Figure 5.

Method used to determine the ligand dissociation-constant of rapamycin with FKBP-12. Top: Equilibrium denaturation of FKBP-12. Bottom: Equilibrium denaturation of FKBP-12 in the presence of rapamycin. The equilibrium is shifted more towards the native state as the ligand stabilizes it. The native FKBP-12 structure shown and the interaction of rapamycin was rendered in the PyMOL molecular graphics system³⁸ from the protein databank file 1fkb. The denatured state is schematic only. The dissociation constant for rapamycin binding can be derived from the change in the free energy of protein unfolding using Eq. (4). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]