Fig. 2.
Changes in the size of the active site entrance. On moving from Eopen (3A3W: view A is to the side of the active site; view C is directly above the active site) to Eclosed (2R1N: view B is to the side of the active site; D is directly above the active site), the active site entrance closes dramatically, with the shortest distance between the van der Waals radii of L271 and F132 contracting from 10 Å to 4.1Å. It is clear that the size of the substrate (7.7 Å in the shortest dimension) would make rapid diffusion into, or out of, the enzyme as Eclosed unlikely.