25. Protein and Amino Acid Metabolism

Which amino acid provides the NH₄⁺ for the formation of carbamoyl phosphate?

Which of the following amino acids is not a part of the urea cycle?

What is the function of aspartate in the urea cycle?

Which one of the following sentences is an incorrect description of a reaction in the urea cycle?

Which of the following metabolites is hydrolyzed in the urea cycle to produce ornithine and urea?

Which enzyme catalyzes the reaction of citrulline with aspartate to produce argininosuccinate?

Write the total number of amino acid metabolites in the urea cycle. How many of those amino acids are not found in proteins?

Fumarate from step 3 of the urea cycle may be recycled into aspartate for use in step 2 of the cycle. The sequence of reactions for this process is

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b. <IMAGE>

c. <IMAGE>

Classify each reaction as one of the following:

1. Oxidation

2. Reduction

3. Transamination

4. Elimination

5. Addition

In the liver, the relative activity of ornithine transcarbamylase is high, that of argininosuccinate synthetase is low, and that of arginase is high. Why is it important that ornithine transcarbamylase activity be high in the liver? What might be the consequence if arginase activity is low or defective?

From what two amino acids do the nitrogens in urea arise? (Hint: See Figure 25.3.)

The net reaction for the urea cycle shows that three ATPs are hydrolyzed; however, the total energy “cost” is four ATPs. Explain why this is true.