Lecture 33 Flashcards
What amino acid(s) do transaminases make when moving NH4+?
Glutamate from α-ketogluterate
What happens to ammonia after transamination?
In the liver, it becomes urea.
When not in the liver, it becomes Gln, then is transported to the liver mitochondria where it releases ammonia to form Glu in a reaction catalyzed by glutaminase.
Complete the reaction of Gln from other tissues in the liver mitochondria.
What is the enzyme? The products? Other parts of reaction?
Gln –> Glu + ammonia
Enzyme: glutaminase
Addition of water
Complete the reaction of Glu in the liver.
Glu –> α-KG + ammonia
NADP+ is reduced.
No enzyme listed.
Complete the reaction of ammonia in extrahepatic tissues.
Glu + ammonia –> Gln
Enzyme: glutamine synthetase
ATP hydrolyzed to ADP + inorganic phosphate
What, in the context of amino acid catabolism, makes Glu unique?
It’s the only one that undergoes rapid oxidative deamination. (removal of ammonia to form α-KG)
What is Ala’s function in amino acid catabolism?
It carries ammonia from muscle to liver–glucose-alanine cycle
What are the two goals of the glucose-alanine cycle?
- amino groups are transferred to the liver for urea production.
- burden of gluconeogenesis is off-loaded onto the liver.
What happens in the liver to amino groups not needed for biosynthesis?
They are converted into urea in the urea cycle.
Where do the two N’s in urea come from?
One comes from reactions of Glu and Gln removing the ammonia to form α-KG and Glu.
The other comes from Glu in mitochondria, but via Asp.
How does each N enter the urea cycle?
One enters via OA –> Asp when Glu is converted to α-KG.
The other enters via carbamoyl phosphate.
What analogy can be made between carbamoyl phosphate in the urea cycle to another compound?
Carbamoyl phosphate : acetyl-CoA
urea cycle: TCA cycle
What are the intermediates of the urea cycle?
O! Caroline An Abominable Organizer!
Ornithine
citrulline
Asp
argininosuccinate
ornitine (again!)
We also get out Arg and fumarate and (of course) urea.
How are some of the other urea cycle intermediates like amino acids?
Ornithine is like Lys, just one CH2 shorter in the side chain.
The other AAs are essential for N metabolism.
Where do the reactions of the urea cycle happen?
Step 1 (ornithine –> citrulline) in the mitochondria
Citrulline then is transported to the cytosol where all of the other ones take place.
Then, ornithine is transported back into the mitochondria (naturally, to finish the cycle)
This means urea is produced in the cytosol. Finally, notice Asp is needed for reaction 2…it is made in the mitochondria (from OA in the TCA cycle) and then transported to the cytosol.
What is the cost of urea production?
4 ATP
2 to synthesize carbamoyl phosphate.
2 to synthesize argininosuccinate.
What is the NET cost of urea production?
1.5 ATP.
This is because NADH is produced when fumarate drops out, becomes malate and then OA (this reaction reduces NAD+) which this then used to make the Asp in the cycle.
What are some regulators of the urea cycle?
long-term high protein diets and starvation cause increased synthesis of urea cycle enzymes.
in the short term, N-acetylglutamate is like F-2,6-BP in glycolysis as a key negative regulator.
How does N-acetylglutamate regulate the urea cycle?
It is an allosteric activator of carbamoyl phosphate synthetase I.
Arginine UP regulates the production of N-acetylglutamate from acetyl-CoA + glutamate.
What molecules control entry of ammonia into the urea cycle?
acetyl Co-A
glutamate
arginine
Which species are ammonotelic and what does that mean?
Aquatic species like bony fish. This means they excrete amino nitrogen as ammonia which is diluted in the water.
Which species are ureotelic and what does that mean?
Most terrestrial animals; they excrete amino nitrogen in the form of urea after they go through the urea cycle in the liver…urea is then excreted into the urine
Which species are uricotelic and what does that mean?
birds and reptiles; excrete nitrogen as uric acid
What do plants do with amino groups?
recycle them…very rarely excrete nitrogen
Where do liver mitochondria get ammonia for the urea cycle?
From ammonia in the mitochondrial matrix and from the portal vein in the intestine from bacterial oxidation of amino acids
What are the functions of carbamoyl phosphate synthetase?
CPS I catalyzes the ATP-dependent reaction forming carbamoyl phosphate in the matrix for the urea cycle.
CPS II has a separate function in pyrimidine biosynthesis.
Why phosphorylate carbamoyl?
To activate it for donation of carbamoyl in he urea cycle.
Ornithine in the urea cycle is like _____ in the citric acid cycle because _____
oxaloacetate, because it accepts material at each turn in the cycle
What are the enzymes in the urea cycle?
- ornithine transcarbamoylase
- argininosuccinate synthetase using ATP and a citrullyl-AMP intermediate
- argininosuccinase
- arginase
Are any steps in the urea cycle reversible?
Yes; the conversion of argininosuccinate to free arginine and fumarate
Fumarate comes back to the TCA cycle in the mitochondria.
Which intermediates in the urea cycle enter the general pool of metabolites?
Remember that in the urea cycle channeling of enzymes happens and only urea is released into the general pool of metabolites.
How can the urea and TCA cycles communicate?
They communicate via the transport of key intermediates between the mitochondrion and cytosol.
TCA cycle enzymes like fumarase and malate dehydrogenase are present as isozymes in the cytosol.
What’s the fate of fumrate from the urea cycle?
It is either…
- metabolized in the cytosol
- turned into malate, then transported to the mitochondria for the TCA cycle
What is the fate of aspartate formed in transamination?
It is transported to the cytosol, becomes a N-donor in the urea cycle’s argininosuccinate synthetase reaction.
What links the urea and TCA cycles?
The argininosuccinate shunt
What does N-acetylglutamate synthase do in non-human animals?
It catalyzes the first steps of de novo Arg synthesis from Glu.
What determines levels of N-acetylglutamate?
The concentration of Glu and acetyl-CoA (substrates for synthesis) and Arg (activates NAG synthase)
Why are 2 ATP equivalents needed to make argininosuccinate?
Because ATP here becomes PPi.
What are the effects of urea-forming defects?
lots of free ammonia is exported to the blood
possible to have a defect only with one enzyme (observed in build-up of products at one step)
Why can’t protein-free diets treat urea cycle defects?
Humans can’t make half of the 20 common amino acids and need the essential AAs.
What is a viable treatment for urea cycle defects?
Benzoate or phyenylbutyrate treatment
What happens to benzoate in the body?
It uses up ammonia in generating glycine.
First benzoate –> benzoyl CoA and glycine –> hippurate
Then the regeneration of glycine
What happens to phenylbutyrate in the body?
Ammonia is used up making Glu.
phenylbutyrate –Beta-oxid.–> phenyl acetate –> phenylacetyl-CoA + Gln –> phenylacetylglutamine
What are enzyme-specific therapies for urea cycle defects?
N-AGS df. means need new activator for CPS I, can use carbamoyl glutamate, N-AG analog
ornithine transcarbamoylase, AS S, A succinase, Arg supplementation
Arg df.– exclude Arg
