847 - Nitrogen Fixation Flashcards
What is diazotrophy?
A form of nitrogen fixation in which the bacteria converts N2 or nitrates into ammonia for incorporation into organic molecules.
What is the chemical formula of a nitrate?
NO3-
What is the chemical formula of a nitrite?
NO2-
What is nitrification?
The production of Nitrates and Nitrites from ammonia.
Which anaerobic bacteria convert ammonia to nitrites?
Nitrosomonas
Which anaerobic bacteria convert nitrites to nitrates?
Nitrobacter
What is denitrification?
Reduction of nitrates to N2, often by nitrogen respiring bacteria.
What bacteria are capable of nitrogen fixation?
Azotobacter, Rhizobium, Klebsiella, some Cyanobacteria
How do most diazotrophic organisms live?
As symbionts, providing organic nitrogen to larger organisms.
How much of the planetary total of nitrogen fixation is done by diazotrophy?
60%
Other than diazotrophy, how is nitrogen fixed?
Lightning and the Haber-Bosch process.
Is nitrogen exergonic or endogonic?
Highly exergonic, DG = -33.5kJ/mol
Why is atmospheric nitrogen largely inert?
Because N2 is very stable, so requires a high activation energy (230-400kJ/mol) to react even though it is highly exergonic.
What are the reaction conditions of the Haber-Bosch process?
500 C, 300 atm, Iron catalyst.
What enzyme is responsible for nitrogen fixation?
Nitrogenase metalloenzymes.
What is the turnover of nitrogenase?
5/s
How does the cell compensate for the low turnover number of nitrogenase?
Very high expression, 20% of total cell protein.
What is the main reason for the low rate of nitrogenase?
Lack of specificity.
What alternative substrates can nitrogenase react?
CO
Ethyl Alkynes
CN-
H+
Describe the macrostructure of nitrogenase.
Two nitrogenase reductase homodimers attached to a single dinitrogenase heterotetramer.
What are the features of dinitrogenase?
a2b2 heterotetramer
2 Fe-Mo clusters
2 8Fe-7S P clusters
AKA MoFe protein
What are the features of dinitrogenase reductase?
Homodimeric ATPases
Single 4Fe-4S cys ligated metallocentre
AKA Fe Protein
What is the simplified equation for nitrogen fixation?
2(N2) + 3(H2) → 2(NH3)
How many electrons are required by the nitrogenase reaction and where do they originate?
8 electrons gained from ferredoxin or flavodoxin molecules that have been reduced by photosynthesis or pyruvate oxidation.
How many ATP → ADP hydrolysings are required for a nitrogen fixation?
16
What is the fate of the electrons used in the reaction?
Six are included in the primary product, two others are used to produce H2 and a by-product through combination with 2H+.
What is the overall reaction for nitrogenase action?
N2 + 8e- + 8H+ + 16ATP → 2(NH3) + H2 + 16(ADP) +16(Pi)
What is the role of the Fe proteins (dinitrogenase reductase)?
Funneling electrons and ATP to the nitrogenase (MoFe protein) by sequential reduction/ATP binding.
What is the Fe protein cycle?
Fe protein binds 2 ATP and reduced by 1 electron. This binds to one side of the nitrogenase where the electron is passed to the main enzyme and the ATP are hydrolysed.
How many Fe Protein cycles are needed for a single nitrogen fixation reaction?
Eight in total, four from each Fe protein attached to the nitrogenase.
What causes nitrogenase degradation?
Oxygen
What is the half life of nitrogenase in aerobic conditions?
30s
How do azotobacter protect against aerobic conditions?
A slime shield preventing O2 entry.
A molecular chaperone-like protein called Shethna that binds nitrogenase.
Very fast oxidative metabolism, with an ETC that can be uncoupled in oxidative stress.
How do Cyanobacteria protect against aerobic conditions?
Hyperactive photosynthesis for fast O2 removal.
Anabaena cyanobacteria produce anaerobic heterocysts in their filamentous colonies.
How are heterocysts kept anaerobic?
3 cell walls
PSII repression
Fast oxidative metabolism
How do Anabaena heterocysts interact with the other cyanobacteria?
Form symbiotically in the filamentous colony, provide nitrogen to surrounding cells and receive carbon products from neighbours as they cannot fix CO2.
How do Rhizobium protect against aerobic conditions?
Live in leguminous root nodules with impermeable cell walls and a layer of cells in between them and the plant called the nodule cortex which has high-affinity Cyt c oxidase for fast O2 reduction as well as high [leghaemoglobin] for O2 binding and N2 supply.
