Protein camouflage in cytochrome c–calixarene complexes

Small molecules that recognize protein surfaces are important tools for modifying protein interaction properties. Since the 1980s, several thousand studies concerning calixarenes and host–guest interactions have been published. Although there is growing interest in protein–calixarene interactions, only limited structural information has been available to date. We now report the crystal structure of a protein–calixarene complex. The water-soluble p-sulfonatocalix[4]arene is shown to bind the lysine-rich cytochrome c at three different sites. Binding curves obtained from NMR titrations reveal an interaction process that involves two or more binding sites. Together, the data indicate a dynamic complex in which the calixarene explores the surface of cytochrome c. In addition to providing valuable information on protein recognition, the data also indicate that the calixarene is a mediator of protein–protein interactions, with potential applications in generating assemblies and promoting crystallization