Describe the urea cycle and its connection to amino acid metabolism.

The urea cycle converts toxic ammonia (from amino acid deamination) into urea for excretion. It occurs partly in mitochondria and partly in cytoplasm of hepatocytes. Ammonia combines with CO2 to form carbamoyl phosphate (mitochondrial), which combines with ornithine to form citrulline. Citrulline exits to cytoplasm, combines with aspartate to form argininosuccinate, then arginine, which is cleaved to release urea and regenerate ornithine. Net reaction: 2NH3 + CO2 + 3ATP -> urea + 2ADP + AMP + 4Pi. The cycle connects to TCA cycle via fumarate (from argininosuccinate cleavage). Cycle defects cause hyperammonemia, which is neurotoxic.