Final answer:
The rate-limiting enzyme for the urea cycle is Carbamoyl Phosphate Synthetase I (CPS-1), which catalyzes the first step in the formation of carbamoyl phosphate, a critical step that determines the pace of the urea cycle.
Step-by-step explanation:
The rate-limiting enzyme for the urea cycle is Carbamoyl Phosphate Synthetase I (CPS-1). This enzyme catalyzes the first step in the urea cycle, which is the formation of carbamoyl phosphate from ammonium ion and bicarbonate. This reaction takes place in the liver mitochondria, requires energy in the form of ATP and a molecule of N-acetyl glutamate as a cofactor for enzyme activation. The importance of CPS-1 is reflected in the fact that its activity sets the pace for the entire cycle, and hence, it is considered the rate-limiting step.
Although other enzymes like ornithine transcarbamylase (OTC) and argininosuccinate synthetase play significant roles in the urea cycle, they are not rate-limiting. Deficiencies in these enzymes can cause health issues, such as the accumulation of toxic levels of ammonia in the body, but it is the activity of CPS-1 that ultimately controls the rate of the urea cycle.