Question

In the formation of urea from ammonia, all of the following are correct except A. aspartate supplies one of the nitrogens found in urea. B. this is an energy-expensive process, utilizing several ATPs. C. the rate of the cycle fluctuates with the diet. D. fumarate is produced. E. ornithine transcarbamoylase catalyzes the rate-limiting step.

Short answer

A. Aspartate supplies one of the nitrogens found in urea. B. The urea cycle is energy-expensive as it utilizes several ATPs. C. The rate of the cycle depends on the diet. D. Fumarate is a byproduct of the urea cycle. E. Ornithine transcarbamoylase catalyzes the rate-limiting step. Answer: E. Ornithine transcarbamoylase catalyzes the rate-limiting step (Incorrect).

Step-by-step solution

Understand the urea cycle

The urea cycle is a series of biochemical reactions that occur in the liver and serves to detoxify ammonia by converting it into urea. The urea cycle involves various intermediate products and enzymes, including aspartate, ATP, fumarate, and ornithine transcarbamoylase.

Analyze each statement

We will now analyze each statement to identify the incorrect one. A. Aspartate does indeed supply one of the nitrogens found in urea, as it reacts with citrulline to form argininosuccinate. B. The urea cycle is an energy-expensive process, as it utilizes several ATPs (four ATPs are consumed per urea molecule produced). C. The rate of the cycle does fluctuate with the diet, as consumption of high-protein diets leads to an increased production of ammonia, which must then be detoxified via the urea cycle. D. Fumarate is produced during the conversion of argininosuccinate into arginine. This means fumarate is a byproduct of the urea cycle. E. Ornithine transcarbamoylase catalyzes the reaction between carbamoyl phosphate and ornithine to form citrulline. However, this is not the rate-limiting step of the urea cycle. The rate-limiting enzyme is actually N-acetylglutamate synthase (NAGS), which catalyzes the formation of N-acetylglutamate (NAG) used to activate carbamoyl phosphate synthetase I (CPS-I).

Identify the incorrect statement

Since statement E is incorrect, as the rate-limiting step of the urea cycle is not catalyzed by ornithine transcarbamoylase, the answer is: E. Ornithine transcarbamoylase catalyzes the rate-limiting step (Incorrect).