AP Biology Questions on Protein Metabolism and the Urea Cycle

📚 Quick Study Guide

• 🧬 Protein Metabolism Overview: Proteins are broken down into amino acids. These amino acids can be used to synthesize new proteins or be catabolized for energy. Catabolism involves removing the amino group ($NH_2$) through deamination or transamination.
• 🧪 Deamination: The removal of an amino group from a molecule. This process produces ammonia ($NH_3$), which is toxic and must be converted to a less toxic form.
• 🔄 Transamination: The transfer of an amino group from one amino acid to a keto acid. This process is crucial for synthesizing non-essential amino acids.
• 🌍 Urea Cycle Location: Primarily occurs in the liver.
• 🔢 Urea Cycle Purpose: Converts toxic ammonia ($NH_3$) into urea ($ (NH_2)_2CO $), a less toxic compound that can be excreted in urine.
• 💡 Urea Cycle Steps (Simplified):
  1. Ammonia enters the cycle.
  2. Ammonia, carbon dioxide, and ATP react to form carbamoyl phosphate.
  3. Carbamoyl phosphate reacts with ornithine to form citrulline.
  4. Citrulline reacts with aspartate to form argininosuccinate.
  5. Argininosuccinate is cleaved to form arginine and fumarate.
  6. Arginine is hydrolyzed to form urea and ornithine, regenerating ornithine to continue the cycle.
• 📝 Key Enzymes: Carbamoyl phosphate synthetase, ornithine transcarbamoylase, argininosuccinate synthetase, argininosuccinate lyase, and arginase.
• 🌱 Regulation: The urea cycle is regulated by substrate availability (ammonia) and enzyme induction (increased synthesis of urea cycle enzymes).

Practice Quiz

1. Which of the following is the primary purpose of the urea cycle?
   1. To synthesize amino acids.
   2. To break down fatty acids.
   3. To convert ammonia to a less toxic form.
   4. To produce ATP.
2. In which organ does the urea cycle primarily take place?
   1. Kidney
   2. Liver
   3. Pancreas
   4. Spleen
3. What is the immediate precursor to urea in the urea cycle?
   1. Ammonia
   2. Ornithine
   3. Arginine
   4. Citrulline
4. What is the role of transamination in amino acid metabolism?
   1. Removal of amino groups to produce ammonia.
   2. Transfer of amino groups from one amino acid to a keto acid.
   3. Synthesis of urea.
   4. Breakdown of proteins into amino acids.
5. Which of the following molecules is regenerated in each turn of the urea cycle?
   1. Urea
   2. Citrulline
   3. Arginine
   4. Ornithine
6. Carbamoyl phosphate synthetase catalyzes the reaction between ammonia, carbon dioxide, and ATP to form carbamoyl phosphate. This occurs during which stage of the urea cycle?
   1. The beginning.
   2. The middle.
   3. The end.
   4. It doesn't occur in the urea cycle.
7. What happens to fumarate, which is produced as a byproduct of the urea cycle?
   1. It is converted back into urea.
   2. It enters the citric acid cycle.
   3. It is excreted in urine.
   4. It is used to synthesize proteins.