Fig. 6.

Three forms of HIV-1 protease: closed (blue, PDB ID code 1HVR), semiopen (red, PDB ID code1HHP), and open (cyan, our model). We observe opening/closing wherein the flap elbows and exposed ends of fulcrum and cantilever move down and toward the terminal β-sheet dimer interface (indicated by yellow arrows). Examination of closed and semiopen crystal structures reveals differences in those regions that are smaller in magnitude but in agreement with the direction of changes seen during opening.