Single amino acid residues control potential-dependent inactivation of an inner membrane bc -cytochrome

Abstract

During extracellular electron transfer, Geobacter sulfurreducens constitutively expresses the bc- cytochrome CbcL, yet cells containing only this menaquinone oxidase fail to respire above –0.1 V vs . SHE. By identifying mutations within cbcL that permit growth at higher potentials, we provide evidence that this cytochrome is regulated by redox potential. Strains expressing CbcL V205A , CbcL V205G , and CbcL F525Y were capable of growth with high potential electron acceptors including Fe(III) citrate, Mn(IV) oxides, and electrodes poised at +0.1 V vs . SHE. Electrochemical characterization of wild type CbcL revealed oxidative inactivation of electron transfer above -0.1 V, while CbcL V205A , CbcL V205G , and CbcL F525Y remained active. Growth yields of CbcL V205A , CbcL V205G , and CbcL F525Y were only 50% of WT, consistent with CbcL-dependent electron transfer conserving less energy. These data support the hypothesis that CbcL has evolved to rapidly shut off in response to redox potential to divert electrons to higher yield oxidases that coexist in the Geobacter membrane. TOC image and caption: Tunnel diode behavior: Electron flux from cells utilizing the menaquinone oxidase CbcL is attenuated by increased redox potential, preventing use of this low-efficiency pathway when driving forces are high enough to conserve energy via other oxidases. Single amino acid substitutions eliminate this switch-off effect and allow function at all potentials.