Final answer:
Ornithine and citrulline are vital non-protein amino acids involved in the urea cycle for detoxifying ammonia by converting it to urea. Ornithine is formed when arginase cleaves arginine and is later used to synthesize citrulline, which passes through the mitochondrial membrane into the cytosol to continue the cycle. Dysfunctions in this pathway can lead to conditions such as citrullinemia.
Step-by-step explanation:
Ornithine and citrulline are non-protein amino acids that play important roles in the urea cycle, a process that removes ammonia from the body by converting it into urea. Ornithine is regenerated from arginine during the cleavage of arginine into ornithine and urea, a reaction catalyzed by the enzyme arginase. Citrulline is then synthesized from ornithine and carbamoyl phosphate in the mitochondria, a reaction aided by the enzyme ornithine transcarbamylase. This synthesis of citrulline is crucial as citrulline is permeable to the mitochondrial membrane, allowing it to diffuse into the cytosol and continue the urea cycle.
The urea cycle includes several reactions, with the first step catalyzed by ornithine transcarbamylase where the -COO group of ornithine replaces phosphate from carbamoyl phosphate to form citrulline. Citrulline then moves to the cytoplasm where further reactions occur, such as the synthesis of arginosuccinate by combining citrulline with aspartate, a reaction requiring ATP and catalyzed by arginosuccinate synthase. Eventually, the cycle leads to the formation of urea, which is released into the blood, while ornithine is transported back to the mitochondria to initiate the next cycle. An inability to process citrulline properly can lead to a condition known as citrullinemia, where excess citrulline is excreted in the urine.