Stereoselectivity and stereospecificity of the alpha,beta-dihydroxyacid dehydratase from Salmonella typhimurium

1. 1. In addition to the known 2R,3R- and 2R, 3S-2,3-dihydroxy-3-methylpentanoic acids (DHI), the 2S,3S- and 2S,3R-isomers were prepared. 2S-2,3-Dihydroxy-3-methylbutanoic acid (DHV) was also prepared in addition to the known 2R-isomer. 2. 2. The six dihydroxy acids were examined for their ability to promote the growth of isoleucine-valine ( ilv )-requiring strains of Salmonella typhimurium and to serve as substrates fro the α,β-dihydroxyacid dehydratase of the same organism. 3. 3. Only 2R,3R-2,3-dihydroxy-3-methylpentanoic and 2R-2,3-dihydroxy-3-methylbutanoic acids supported growth of the ilv strains of S. typhimurium . 4. 4. α,β-Dihydroxyacid dehydratase utilized the three isomers with the 2R-configuration as substrates but not those with the 2S-configuration. 5. 5. In an additional growth study that utilized the 3R- and 3S-isomers of 3-methyl-2-oxopentanoic acid, the α-keto acid analogue of isoleucine, only the 3S-isomer supported growth. 6. 6. It is concluded that the mechanism of action of the dehydratase is stereospecific in that the proton that is attached to C-3 of the substrate occupies the same stereochemical position as the departing hydroxyl group (Fig. 6).