3.D.10.1.3
The diheme-containing quinol:fumarate reductase (QFR), FrdABC (Madej et al. 2009).  This enzyme complex is required for fumarate respiration using formate or sulfide as electron donor.  It mediates transmembrane electron transfer by proton transfer via a compensatory transmembrane proton transfer pathway ('E-pathway') (Lancaster et al. 2008). This is necessary because, although the reduction of fumarate by menaquinol is exergonic, it is not exergonic enough to support the generation of a pmf. This compensatory E-pathway appears to be required by all dihaem- containing QFR enzymes. The conservation of an essential acidic residue on transmembrane helix V (Glu-C180 in W. succinogenes QFR) provides a key for the sequence-based discrimination of these QFR enzymes from the dihaem-containing SQR enzymes (Lancaster et al. 2008). This enzyme complex may mediate transmembrane electron transfer coupled to protolytic reactions on opposite sides of the membrane and by transmembrane proton transport (Madej et al. 2006).