Urea Cycle
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Urea Cycle
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The Urea Cycle is the liver’s method to remove excess nitrogen. In the urea cycle, this nitrogen starts in the form of ammonia, which is created by amino acid breakdown. This ammonia combines with carbon dioxide to form carbamoyl phosphate. This reaction is catalyzed by the mitochondrial enzyme, CPS1. Carbamoyl phosphate is then combined with ornithine to form citrulline. Just like CPS I, ornithine transcarbamylase is also found in the mitochondria. Next, citrulline and aspartate are converted to argininosuccinate, and this reaction occurs via the argininosuccinate synthetase enzyme. Next, argininosuccinate is then broken down by argininosuccinase, to form arginine and fumarate. Lastly, arginase splits arginine into urea and ornithine. The urea is excreted in the urine, thereby getting rid of nitrogen, whereas the ornithine is regenerated, thus completing the cycle.