free O₂ concentration low while still allowing enough for respiration

Rhizobium

 are soil bacteria that form a mutualistic symbiosis with legume plants

What do plants get from living symbiotically with rhizobuim

fixed nitrogen (as ammonia) it can use to make amino acids

What do bacteria get from plants

protected niche and a steady supply of carbon (sugars) from the plant.

How is Rhizobium/plant symbiosis estabolished

1. legume roots secrete flavonoids

2. flavonoids induce rhixo to make nod factors

3. nod factors trigger connection

4. plant cells proliferate to make a root nodule (organ w bacteria)

5. inside nodule bacteria become bacteriods (fix nitrogen)

Leghemoglobin

a plant-encoded, bacteria-induced protein

Leghemoglobin

buffer oxygen levels inside the nodule enough

Fixed nitrogen (NH₃/NH₄⁺) is exported from bacteroids to the plant,

 where it's assimilated into glutamine/glutamate (via glutamine synthetase/GOGAT)