Voltammetry of a ‘protein on a rope’

Abstract A periplasmic electron-transfer protein, cytochrome c 555 m from Aquifex aeolicus contains a 62-residue N-terminal extension by which it is anchored to the membrane – most probably via a thioester bond to its N-terminal cysteine. This linker can act as a ‘rope’ to tether the protein close to its reaction partners. Mimicking this principle, a recombinant cytochrome c 555 m , expressed in Escherichia coli , has been attached covalently to a gold electrode modified with 6-mercaptohexan-1-ol. The ‘tethered’ cytochrome c 555 m displays remarkably fast electron-transfer kinetics, with an electrochemical exchange rate constant k 0 of 1.4×10 4 s −1 . The results show that fast electron transfer is associated with weak interactions: importantly, the tethered cytochrome can explore many different orientations without escaping into solution.