Stereochemistry and fate of hydrogen atoms in the diol-dehydratase-catalyzed dehydration of meso-butane-2,3-diol

The transformation of meso-butane-2,3-diol into butan-2-ol by a strain of Lactobacillus brevis occurs through a diol-dehydratase-catalyzed conversion of the diol into butan-2-one, which is then reduced to the secondary alcohol by dehydrogenases. Experiments performed with deuterated meso-butane-2,3-diols showed that the dehydration reaction brings about an inversion of configuration at the (R)-configured C-atom of mesobutane-2,3-diol as a consequence of the substitution of the OH group by a H-atom; at the same time, the H-atom already bound to the (R) C-atom is retained in the resulting methylene group. The H-atom replacing the OH group was assessed to come from the medium since the H-atom at the (S)-configured C-atom was completely lost. By contrast, in the case of the conversion of (RS)-propane-1,2-diol into propan-1-ol under the same fermentation conditions, an extensive H-transfer (ca. 80%) from the primary-alcohol function to the adjacent C-atom was observed. This fact is taken as an indication of different modes in which the two substrates are processed by the enzyme (despite the same stereochemical outcome). A speculative hypothesis is presented to interpret such a dissimilarity.