NITROGEN METABOLISM Flashcards
How is ammonia generated?
from the deamination of proteins
Where does the formation of glutamate from alpha-ketoglutarate occur?
Mitochondrial cells within the brain
What is the major site of the urea cycle?
Liver
What is urea composed of?
2 amino groups (one form ammonium & 1 from asparate) and a molecule of CO2
This can then be diffused into blood, extracted by the kidneys and secreted in urine
What is the first step, prior to the urea cycle?
The synthesis of a carbamoyl group in the form of carbomoyl phosphate.
The formation of this molecule requires 2 ATP + CO2 + NH4+ + water
Catalysed by carbomyl phosphate synthase
First step of Urea cycle
Condensation of Ornithin and Carbomoyl phosphate to form Citruline.
Condensation reaction
2 small molecules combine to create 1 large molecule with water as a by-product
Second step of urea cycle
Condensation of Citruline with aspartate to form Argininosuccinate (1 molecuel of ATP needed)
Third step of urea cycle
Argininosuccinate is cleaved into the amino acid Arginine and fumarate
Final step in urea cycle
reformation of Ornithine and the liberation of urea from the hydrolysis of Arginine
Where does the urea cycle occur?
Most of the reaction occurs in the cytoplasm some of the reaction occurs in mitochondria
Where does the formation of ammonia from glutamate occur?
Mitochondria
Where does the formation of carbomyl phosphate occur?
Mitochondria
Where does the condensation reaction of ornithine and carbomyl to citrulline occur?
mitochondria
Citrulline-orinithine exchanger
Membrane bound protein responsible for the coordinated transport of ornithine into the mitochondrial matrix and citrulline out to the cytoplasm
Is fumarate synthesised in the citric acid cycle or the urea cycle
The urea cycle but it is used in the citric acid cycle
hydrated to malate and oxidised to oxaloacetate
What are the possible fates of oxaloacetate?
- transamination to aspartate (which in turn may donate an amino group to the urea cycle),
- conversion to glucose via gluconeogenesis,
- condensation with acetyl-CoA to form citrate in the citric acid cycle
- conversion to pyruvate (which may be converted to acetyl-CoA for ketogenesis etc).
What is the stoichiometric relationship between ammonia and aspartate?
The urea cycle must have the donation of 1 amino group from ammonia and 1 from aspartate to function properly. Therefore, there is a stoichiometric relationship between ammonia and aspartate, which must be maintained.
why is the glutamate common intermediate important in maintaining the stoichiometric relationship between aspartate and ammonia?
, the deamination of an amino acid to form glutamate is a common step in the catabolism of many amino acids. Glutamate can then be deaminated to form ammonium catalysed by glutamate dehydrogenase. Alternatively, glutamate may also undergo transamination with oxaloacetate to form aspartate, catalysed by aspartate transaminase. Hence an excess or deficiency of one of the amino group donator may be compensated for by the formation of the deficient component from the other, through the glutamate intermediate.
How much ATP is used in the urea cycle?
4
How do ruminants minimise energetic cost of urea cycle?
by secreting most of the urea synthesised back into the rumen where it can be utilised by rumen microflora for amino acid synthesis. Similarly, desert dwelling ruminants (ie. the camel) used this strategy to avoid the water loss associated with urea excretion.
