What is the crystal structure of HIV protease?

• A. Monomer
• B. Heterodimer
• C. Trimer
• D. Symmetric dimer of two identical subunits

Answer: (D)

HIV protease is a symmetric homodimeric enzyme: two identical subunits come together to form the active enzyme. The active site sits at the interface between these subunits and is created by residues from both monomers, including the catalytic aspartates from each subunit. Because the two subunits are identical and arranged in a mirror-like fashion, the crystal structure is best described as a symmetric dimer of two identical subunits. This dimerization is essential for activity—monomeric forms lack the complete active site geometry—whereas a trimer, monomer, or heterodimer would not provide the same intersubunit arrangement necessary for catalysis and for inhibitor binding that spans the dimer interface.