Recombinant Saccharomyces cerevisiae Pyruvate decarboxylase isozyme 3 (PDC6), partial

Minor of three pyruvate decarboxylases (PDC1, PDC5, PDC6) implicated in the nonoxidative conversion of pyruvate to acetaldehyde and carbon dioxide during alcoholic fermentation. Most of the produced acetaldehyde is subsequently reduced to ethanol, but some is required for cytosolic acetyl-CoA production for biosynthetic pathways. The enzyme is also one of five 2-oxo acid decarboxylases (PDC1, PDC5, PDC6, ARO10, and THI3) able to decarboxylate more complex 2-oxo acids (alpha-keto-acids) than pyruvate, which seem mainly involved in amino acid catabolism. Here the enzyme catalyzes the decarboxylation of amino acids, which, in a first step, have been transaminated to the corresponding 2-oxo acids. In a third step, the resulting aldehydes are reduced to alcohols, collectively referred to as fusel oils or alcohols. Its preferred substrates are the transaminated amino acids derived from threonine (2-oxobutanoate), norvaline (2-oxopentanoate), valine (3-methyl-2-oxobutanoate, also alpha-keto-isovalerate), isoleucine ((3S)-3-methyl-2-oxopentanoate, also alpha-keto-beta-methylvalerate), phenylalanine (phenylpyruvate), and tryptophan (3-(indol-3-yl)pyruvate), whereas transaminated leucine is no substrate. In a side-reaction the carbanionic intermediate (or active aldehyde) generated by decarboxylation or by activation of an aldehyde can react with an aldehyde via condensation (or carboligation) yielding a 2-hydroxy ketone, collectively called acyloins. The expression level of this protein in the presence of fermentable carbon sources is so low that it cannot compensate for the other two pyruvate decarboxylases to sustain fermentation.