Substrate Specificity of the 3-Methylmercaptopropionyl Coenzyme A Dehydrogenase (DmdC1) from Ruegeria pomeroyi DSS-3.

The acyl-coenzyme A (CoA) dehydrogenase family enzyme DmdC catalyzes the third step in the dimethylsulfoniopropionate (DMSP) demethylation pathway, the oxidation of 3-methylmercaptopropionyl-CoA (MMPA-CoA) to 3-methylthioacryloyl-CoA (MTA-CoA). To study its substrate specificity, the recombinant DmdC1 from Ruegeria pomeroyi was characterized. In addition to MMPA-CoA, the enzyme was highly active with short-chain acyl-CoAs, with values for MMPA-CoA, butyryl-CoA, valeryl-CoA, caproyl-CoA, heptanoyl-CoA, caprylyl-CoA, and isobutyryl-CoA of 36, 19, 7, 11, 14, 10, and 149 μM, respectively, and values of 1.48, 0.40, 0.48, 0.73, 0.46, 0.23, and 0.01 s, respectively. Among these compounds, MMPA-CoA was the best substrate. The high affinity of DmdC1 for its substrate supports the model for kinetic regulation of the demethylation pathway. In contrast to DmdB, which catalyzes the formation of MMPA-CoA from MMPA, CoA, and ATP, DmdC1 was not affected by physiological concentrations of potential effectors, such as DMSP, MMPA, ATP, and ADP. Thus, compared to the other enzymes of the DMSP demethylation pathway, DmdC1 has only minimal adaptations for DMSP metabolism compared to other enzymes in the same family with similar substrates, supporting the hypothesis that it evolved relatively recently from a short-chain acyl-CoA dehydrogenase involved in fatty acid oxidation. We report the kinetic properties of DmdC1 from the model organism and close an important gap in the literature. While the crystal structure of this enzyme was recently solved and its mechanism of action described (X. Shao, H. Y. Cao, F. Zhao, M. Peng, et al., Mol Microbiol 111:1057-1073, 2019, https://doi.org/10.1111/mmi.14211), its substrate specificity and sensitivity to potential effectors was never examined. We show that DmdC1 has a high affinity for other short-chain acyl-CoAs in addition to MMPA-CoA, which is the natural substrate in DMSP metabolism and is not affected by the potential effectors tested. This evidence supports the hypothesis that DmdC1 possesses few adaptations to DMSP metabolism and likely evolved relatively recently from a short-chain acyl-CoA dehydrogenase involved in fatty acid oxidation. This work is important because it expands our understanding of the adaptation of marine bacteria to the increased availability of DMSP about 250 million years ago.