Auto‐regulated Protein Assembly on a Supramolecular Scaffold.

Controlled protein assembly provides a means to regulate function. Supramolecular building blocks, including rigid macrocycles, are versatile triggers of protein assembly. Now it is shown that sulfonato‐calix[8]arene (sclx8) mediates the formation of cytochrome c tetramers in solution. This tetramer spontaneously disassembles at ≥2 equivalents of sclx8, providing a remarkable example of auto‐regulation. Using X‐ray crystallography the sclx8 binding sites on cytochrome c were characterized. Crystal structures at different protein–ligand ratios reveal varying degrees (up to 35 %) of protein surface coverage by the flexible calixarene and suggest a mechanism for oligomer disassembly. The solution structure of the oligomer was characterized by small‐angle X‐ray scattering. Overall, the data indicate calixarene‐controlled protein assembly and disassembly without the requirement for a competitive inhibitor, and point to protein encapsulation by a flexible macrocycle. Calixarene‐controlled assembly and disassembly of a protein tetramer is demonstrated. The flexible sulfonato‐calix[8]arene acts as a scaffold for the assembly of cytochrome c. High concentrations of the calixarene mask the protein surface and prevent oligomerization, providing a means to control assembly without the need for competitive inhibitors. [ABSTRACT FROM AUTHOR]