The specificities and configurations of ternary complexes of yeast and liver alcohol dehydrogenases

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Abstract
Some aspects of the substrate specificities of liver and yeast alcohol dehydrogenases were investigated with pentan-3-ol, hepatan-4-ol, ([plus or minus])-butan-2-ol, ([plus or minus])-butan-2-ol, ([plus or minus])-hexan-3-ol and ([plus or minus])-octan-2-ol as potential substrates. The liver enzyme is active with all substrates tested, including both isomers of each optically active alcohol. In contrast, the yeast enzyme is completely inactive towards those 2ndary alcohols where both alkyl groups are larger than methyl and active with only the (+)-isomers of butan-2-ol and octan-2-ol. The absence of sterospecificity of liver alcohol dehydrogenase towards optically active 2ndary alcohols and its broad specificity towards 2ndary alcohols in general are explained in terms of an alkyl-binding site that will react with a variety of alkyl groups and the ability of the enzyme to accommodate a fairly large unbound alkyl group in an active enzyme-NAD+-secondary alcohol ternary complex. The absolute optical specificity of the yeast enzyme towards n-alkylmethyl carbinols and its unreactivity towards pentan-3-ol, hexan-4-ol are explained by its inability to accept alkyl groups larger than methyl in the unbound position in a viable ternary complex. Comparison of the known configurations of the n-alkylmethyl carbinols and [1-H2]ethanol and [ 1-H3]geraniol, which were used in stereospecificity studies with these enzymes by other workers, provides strong evidence for which alkyl group of the sub-strate is bound to the enzyme in the oxidation of n-alkylmethyl carbinols. The conclusions reached are, for butan-2-ol oxidation with the liver enzyme, confirmed by deductions from kinetic data obtained with (+)-butan-2-ol and a sample of butan-2-ol containing 66% of ([long dash])-butan-2-ol. Initial-rate parameters for the oxidations of (+)-butan-2-ol, 66% (-)-butan-2-ol and pentan-3-ol by NAD with liver alcohol de-hydrogenase are presented. The data are completely consistent with a general mechanism of catalysis previously proposed for this enzyme.