Supramolecular Hemoprotein Assembly with a Periodic Structure Showing Heme–Heme Exciton Coupling

A supramolecular assembly of units of cytochrome b 562 with externally attached heme having intermolecular linkages formed via the heme–heme pocket interaction was investigated in an effort to construct a well-defined structure. The engineered site for surface attachment of heme at Cys80 in an N80C mutant of cytochrome b 562 provides the primary basis for the formation of the periodic assembly structure, which is characterized herein by circular dichroism (CD) spectroscopy and high-speed atomic force microscopy (AFM). This assembly represents the first example of the observation of a split-type Cotton effect by heme–heme exciton coupling in an artificial hemoprotein assembly system. Molecular dynamics simulations validated by simulated CD spectra, AFM images, and mutation experiments reveal that the assembly has a periodic helical structure with 3 nm pitches, suggesting the formation of the assembled structure is driven not only by the heme–heme pocket interaction but also by additional secondary hydrogen bonding and/or electrostatic interactions at the protein interfaces of the assembly.