USMLE Step 1


Biochemical Pathways

Urea Cycle
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Urea Cycle

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The Urea Cycle is a biochemical pathway that occurs in the liver to eliminate excess nitrogen as urea. Nitrogen is delivered in high amounts to the liver (refer to Cahill Cycle), and begins in this pathway as ammonia.

Ammonia combines with carbon dioxide to form carbamoyl phosphate, in a reaction catalyzed by the mitochondrial enzyme, CPS1.

Carbamoyl phosphate is then combined with ornithine to produce citrulline, in a reaction catalyzed by another mitochondrial enzyme, ornithine transcarbamylase. Notably, Ornithine Transcarbamylase Deficiency can block the urea cycle, leading to hyperammonemia.

Citrulline and aspartate are converted into argininosuccinate, and this reaction occurs via the argininosuccinate synthetase enzyme. Argininosuccinate is then broken down by argininosuccinase, to form arginine and fumarate.

Arginase splits arginine into urea and ornithine. The urea is excreted in the urine, thereby eliminating nitrogen, whereas the ornithine is regenerated, completing the urea cycle.

Find Urea Cycle and more Biochemical Pathways among Pixorize's visual mnemonics for the USMLE Step 1 and NBME Shelf Exams.