The pathway of formation of n-butyl and n-amyl alcohols by a mutant strain of Saccharomyces cerevisiae

Abstract

A mutant strain of Saccharomyces cerevisiae was found which produces relatively large amounts of n-propyl and n-butyl alcohols but none of the other higher alcohols usually associated with alcoholic fermentation by yeasts. Tracer studies with this mutant are reported which are consistent with the hypothesis that n-butyl alcohol is formed by the familiar Ehrlich-Neubauer-Fromherz pathway from α-keto- n-valeric acid which in turn is synthesized from α-ketobutyric acid by a pathway analogous to the pathway by which α-ketoisocaproic acid is synthesized from α-ketoisovaleric acid in the synthesis of leucine; i.e., it is proposed that α-ketobutyric acid condenses with acetyl-CoA to form α-aceto-α-hydroxybutyric acid which, after dehydration, rehydration, oxidation, and decarboxylation, yields α-keto- n-valeric acid. If norvaline is added as a source of α-keto- n-valeric acid to the wild type, n-butyl alcohol is formed. If the same compound is added to a fermentation by the mutant, the formation of n-butyl alcohol is slightly stimulated and, in addition, a new alcohol is formed, namely, n-amyl alcohol. Tracer studies indicate that this alcohol is formed from α-ketovaleric acid by a pathway analogous to the pathway of formation of n-butyl alcohol from α-ketobutyric acid. The addition of norleucine, as a source of α-keto- n-caproic acid, to a fermentation by the mutant, stimulates the formation of n-amyl alcohol but does not cause detectable amounts of n-hexyl alcohol to be formed. The several acetotransferase activities of yeasts are discussed.