Voltammetry of a ‘protein on a rope’

A periplasmic electron-transfer protein, cytochrome c555m 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 c555m, expressed in Escherichia coli, has been attached covalently to a gold electrode modified with 6-mercaptohexan-1-ol. The ‘tethered’ cytochrome c555m displays remarkably fast electron-transfer kinetics, with an electrochemical exchange rate constant k0 of 1.4×104 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.