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Biochem Final > Urea Cycle > Flashcards

Urea Cycle Flashcards

1
Q

Where does the urea cycle occur (organ)?

A

liver

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2
Q

Where does the ammonium ions come from?

A
  • catabolism of amino acid

- gut bacteria makes a lot as well

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3
Q

What amino acids can undergo deamination without aminotransferases?

A

Ser, Cys, His, and Thr

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4
Q

What is the rate-limiting step of the urea cycle?

A

formation of carbamoyl phosphate catalyzed by carbamoyl phosphate synthetase (step 1)

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5
Q

What are the substrates for carbamoyl phosphate synthetase?

A

HCO3- + 2ATP + NH4+

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6
Q

Where is carbamoyl phosphate synthetase located in the cell?

A

mitochondria

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7
Q

What is the allosteric acitvator of carbamoyl phosphate synthetase?

A

N-acetylglutamate (synthesized from N-acetylglutamate synthetase, which is activated by arginine, from a later step)

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8
Q

What is step 2 of the urea cycle?

A

Ornithine + Carbamoyl Phosphate»Citrulline

Enzyme: Ornithine Transcarbamoylase

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9
Q

Where is citrulline located?

A

Once synthesized, it is transported out of the mitochondria in exchange for ornithine

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10
Q

What is step 3 of the urea cycle?

A

Citrulline + Aspartate + ATP»Argininosuccinate

Enzyme: Argininosuccinate Synthetase

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11
Q

Where is step 3 occuring?

A

cytoplasm (remember that citrulline was exported out)

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12
Q

What is step 4 of the urea cycle?

A

Argininosuccinate»Arginine + Fumarate

Enzyme: Argininosuccinate Lyase

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13
Q

What is the final step of the urea cycle?

A

Arginine + H2O»Ornithine + Urea

Enzyme: Arginase

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14
Q

Where do the ammonium ions come from in the molecule of urea?

A

2 ammonium ions: 1 free NH4+ and 1 from aspartate

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15
Q

High levels of arginine are indicative of what?

A

elevated peripheral blood ammonia levels, thus urea cycle is active

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16
Q

When enterocytes take up glutamine from the diet, what do they do with it? And then what happens?

A

spits out ornithine and citrulline
- ornithine goes to the liver
- citrulline is taken up by the kidneys and goes through the same process as the liver to make arginine, then excretes it to circulation
This is the intestinal-renal axis

17
Q

What organ can complete the entire urea cycle?

A

only the liver

18
Q

Where is arginine made?

A

kidneys via the intestinal-renal axis

19
Q

Why is arginine considered an essential amino acid if we can make it?

A
  • the intestinal-renal axis is not well developed until a certain age therefore infants cannot synthesize arginine
  • also as we age, it becomes essential because certain conditions impact the function of small intestines or kidneys, leading to pressure ulcers
20
Q

Defects in which enzymes cause severe hyperammonemia and are often lethal?

A

Carbamoyl phosphate synthetase I and Ornithine transcarbamoylase

21
Q

Why are defects in other urea cycle enzymes less harmful? Which enzymes are these?

A
  • the intermediates are less toxic to the body and can be excreted by urine
  • argininosuccinate synthetase, argininosuccinate lyase, and arginase
22
Q

What condition is caused by defect in arginosuccinate synthetase?

A

Citrullinemia

- citrulline gets excreted, but that’s only one nitrogen, so supplement

23
Q

What condition is caused by defect in argininosuccinate lyase?

A

Argininosuccinic Aciduria

24
Q

What condition is caused by defect in arginase?

A

Hyperargininemia

25
Q

What are some treatment option for conditions that deal with early cycle enzymes?

A

protein restriction (difficult for growth)
hemodialysis
treatment with phenylbutyrate and benzoate

26
Q

How does the treatment with phenylbutyrate and benzoate work?

A

phenylbutyrate and benzoate both form water-soluble conjugates with glutamate and glycine, respectively, that the kidneys can eliminate easily

27
Q

What is the treatment for argininosuccinic aciduria?

A
  • defective enzyme is argininosuccinate lyase

- arginine supplement provides the body a way to eliminate urea. the substrate arginosuccinate gets excreted in urine

28
Q

What is the treatment for citrullinemia?

A
  • defective enzyme is argininosuccinate synthetase
  • arginine supplement supports citrulline synthesis, which will get eliminated, but it only carrier one ammonium ion so either phenylbutyrate or benzoate should also be prescribed
29
Q

What other conditions besides urea cycle defects can lead to toxic levels of ammonia accumulation?

A

liver disease because urea cycle occurs there

30
Q
  • early sign is swelling of brain due to glutamate taken up by astrocytes and converted to glutamine.
  • lethargy, stupor, vomiting, and convulsions
A

hyperammonemia

ammonia is a potent neurotoxin

Biochem Final (21 decks)

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