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MB 414 > Nitrogen Metabolism > Flashcards

Nitrogen Metabolism Flashcards

1
Q

Nitrogen in the atmosphere

A
  • most abundant element in air (80%)

- occurs as dinitrogen

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2
Q

Nitrogen fixers

A
  • convert N2 to ammonia
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3
Q

Nitrogen fixation

A
  • conversion of N2 into NH3-
  • the “fixed” nitrogen is immediately incorporated into organic compounds
  • only prokaryotes fix nitrogen
  • NO nitrogen fixing plants or animals.
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4
Q

Ammonification

A
  • release of ammonia into the environment from N-containing organic compounds
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5
Q

Nitrification

A
  • oxidation of ammonia by aerobic bacteria
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6
Q

Denitrification

A
  • reduction of nitrate to N2 by anaerobic respiration
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7
Q

biological Nitrogen fixation reaction

A
  • N2 + 8H+ + 8e- + 16-32 ATP -> 2 NH3+ H2 + 16-32 (ADP + Pi)
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8
Q

Non biological nitrogen fixation

A
  • N2 + 3H2 -> 2 NH3
  • Haber-Bosch reaction
  • accounts for 1/5 of nitrogen fixed per year
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9
Q

Why is nitrogen fixation so expensive?

A
  • N2 is joined by a very stable triple bond that is difficult to break.
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10
Q

Who fixes nitrogen?

A
  • Free living heterotrophs - Azotobacter and Pseudomonas
  • Phototrophs - Anabaena, Nostoc, Rhodospirrilum, Rhodobacter
  • Plant symbiotic bacteria
    • Rhizobium - leguminous plant
    • Frankia - angiosperms
    • Azospirillum - grass, maize
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11
Q

Nitrogenase

A
  • carries out nitrogen fixation
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12
Q

nitrogen is reduced at

A
  • molybdenum-iron cofactor

- reducing power stored in P-cluster - stores electrons

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13
Q

Nitrogenase reguluation

A
  • you must make 26 proteins to support N2 fixation
    • protein synthesis is expensive
  • it takes 16-32 ATP + NADH per N2 fixed
  • regulated at transcriptional level and post-translational level
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14
Q

Transcriptional Regulation

A
  • NtrB and NtrC two component regulatory system
  • NtrB is the sensor - senses N status of the cell
  • NtrC is the response regulate - activates transcription of NifA, NifL
  • NifA - positive regulator
  • NifL - negative regulator
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15
Q

nitrogen status measured by

A
  • measured to the ratio of alpha-ketoglutarate/glutamate via PII
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16
Q

Transcriptional regulation process

A
  • nitrogenase extremely sensitive to O2. Only turn on if O2 is low
  • in presence of high levels of O2, NifL will bind to NifA and not allow it to turn on transcription of nitrogenase operon
  • If O2 levels are low, NifL will not bind NifA, and it can activate transcription
17
Q

Post translational regulation

A
  • the cell shuts off nitrogenase whenever ammonia feels increase by modifying an arginine residue in the active site of the iron protein.
  • done by adding an ADP ribose via DRAT.
  • when ammonia levels drop, the ADP ribose removed by DRAG.
18
Q

Nitrogenase and O2

A
  • nitrogenase is rapidly inactivated by exposure to O2
  • The FeMoCo center binds to O2 tightly, and the entire enzyme must be degraded and made again
  • Many N2 fixers have developed mechanisms to protect nitrogenase from O2
19
Q

Azotobacter vinelandii

A
  • obligate aerobe, but can fix N2
  • to protect nitrogenase from inactivation use
    • respiratory protection
    • conformational protection
20
Q

Respiratory protection

A
  • Azotobacter keeps its cytoplasm at very low O2 concentration through its ETC
  • At high O2 levels, it will turn on an alternative terminal oxidase, cytochrome bd, that has a high affinity for O2 and a very rapid rate of O2 reduction
  • but it’s not a proton pump, so less energy available
21
Q

Conformational protection

A
  • if O2 is too high, FeSII will bind to nitrogenase and protect the FeMO cluster from O2 inactivation
  • When FeSII is bound to the protein, N2 also does not have access and nitrogen fixation stops
  • When O2 drops, FeSII dissociates and nitrogen fixation proceeds.
22
Q

Cyanobacteria

A
  • cannot fix nitrogen and photosynthesize at the same time because O2 will damage nitrogenase
  • when these bacteria need to fix nitrogen, they do so in a heterocyst and share nitrogen with neighbors who share their fixed CO2
  • no photosynthesis occurs in a heterocyst so nitrogenase is not inhibited.
23
Q

symbiotic bacteria

A
  • nitrogen fixing bacteria live in symbiosis with plants

- environment inside the nodule kept at low O2 by leghemoglobin

24
Q

lock and key mechanism

A
  • only bacteria that secrete the correct polysaccharide “key” are able to colonize each host.
  • plant secretes flavonoids that stimulate bacteria to start producing Nod factors
  • bacteria make polysaccharide which induces root hair curling and allows the bacteria to invade into the cortex cells.
  • will invade cell and divide rapidly producing a nodule on the root.
25
Q

Ammoniafication

A
  • mineralization
  • once N2 is fixed to ammonia, it is almost immediately incorporated into amino acids
  • when an organism is growing on high nitrogen:carbon ratio diet
  • will excrete ammonia
26
Q

nitrification

A
  • aerobic process

- bacteria of the genus nitrosomas and nitrobacter oxidize ammonia to nitrite and nitrate

27
Q

Nitrosomas

A
  • gets energy from oxidation of ammonia to nitrite
  • does in two steps
  • first step produces hydroxylamine
  • then oxidizes to nitrite
28
Q

nitrobacter

A
  • uses nitrite as energy source to produce nitrate
29
Q

Denitfification

A
  • nitrate (NO3) to Dinitrogen (N2)
  • anaerobic process
  • nitrate - nitrite - nitric oxide - nitrous oxide - dinitrogen

MB 414 (26 decks)

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