Final answer:
The rate limiting enzyme for the urea cycle is carbamoyl phosphate synthase I (CPS1), which is essential for converting ammonia into carbamoyl phosphate, the first step in the urea cycle that takes place in the mitochondria.
Step-by-step explanation:
The rate limiting enzyme for the urea cycle is carbamoyl phosphate synthase I (CPS1). This enzyme initiates the cycle by catalyzing the condensation of ammonia with bicarbonate to form carbamoyl phosphate, which requires the expenditure of two ATP molecules. Since it is the first step in the cycle and has the highest energy requirement, CPS1 sets the pace for the entire cycle. It is found in the mitochondria, where the first two steps of the urea cycle occur, while the remaining three steps occur in the cytosol. As blood urea nitrogen (BUN) levels are an indicator of kidney function, proper activity of the urea cycle is critical for detoxifying ammonia and maintaining metabolic balance.
A helpful mnemonic to remember the rate limiting enzyme is "Can't Proceed Swiftly" where 'Can't' stands for Carbamoyl phosphate, 'Proceed' for Phosphate, and 'Swiftly' for Synthase I, indicating that without CPS1, the urea cycle cannot proceed efficiently.