HIV protease, a dimeric aspartyl protease. The protease is a dimer of identical subunits, shown in blue and yellow, consisting of 99 amino acids each. Notice the placement of active-site aspartic acid residues, one from each chain, which are shown as ball-and-stick structures. The flaps will close down on the binding pocket after substrate has been bound.
[Drawn from 3PHV.pdb.]