Which of the following statements about the protein buffer system is correct?

• A. The protein buffer system is only effective in the extracellular fluid.
• B. Proteins buffer pH changes by directly producing bicarbonate ions.
• C. Proteins can act as buffers because their amino acid side chains can accept or donate hydrogen ions.
• D. Only enzymes, not structural proteins, participate in buffering pH.

Answer: (C)

Verified step by step guidance

1. Understand the concept of a buffer system: A buffer system helps maintain pH stability by minimizing changes in hydrogen ion (H⁺) concentration. Proteins can act as buffers because they have amino acid side chains that can accept or donate H⁺ ions, depending on the pH of the environment.
2. Analyze the structure of proteins: Proteins are made up of amino acids, which have functional groups such as amino (-NH₂) and carboxyl (-COOH) groups. These groups, along with certain side chains (R groups), can participate in buffering by either accepting or releasing H⁺ ions.
3. Evaluate the first statement: 'The protein buffer system is only effective in the extracellular fluid.' This is incorrect because proteins are present in both intracellular and extracellular fluids, and they can function as buffers in both environments.
4. Evaluate the second statement: 'Proteins buffer pH changes by directly producing bicarbonate ions.' This is incorrect because proteins do not produce bicarbonate ions. The bicarbonate buffer system is a separate mechanism involving carbonic acid (H₂CO₃) and bicarbonate (HCO₃⁻).
5. Evaluate the third statement: 'Proteins can act as buffers because their amino acid side chains can accept or donate hydrogen ions.' This is correct because the side chains of certain amino acids, such as histidine, can act as weak acids or bases, allowing proteins to buffer pH changes effectively.