4 Transamination In transamination: Amino acids are degraded in the liver. An amino group is transferred from an amino acid to an -keto acid, usually -ketoglutarate. The reaction is catalyzed by a transaminase or aminotransferase. A new amino acid, usually glutamate, and a new -keto acid are formed.
6 Oxidative Deamination Oxidative deamination: Removes the amino group as an ammonium ion from glutamate. Provides -ketoglutarate for transamination. NH 3 + Glutamate | dehydrogenase - OOC—CH—CH 2 —CH 2 —COO - + NAD + + H 2 O Glutamate O || - OOC—C—CH 2 —CH 2 —COO - + NH 4 + + NADH -Ketoglutarate
7 Urea Cycle The urea cycle: Detoxifies ammonium ion from amino acid degradation. Converts ammonium ion to urea in the liver. O || H 2 N—C—NH 2 urea Provides 25-30 g urea daily for urine formation in the kidneys.
8 Carbamoyl Phosphate In the mitochondria, an ammonium ion reacts with CO 2 from the citric acid cycle, 2 ATP, and water. NH 4 + + CO 2 + 2ATP + H 2 O O O || || H 2 N—C—O—P—O - + 2ADP + P i | O - Carbamoyl phosphate
9 Reaction 1 Transfer of Carbamoyl Group The carbamoyl group is transferred to ornithine to form citrulline. Citrulline moves across the mitochondrial membrane into the cytosol.
10 Reaction 2 Condensation with Aspartate In the cytosol, citrulline combines with aspartate. Hydrolysis of ATP to AMP provides energy. The N in aspartate is part of urea. Cytosol
11 Reaction 3 Cleavage of Fumarate Fumarate: Is cleaved from argininosuccinate. Enters the citric acid cycle.
12 Reaction 4 Hydrolysis Forms Urea Hydrolysis of arginine: Forms urea. Forms ornithine, which returns to the mitochondrion to pick up another carbamoyl group to repeat the urea cycle.