FRANCIS LEROY & SEMPOT AMAURY, BIOCOSMOS / SCIENCE PHOTO LIBRARY FRANCIS LEROY & SEMPOT AMAURY, BIOCOSMOS / SCIENCE PHOTO LIBRARY
Urea cycle, animation. The urea cycle produces urea from ammonia, a toxic by-product of many biological processes. Urea is able to be safely excreted via the kidneys as urine. In the mitochondrial matrix of liver cells a carbamoyl group is transferred to ornithine by the enzyme ornithine transcarbamoylase to form citrulline. In the cell cytosol argininosuccinate synthetase catalyses the condensation of citrulline and aspartate to form argininosuccinate. This is then cleaved by argininosuccinase to arginine and fumarate. Fumarate is fed into the citric acid cycle and arginine continues in the urea cycle. Arginine is hydrolysed to urea and ornithine by arginase and the cycle is able to start over.
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