Synthesis and biophysical characterization of bioengineered cytochrome nanowires

Geobacter bacteria produce multiheme c-type cytochrome nanowires that are involved in long-range extracellular electron transfer. The ability of these protein nanowires to conduct electrical current makes them promising candidates for electronic devices, offering several functional and sustainable advantages over traditional materials. Therefore, this study focused on synthesizing hybrid protein fibers that mimic the natural Geobacter extracellular nanowires. To achieve this, a mutated PpcA triheme protein variant was used as the building block, with thiol-ene coupling employed to bind the protein molecules. This engineered PpcA variant (PpcAK9CK22C) maintained a structure similar to that of the native protein. Thermal denaturation studies revealed a two-state process, with a melting temperature of 62 ± 1 °C and an enthalpy change of 61 ± 2 kcal/mol. The new protein nanowires showed a lower heme group content than the precursor protein and displayed distinct secondary structure features, with a slight reduction in helical content and an increase in β-sheet and unordered structures. Their thermal stability also differed, as it could not be described by the same model applied to the PpcA variant. Despite these differences, the nanowires retained their ability to undergo redox cycling. Morphologically, they consisted of linear single-protein filaments extending over 300 nm in length.