847 - Nitrogen Fixation

Question 1

What is diazotrophy?

Answer 1

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

Question 2

What is the chemical formula of a nitrate?

Answer 2

NO3-

Question 3

What is the chemical formula of a nitrite?

Answer 3

NO2-

Question 4

What is nitrification?

Answer 4

The production of Nitrates and Nitrites from ammonia.

Question 5

Which anaerobic bacteria convert ammonia to nitrites?

Answer 5

Nitrosomonas

Question 6

Which anaerobic bacteria convert nitrites to nitrates?

Answer 6

Nitrobacter

Question 7

What is denitrification?

Answer 7

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

Question 8

What bacteria are capable of nitrogen fixation?

Answer 8

Azotobacter, Rhizobium, Klebsiella, some Cyanobacteria

Question 9

How do most diazotrophic organisms live?

Answer 9

As symbionts, providing organic nitrogen to larger organisms.

Question 10

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

Answer 10

60%

Question 11

Other than diazotrophy, how is nitrogen fixed?

Answer 11

Lightning and the Haber-Bosch process.

Question 12

Is nitrogen exergonic or endogonic?

Answer 12

Highly exergonic, DG = -33.5kJ/mol

Question 13

Why is atmospheric nitrogen largely inert?

Answer 13

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

Question 14

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

Answer 14

500 C, 300 atm, Iron catalyst.

Question 15

What enzyme is responsible for nitrogen fixation?

Answer 15

Nitrogenase metalloenzymes.

Question 16

What is the turnover of nitrogenase?

Answer 16

5/s

Question 17

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

Answer 17

Very high expression, 20% of total cell protein.

Question 18

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

Answer 18

Lack of specificity.

Question 19

What alternative substrates can nitrogenase react?

Answer 19

CO
Ethyl Alkynes
CN-
H+

Question 20

Describe the macrostructure of nitrogenase.

Answer 20

Two nitrogenase reductase homodimers attached to a single dinitrogenase heterotetramer.

Question 21

What are the features of dinitrogenase?

Answer 21

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

Question 22

What are the features of dinitrogenase reductase?

Answer 22

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

Question 23

What is the simplified equation for nitrogen fixation?

Answer 23

2(N2) + 3(H2) → 2(NH3)

Question 24

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

Answer 24

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

Question 25

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

Answer 25

16

Question 26

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

Answer 26

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

Question 27

What is the overall reaction for nitrogenase action?

Answer 27

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

Question 28

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

Answer 28

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

Question 29

What is the Fe protein cycle?

Answer 29

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.

Question 30

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

Answer 30

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

Question 31

What causes nitrogenase degradation?

Answer 31

Oxygen

Question 32

What is the half life of nitrogenase in aerobic conditions?

Answer 32

30s

Question 33

How do azotobacter protect against aerobic conditions?

Answer 33

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.

Question 34

How do Cyanobacteria protect against aerobic conditions?

Answer 34

Hyperactive photosynthesis for fast O2 removal. | Anabaena cyanobacteria produce anaerobic heterocysts in their filamentous colonies.

Question 35

How are heterocysts kept anaerobic?

Answer 35

3 cell walls PSII repression Fast oxidative metabolism

Question 36

How do Anabaena heterocysts interact with the other cyanobacteria?

Answer 36

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

Question 37

How do Rhizobium protect against aerobic conditions?

Answer 37

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.