Stereospecificity of Corynebacterium glutamicum 2,3-butanediol dehydrogenase and implications for the stereochemical purity of bioproduced 2,3-butanediol

The stereochemistry of 2,3-butanediol (2,3-BD) synthesis in microbial fermentations is important for many applications. In this work, we showed that Corynebacterium glutamicum endowed with the Lactococcus lactis genes encoding α-acetolactate synthase and decarboxylase activities produced meso-2,3-BD as the major end product, meaning that (R)-acetoin is a substrate for endogenous 2,3-butanediol dehydrogenase (BDH) activity. This is curious in view of the reported absolute stereospecificity of C. glutamicum BDH for (S)-acetoin (Takusagawa et al. Biosc Biotechnol Biochem 65:1876–1878, 2001). To resolve this discrepancy, the enzyme encoded by butA^Cg was produced in Escherichia coli and purified, and the stereospecific properties of the pure protein were examined. Activity assays monitored online by ¹H-NMR using racemic acetoin and an excess of NADH showed an initial, fast production of (2S,3S)-2,3-BD, followed by a slow (∼20-fold lower apparent rate) formation of meso-2,3-BD. Kinetic parameters for (S)-acetoin, (R)-acetoin, meso-2,3-BD and (2S,3S)-BD were determined by spectrophotometric assays. V_max values for (S)-acetoin and (R)-acetoin were 119 ± 15 and 5.23 ± 0.06 μmol min⁻¹ mg protein⁻¹, and K_m values were 0.23 ± 0.02 and 1.49 ± 0.07 mM, respectively. We conclude that C. glutamicum BDH is not absolutely specific for (S)-acetoin, though this is the preferred substrate. Importantly, the low activity of BDH with (R)-acetoin was sufficient to support high yields of meso-2,3-BD in the engineered strain C. glutamicum ΔaceEΔpqoΔldhA(pEKEx2-als,aldB,butA^Cg). Additionally, we found that the BDH activity was nearly abolished upon inactivation of butA^Cg (from 0.30 ± 0.03 to 0.004 ± 0.001 μmol min⁻¹ mg protein⁻¹), indicating that C. glutamicum expresses a single BDH under the experimental conditions examined.