urea cycle Flashcards
Trapping ammonia into urea (transamination): 3 steps
- Transferring amino group from AA to alpha ketogluterate, producing alpha ketoacid and glutamate (containing the nh2 group)
- glutamate transferring nh2 to nad+ producing
- Nadh and NH4
- NADH + NH4
- NH4 enters urea cycle
Where does the urea cycle occur?
in the liver?
1st step in urea synthesis
____ + ____ —> _______
CO2 + NH4 —> carbamoyl phosphate
C=O attached to a O3PO and NH2
Where do the atoms in the carbamoyl phosphate come from?
CO2 metabolism, and NH4 from deamination
What is the enzyme responsible for transferring CO2 to NH4?
carbomyl phosphate synthetase
2nd step in urea cycle
combining carbomyl phosphate with ornithine to produce citrulline
Where does the 2nd step occur?
mito
Where does the 1st step occur?
mito
T/F citrulline can be located in both the mito and the cytoplasm?
True
3rd step in urea cycle
Citrulline is combined with aspartate to produce argininosuccinate
Argininosuccinate is the product of
aspartate and citrulline
What does aspartate contribute to the urea cycle
-NH2
What catalyzes the formation of argininosuccinate?
argininosuccinate synthase
argininosuccinate —->
Arginine and Fumarate
What enzymes catalyze the 1st, 2nd, 3rd, 4th, 5th steps in the urea cycle?
- Carbamoyl phosphate synthetase (CPS1)
- Ornithine transcarbamolyase (OTC)
- Argininosuccinate synthetase
- Argininosuccinate lyase
- Argininase (breaks arginine into ornithine and urea)
Where does transamination occur?
muscle
The nitrogen for the urea cycle comes from where?
funneled from transamination reactions
What is the significance of aspartate and fumarate in the urea cycle? What happens to the two?
Fumarate —> malate–> oxaloacetate –> aspartate and glucose (if it wishes)
What are the two “funneling” events of ammonias?
- coming from glutamate in transamination reactions
- coming from aspartate from TCA cycle
Transamination rxns = glutamate or aspartate
Fumerate –>
aspartate
transamination rxns =
amino acids funnel amino groups by transamination reactions generating aspartate or glutamate
glutamate releases free ammonia
aspartate releases it directly into urea cycle
“Funneling” by transaminations come from
muscle: exercise causes high protein degradation producing alanine
alanine goes to the liver
which amino acid is the most produced by muscle and brain, and transferred to the liver?
alanine
