PYRUVATE DECARBOXYLASE CATALYZES DECARBOXYLATION OF BRANCHED-CHAIN 2-OXO ACIDS BUT IS NOT ESSENTIAL FOR FUSEL ALCOHOL PRODUCTION BY SACCHAROMYCES-CEREVISIAE

The fusel alcohols 3-methyl-1-butanol, 2-methyl-1-butanol, and 2-methy l-propanol are important flavor compounds in yeast-derived food produc ts and beverages. The formation of these compounds from branched chain amino acids is generally assumed to occur via the Ehrlich pathway, wh ich involves the concerted action of a branched-chain transaminase, a decarboxylase, and an alcohol dehydrogenase. Partially purified prepa rations of pyruvate decarboxylase (EC 4.1.1.1) have been reported to c atalyze the decarboxylation of the branched chain 2-oxo acids formed u pon transamination of leucine, isoleucine, and valine, Indeed, in a co upled enzymatic assay with horse liver alcohol dehydrogenase, cell ext racts of a wild-type Saccharomyces cerevisiae strain exhibited signifi cant decarboxylation rates with these branched-chain 2-oxo acids, Deca rboxylation of branched-chain 2-ore acids was not detectable in cell e xtracts of an isogenic strain in which all three PDC genes had been di srupted, Experiments with cell extracts from S. cerevisiae mutants exp ressing a single PDC gene demonstrated that both PDC1- and PDC5-encode d isoenzymes can decarboxylate branched-chain 2-oxo acids. To investig ate whether pyruvate decarboxylase is essential for fusel alcohol prod uction by whole cells, wild-type S. cerevisiae and an isogenic pyruvat e decarboxylase-negative strain were grown on ethanol with a mixture o f leucine, isoleucine, and valine as the nitrogen source. Surprisingly , the three corresponding fusel alcohols were produced in both strains . This result proves that decarboxylation of branched-chain 2-ore acid s via pyruvate decarboxylase is not an essential step in fusel alcohol production.