FIG 1.

Exogenous and endogenous fumarate respiration (FR) by E. coli. For endogenous FR (fumarate produced during hexose fermentation) up to 15% of the PEP formed during hexose fermentation (37) is carboxylated to yield OAA, which is then converted by the reductive branch of the anaerobic citric acid cycle to succinate. For exogenous FR, l-aspartate, l-malate, fumarate, l-tartrate, or citrate are taken up by antiporters from the medium, and succinate is excreted in an electroneutral antiport. Enzymes and feeding reactions for fumarate formation are shown in blue and red, respectively, joint reactions in green. Details are described in the text and in reviews (3, 8, 31, 35). At concentrations > 0.1 mM fumarate, l-aspartate or l-malate, the transporters DcuA, DcuB, and DcuC are able to replace each other. DcuA, DcuB, and DcuC are present in S. Typhimurium as well (19), whereas citrate and tartrate are used only by E. coli (6, 46), but not by S. Typhimurium for FR (47, 48, 100). AspA, aspartase; CitT, citrate/succinate antiporter; CL, citrate lyase; DcuA, C4-DC antiporter DcuA; DcuB, C4-DC antiporter DcuB; DcuC, C4-DC transporter DcuC; FR, fumarate respiration; Frd, fumarate reductase FrdABCD; FumB, fumarase B; Hyb, hydrogenase; Nuo, NuoA-N; PtsG, glucose transporter of the phosphotransferase system; TtdAB, tartrate dehydratase; TtdT, l-tartrate/succinate antiporter; MK, menaquinone; MKH₂, menaquinol.