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Propanediol dehydratase was purified from cell extracts of Propionibacterium freudenreichii by protamine sulfate treatment, ammonium sulfate fractionation, DEAE-cellulose column chromatography, and Sepharose 4B column chromatography.The enzyme was purified approximately 635-fold with overall recovery of 23% and proved to be almost homogeneous by polyacrylamide gel electrophoresis.The molecular weight of the enzyme was estimated to be 230,000 by gel filtration. The enzyme showed the highest activity around pH 9.2. The enzyme dehydrated DL-1,2-propanediol, 1,2-ethanediol, DL-1,2-butanediol, DL-2,3-butanediol, and DL-1,3-butanediol. The K_m values for 1,2-propanediol, 1,2-ethanediol, and coenzyme B_<12> were 0.18 mM, 1.79 mM, and 0.81 μM, respectively. Propanediol dehydratase was inhibited by sulfhydryl compounds.