Nitrogen Fixation

Question 1

Nitrogen fixation

Answer 1

Bacteria convert N2 to NH3 / NH4+

Question 2

Nitrification

Answer 2

Bacteria oxidize ammonia (NH3) into nitrite (NO2) and nitrate (NO3)

Question 3

Denitrification

Answer 3

Nitrate and Nitrite is reduced to N2 under anaerobic conditions

Question 4

Assimilation

Answer 4

Plants reduce NO3 to NO2 and then NO2 to NH4+ by nitrite and nitrate reductases

Question 5

Describe nitrate assimilation

Answer 5

• Carried out by nitrate reductase
• Involves Mo cofactor
• 2e are donated from NADH to nitrate reductase
• 2e make their way to Mo cofactor at the active site of nitrate reductase
• Mo cofactor transfers 2e to nitrate reducing it to nitrite

Question 6

Describe nitrite assimilation

Answer 6

• Carried out by nitrite reductase
• Located in the chloroplasts of plants
• 6e come from ferredoxin
• Ferredoxin transfers 6e to nitrite reductase
• Nitrite reductase transfers 6e to cofactor siroheme reducing nitrite to ammonium (NH4+)
  NADPH does the same thing as ferredoxin in humans

Question 7

What are the challenges associated with reducing nitrogen?

Answer 7

Due to the triple bond, Nitrogen has an extremely high activation energy

Question 8

How do bacteria such as diazotrophs convert N2 into ammonia?

Answer 8

By utilizing nitrogenase complex which overcomes the high activation energy by consuming lots of ATP

Question 9

Describe the nitrogenase complex

Answer 9

• Works in anerobic environments
• Two central components:
  1. Dinitrogenase reductase
  2. Dinitrogenase
• 3 cofactors:

Question 10

What are the 4 cofactors involved in the nitrogenase complex?

Answer 10

1. 4Fe-4S cluster
2. P cluster
3. FeMo cluster

Question 11

Describe the reduction of dinitrogenase

Answer 11

• Reduced by a transfer of electrons from dinitrogenase reductase
• 8e are transferred from reductase to dinitrogenase one at a time
• Reduced reductase binds to the dinitrogenase and transfers a single electron
• Oxidized reductase dissociates from dinitrogenase in a repeating cycle
• Each cycle requires the hydrolysis of ATP

Question 12

Describe how dinitrogenase converts N2 to NH4+

Answer 12

• 6e are required to fix one molecule of N2
• 2e reduce 2H+ to H2

Question 13

Describe the symbiotic relationship between leguminous plants and nitrogen-fixing bacteria

Answer 13

• leguminous plants have nitrogen-fixing bacteria in their root nodules
• Bacteria have access to large reservoirs of energy (in the form of citric acid cycle intermediates)
• Bacteria are bathed in solution of O2 binding heme protein leghemoglobin
• The plant has a ready supply of reduced N2

Question 14

What is the significance of leghemoglobin?

Answer 14

Leghemoglobin binds all available O2 so that it cannot interfere with N2 fixation
Nitrogenase cofactors must be kept away from O2