HIV Protease Active Site Structure

Steps:

• Identify HIV protease as a homodimer with two 99-amino-acid chains.
• Note that each chain contributes a key residue at position 25 to the catalytic site.
• Recognize Asp25 from each chain forms a dyad essential for enzymatic activity.
• Confirm the role via standard biochemistry of aspartyl proteases.

Why A is correct:

• Asp25 is the catalytic aspartic acid residue, forming the active site dyad per the mechanism of aspartyl proteases.

Why the others are wrong:

• B: Ignores the dimeric nature; single chain lacks full activity.
• C: Position 25 is aspartate, not glycine, which is not catalytic.
• D: Amino acid 25 pairs with the symmetric Asp25 from the other chain, not residue 50.

Not enough information: Question cuts off after "amino acid 25 and," leaving choices ambiguous.

Final answer: A