PROTEIN METABOLISM Prof.Dr.Fügen Aktan

Urea Cycle Living organisms excrete the excess nitrogen in one of 3 ways. excrete ammonia (ammonotelic) excrete uric acid (uricotelic) excrete urea (ureotelic)

Urea is synthesised in the liver via the urea cycle. This cycle begins with ornithine. urea cycle takes place partly in the mitochondria and partially in the cytosol along with ornithine and citrulline being transported beyond the mitochondrial membrane arginase that is needed in the final step of urea creation is only in the liver and missing in all the other tissues.

Carbamoyl phosphate synthase I initiates Urea biosynthesis Carbamoyl phosphate plus Ornithine forms Citrulline citrulline take place in the cytosol. Citrulline plus Aspartate forms Argininosuccinate Cleavage of Argininosuccinate forms Arginine and Fumarate Cleavage of Arginine releases Urea and re-forms Ornithine

Hepatic amino acid metabolism is finely regulated. The liver is largely responsible for maintaining circulating amino acid homeostasis. The liver also plays a critical role in the biosynthesis of key molecules from amino acids, e.g. creatine and glutathione. It also uses amino acids and glutathione in the conjugation of xenobiotics and toxic molecules to ensure their elimination from the body. In addition, amino acids play a crucial regulatory role in controlling the turnover of hepatic proteins.

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