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

Summary Actinobacillus succinogenes is a natural succinate producer, which is the result of fumarate respiration. Succinate production from anaerobic growth with C 4 ‐dicarboxylates requires transporters catalysing uptake and efflux of C 4 ‐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 C 4 ‐dicarboxylates – l ‐malate, fumarate and succinate – but the preferred substrate for uptake was l ‐malate. Under physiological conditions, the C 4 ‐dicarboxylates were reduced to succinate. Therefore, DcuE is proposed as the l ‐malate/succinate antiporter in A. succinogenes .