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BIOC2005 - Metabolism > 847 - Nitrogen Fixation > Flashcards

847 - Nitrogen Fixation Flashcards

1
Q

What is diazotrophy?

A

A form of nitrogen fixation in which the bacteria converts N2 or nitrates into ammonia for incorporation into organic molecules.

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2
Q

What is the chemical formula of a nitrate?

A

NO3-

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3
Q

What is the chemical formula of a nitrite?

A

NO2-

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4
Q

What is nitrification?

A

The production of Nitrates and Nitrites from ammonia.

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5
Q

Which anaerobic bacteria convert ammonia to nitrites?

A

Nitrosomonas

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6
Q

Which anaerobic bacteria convert nitrites to nitrates?

A

Nitrobacter

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7
Q

What is denitrification?

A

Reduction of nitrates to N2, often by nitrogen respiring bacteria.

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8
Q

What bacteria are capable of nitrogen fixation?

A

Azotobacter, Rhizobium, Klebsiella, some Cyanobacteria

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9
Q

How do most diazotrophic organisms live?

A

As symbionts, providing organic nitrogen to larger organisms.

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10
Q

How much of the planetary total of nitrogen fixation is done by diazotrophy?

A

60%

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11
Q

Other than diazotrophy, how is nitrogen fixed?

A

Lightning and the Haber-Bosch process.

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12
Q

Is nitrogen exergonic or endogonic?

A

Highly exergonic, DG = -33.5kJ/mol

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13
Q

Why is atmospheric nitrogen largely inert?

A

Because N2 is very stable, so requires a high activation energy (230-400kJ/mol) to react even though it is highly exergonic.

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14
Q

What are the reaction conditions of the Haber-Bosch process?

A

500 C, 300 atm, Iron catalyst.

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15
Q

What enzyme is responsible for nitrogen fixation?

A

Nitrogenase metalloenzymes.

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16
Q

What is the turnover of nitrogenase?

A

5/s

17
Q

How does the cell compensate for the low turnover number of nitrogenase?

A

Very high expression, 20% of total cell protein.

18
Q

What is the main reason for the low rate of nitrogenase?

A

Lack of specificity.

19
Q

What alternative substrates can nitrogenase react?

A

CO
Ethyl Alkynes
CN-
H+

20
Q

Describe the macrostructure of nitrogenase.

A

Two nitrogenase reductase homodimers attached to a single dinitrogenase heterotetramer.

21
Q

What are the features of dinitrogenase?

A

a2b2 heterotetramer
2 Fe-Mo clusters
2 8Fe-7S P clusters
AKA MoFe protein

22
Q

What are the features of dinitrogenase reductase?

A

Homodimeric ATPases
Single 4Fe-4S cys ligated metallocentre
AKA Fe Protein

23
Q

What is the simplified equation for nitrogen fixation?

A

2(N2) + 3(H2) → 2(NH3)

24
Q

How many electrons are required by the nitrogenase reaction and where do they originate?

A

8 electrons gained from ferredoxin or flavodoxin molecules that have been reduced by photosynthesis or pyruvate oxidation.

25
Q

How many ATP → ADP hydrolysings are required for a nitrogen fixation?

A

16

26
Q

What is the fate of the electrons used in the reaction?

A

Six are included in the primary product, two others are used to produce H2 and a by-product through combination with 2H+.

27
Q

What is the overall reaction for nitrogenase action?

A

N2 + 8e- + 8H+ + 16ATP → 2(NH3) + H2 + 16(ADP) +16(Pi)

28
Q

What is the role of the Fe proteins (dinitrogenase reductase)?

A

Funneling electrons and ATP to the nitrogenase (MoFe protein) by sequential reduction/ATP binding.

29
Q

What is the Fe protein cycle?

A

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.

30
Q

How many Fe Protein cycles are needed for a single nitrogen fixation reaction?

A

Eight in total, four from each Fe protein attached to the nitrogenase.

31
Q

What causes nitrogenase degradation?

A

Oxygen

32
Q

What is the half life of nitrogenase in aerobic conditions?

A

30s

33
Q

How do azotobacter protect against aerobic conditions?

A

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.

34
Q

How do Cyanobacteria protect against aerobic conditions?

A

Hyperactive photosynthesis for fast O2 removal.

Anabaena cyanobacteria produce anaerobic heterocysts in their filamentous colonies.

35
Q

How are heterocysts kept anaerobic?

A

3 cell walls
PSII repression
Fast oxidative metabolism

36
Q

How do Anabaena heterocysts interact with the other cyanobacteria?

A

Form symbiotically in the filamentous colony, provide nitrogen to surrounding cells and receive carbon products from neighbours as they cannot fix CO2.

37
Q

How do Rhizobium protect against aerobic conditions?

A

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.

BIOC2005 - Metabolism (8 decks)

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