Which of the following statements correctly describes the role of pepsin in protein digestion?

• A. Pepsin is an enzyme that breaks proteins down into short peptides in the stomach.
• B. Pepsin begin breaking down proteins in the mouth, as it is released from salivary glands.
• C. Pepsin is a protein itself and therefore is denatured in the stomach due to the acidic environment.
• D. Pepsin's primary function is to denature proteins (causing them to lose their quaternary, tertiary, & secondary structure) but does not cleave (split) them.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the role of pepsin in protein digestion. Pepsin is a proteolytic enzyme that is secreted in an inactive form called pepsinogen by the stomach lining. It becomes active in the acidic environment of the stomach (pH ~1.5-3.5) and begins breaking down proteins into smaller peptides.
2. Step 2: Analyze the first statement: 'Pepsin is an enzyme that breaks proteins down into short peptides in the stomach.' This is correct because pepsin's primary function is to cleave peptide bonds in proteins, producing smaller peptide fragments in the stomach.
3. Step 3: Analyze the second statement: 'Pepsin begins breaking down proteins in the mouth, as it is released from salivary glands.' This is incorrect because pepsin is not active in the mouth. Protein digestion does not begin in the mouth; it starts in the stomach where pepsin is activated.
4. Step 4: Analyze the third statement: 'Pepsin is a protein itself and therefore is denatured in the stomach due to the acidic environment.' This is incorrect because while pepsin is a protein, it is specifically adapted to function in the acidic environment of the stomach and is not denatured by it.
5. Step 5: Analyze the fourth statement: 'Pepsin's primary function is to denature proteins (causing them to lose their quaternary, tertiary, & secondary structure) but does not cleave (split) them.' This is incorrect because pepsin's primary function is to cleave peptide bonds, not just denature proteins. Denaturation is primarily caused by the acidic environment, not by pepsin itself.