Internal cavities of ecDHFR. (a) 0.1 and (b) 220 MPa structures of the M20-closed crystal and (c) 0.1 and (d) 270 MPa structures of the M20-open crystal. Cavities were drawn with the surface-cavity mode of PyMOL using a solvent radius of 1.2 Å. NADP+ and FOL are treated as components of the protein molecule, and water molecules are ignored. NADP+, FOL and several residues referred to in Fig. 3 are shown as stick models. For the M20-open crystal, (c) and (d) correspond to the B chain in the asymmetric unit.