Urea Cycle and Nitrogen Excretion

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11 vocabulary flashcards covering key enzymes, intermediates, and concepts of the urea cycle.

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11 Terms

1
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Arginase

Cytosolic enzyme that hydrolyzes arginine to urea and regenerates ornithine, completing the cycle.

2
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Ammonia (NH3)

N waste product of most aquatic animals. Requires little energy and lots of water to produce

3
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Urea (C=C-(NH2)2)

N waste product of mammals, most amphibians, sharks and some fish. Requires intermediate amounts of energy and water to produce

4
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Uric acid (double ring N structure)

N waste product of many reptiles, birds, insects, and land snails. Requires lots of energy and little water to produce

5
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Urea cycle

  1. Ammonia + Co2 + ATP → carbamoyl phosphate (mitochondria)

  2. Ornithine + carbamoyl phosphate → citrulline (mitochondria)

  3. Citrulline exported to cytosol and combines with Asp using ATP → argininosuccinate

  4. Argininosuccinate splits into fumarate and arginine; fumarate → malate → OAA for gluconeogenesis

  5. Arginine splits into urea + ornithine. Urea eliminated and ornithine back to mitochondria

6
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Urea cycle 1: COLORED

Transamination reaction with AA and a-KG forms a-keto acid and glutamate. Glutamate loses NH3, and reforms a-KG, generating NAD(P)H. NH3 from glutamate combines with 2 ATP + HCO3- → (via carbamoyl phosphate synthetase) carbamoyl phosphate

7
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Urea cycle step 2: CATS

Carbamoyl phosphate + 2 ADP + Pi + Ornithine → (via ornithine transcarbamoylase) citrulline + Pi. Citrulline exported from the liver mitochondria to the cytosol.

8
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Urea cycle step 3: ALWAYS

Citrulline + ATP + Aspartate → (via argininosuccinate synthetase) Arginino-succinate + AMP + PPi

9
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Argininosuccinate synthetase

Citrulline + ATP → Citrullyl-AMP + PPi (leaves) + Aspartate → Argininosuccinate + AMP (leaves)

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Urea cycle step 4 + 5: ASK FOR

Arininosuccinate → (via argininosuccinase) Fumarate + Arginine. Fumarate leaves + forms malate using water and fumarase. Malate + NAD+ → (via malate DH) OAA + NADH (leaves) → gluconeogensis

11
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Urea cycle step 6: AWESOME UMBRELLAS, ORANGE

Arginine + H2O → (via arginase) Urea (to kidney for excretion) + Ornithine; ornithine transported back into liver mitochondria to continue cycle