Urea Cycle and Nitrogen Excretion

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

Arginase

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

Ammonia (NH3)

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

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

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

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

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

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.

Urea cycle step 3: ALWAYS

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

Argininosuccinate synthetase

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

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

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