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 c onversion of the diol into butan-2-one, which is then reduced to the s econdary alcohol by dehydrogenases. Experiments performed with deutera ted meso-butane-2,3-diols showed that the dehydration reaction brings about an inversion of configuration at the (R)-configured C-atom of me sobutane-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 replacin g the OH group was assessed to come from the medium since the H-atom a t the (S)-configured C-atom was completely lost. By contrast, in the c ase of the conversion of (RS)-propane-1,2-diol into propan-1-ol under the same fermentation conditions, an extensive H-transfer (ca. 80%) fr om the primary-alcohol function to the adjacent C-atom was observed. T his fact is taken as an indication of different modes in which the two substrates are processed by the enzyme (despite the same stereochemic al outcome). A speculative hypothesis is presented to interpret such a dissimilarity.