PNNL

Abstract

Single-molecule current-voltage (I–V) spectra were collected using a scanning tunneling microscope for two decaheme c-type cytochromes, OmcA and MtrC, which are outer-membrane proteins from the dissimilatory metal-reducing bacterium Shewanella oneidensis. 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 pronounced 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 scale for comparison with the midpoint potential measured using protein film voltammetry, which showed good correspondence. We conclude that MtrC mediates tunneling current by heme orbital participation. 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.