Mechanisms of electron transfer in two decaheme cytochromes from a metal-reducing bacterium

In this report, we analyze and interpret single-molecule current-voltage (I-V) tunneling spectra collected for two decaheme c-type cytochromes using a scanning tunneling microscope. The cytochromes (OmcA and MtrC) are outer-membrane proteins from the metal-reducing bacterium Shewanella oneidensis and function as metal-reducing enzymes. Although the two cytochromes are similar in heme count, charge-carrying amino acid content, and molecular mass, their I-V spectra are significantly different. The I-V spectra for OmcA show smoothly varying symmetric exponential behavior. These spectra are well fit by a coherent tunneling model that is based on a simple square barrier description of the tunneling junction. In contrast, the I-V spectra for MtrC have significant breaks in slope in the positive tip bias range. Two large peaks in the normalized differential conductance spectra of MtrC were fit to a tunneling model that accounts for the possibility of transient population of empty states stabilized by vibrational relaxation. Reorganization energies deduced for the two features are similar to those normally assigned to metal centers in other metalloproteins. Work function measurements of the cytochrome films were used to convert the energies of these two spectral features to the normal hydrogen electrode (NHE) scale for comparison with the redox potential domain previously measured by protein film voltammetry, which showed good correspondence. We conclude that MtrC mediates tunneling current by discretely resolved heme orbital participation at -81 and -365 mV versus NHE. The difference in tunneling behavior between OmcA and MtrC suggests distinct physiological functions for the two cytochromes; in contrast to OmcA, MtrC appears to be tuned to a specific operating potential.