Transcript Lecture Slides for Nitrogen Metabolism

Nitrogen Metabolism
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Sources of Nitrogen
• Atmospheric N2
• NO3- from lightning and rainfall
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Haber Process (same Haber as in chemical
warfare)
High temperature, high pressure, catalyst
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Nitrogen-fixing bacteria convert N2 to ammonia
• Rhizobium in plant root nodules
• Azotobacter
• Klebsiella
• Some cyanobacteria
• Enzymes require anaerobic conditions.
• Root nodules contain leghemoglobin (i.e.
legume hemoglobin).
• Ties up O2 and passes to respiration.
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Nitrogenase
N2 + 8H+ + 8 e- + 16ATP ⇄ 2NH3 + H2 + 16 ADP
(Requires a lot of energy.)
• Dinitrogenase component (MoFe) reduces N2
• Dinitrogenase reductase component (Fe) rereduces
dinitrogenase
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So how do we use the ammonia?
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:Production of Glutamate
• Glutamate dehydrogenase
• Reductive amination
• Usually running backwards in critters due to low [NH3]
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Glutamine synthetase
Glutamic acid, glutamine, and a-ketoglutarate are primary players in
moving nitrogens.
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Regulation
Glutamine is involved in
the metabolism of:
• Aromatic a.a.s
• Several other a.a.s
• Purines
• Pyrimidines
• Amino sugars
Regulated by feedback
inhibition
Also regulated globally by
signal cascades
• Adenylation of tyrosine
blocks active site
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Asparagine synthetase
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Carbamoyl phosphate synthetase
Carbamoyl phosphate can be made from both ammonia and glutamate
Mitochondrial
Cytosolic
(Bacterial enzyme accepts both substrates)
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Amino acid metabolism
• Different organisms can make different sets of amino acids.
• Those we can’t manufacture are essential amino acids in our diet.
• Lists of essential and non-essential vary.
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Amino acid synthesis: OAA derivatives
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Amino acid synthesis: Pyruvate derivatives
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Amino acid synthesis: Ribose derivatives
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Amino acid synthesis: Erythrose derivatives
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Amino acid synthesis: a-ketoglutarate derivatives
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Amino acid synthesis: 3-phosphoglycerate derivatives
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Amino acid synthesis via transamination
• You’ve seen this before.
 a-Amino groups of amino acids come by transamination
from Glu
• You can synthesize all amino acids from their a-keto
conjugates.
• For the essential amino acids, we can’t produce the aketo acids.
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Also play a role in amino acid degradation
• Above example is part of the degradation path for
alanine.
• Transamination (transfer of the amide NH2 to aketoglutarate) yields pyruvate.
• Pyruvate can be tossed into PDH and the Krebs cycle.
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Amino acid oxidases
• The a-amino group is usually removed by transfer to aketoglutarate to yield glutamate and the a-keto acid.
• Glutamate is then deaminated by glutamate
dehydrogenase.
• There are also amino acid oxidases, using a flavin
electron acceptor.
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Fates of deaminated amino acids (i.e. fate of the
carbons)
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Removal of nitrogenous waste
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Insects, birds, and reptiles primarily excrete uric acid.
Desert mammals like kangaroo rats primarily excrete uric acid.
Oddly, so do dalmatians.
Uric acid is pretty insoluble.
In humans, it is the end product of purine catabolism.
Overproduction or underexcretion can lead to the formation of uric
acid crystals – frequently starting in the gravitationally lowest joints
– i.e. the big toe.
• That condition is gout, which is often extremely painful.
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Gout – a surgical example in the left hand
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Removal of nitrogenous waste – the urea cycle
The universal product of degrading amino acids is ammonia.
• Ammonia is toxic at not very high concentrations.
• You have to get rid of it.
• If you are an aquatic organism, you can just diffuse in into the
environment.
• We convert ammonia to urea.
The basic idea:
• Add the nitrogens to ornithine to (eventually) make arginine.
• Cleave the guanidino group off arginine to leave you with ornithine
and urea.
• Urinate out the urea.
• Throw the ornithine back into the cycle.
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Ammonia transport to the liver
• Urea formation takes place in the liver
• Waste nitrogen is transported through the blood
as Gln or Ala.
• Reconverted to ammonia in the liver
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