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 CbcLV205A, CbcLV205G, and CbcLF525Y 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 CbcLV205A, CbcLV205G, and CbcLF525Y remained active. Growth yields of CbcLV205A, CbcLV205G, and CbcLF525Y 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 captionO_ST_ABSTunnel diode behaviorC_ST_ABSElectron 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. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=134 SRC="FIGDIR/small/506072v1_ufig1.gif" ALT="Figure 1"> View larger version (26K): org.highwire.dtl.DTLVardef@11c9b5corg.highwire.dtl.DTLVardef@11be4c6org.highwire.dtl.DTLVardef@4883bborg.highwire.dtl.DTLVardef@c09223_HPS_FORMAT_FIGEXP M_FIG C_FIG