High-affinity l-malate transporter DcuE of Actinobacillus succinogenes catalyses reversible exchange of C4-dicarboxylates

Actinobacillus succinogenes is a natural succinate producer, which is the result of fumarate respiration. Succinate production from anaerobic growth with C₄ -dicarboxylates requires transporters catalysing uptake and efflux of C₄ -dicarboxylates. Transporter Asuc_1999 (DcuE) found in A. succinogenes belongs to the Dcu family and was considered the main transporter for fumarate respiration. However, deletion of dcuE affected l-malate uptake of A. succinogenes rather than fumarate uptake. DcuE complemented anaerobic growth of Escherichia coli on l-malate or fumarate; thus, the transporter was characterized in E. coli heterologously. Time-dependent uptake and competitive inhibition assays demonstrated that l-malate is the most preferred substrate for uptake by DcuE. The V_max of DcuE for l-malate was 20.04 μmol/gDW·min with K_m of 57 μM. The V_max for l-malate was comparable to that for fumarate, whereas the K_m for l-malate was 8 times lower than that for fumarate. The catalytic efficiency of DcuE for l-malate was 7.3-fold higher than that for fumarate, showing high efficiency and high affinity for l-malate. Furthermore, DcuE catalysed the reversible exchange of three C₄ -dicarboxylates - l-malate, fumarate and succinate - but the preferred substrate for uptake was l-malate. Under physiological conditions, the C₄ -dicarboxylates were reduced to succinate. Therefore, DcuE is proposed as the l-malate/succinate antiporter in A. succinogenes.