Isoleucine degradation (WP178)

While Saccharomyces cerevisiae can use most amino acids as their sole nitrogen source, they can only use a few amino acids as a carbon source to support growth (CITS:[Large86][Cooper82]). This is in contrast to most eukaryotes and some fungi, which can metabolize amino acids completely, utilizing them as sole sources of carbon and nitrogen (CITS:[Stryer88][Large 86]). S. cerevisiae degrade the branched-chain amino acids (iso-leucine, leucine, and valine) and the aromatic amino acids (tryptophan, phenylalanine, and tyrosine) via the Ehrlich pathway (CITS:[Sentheshanmuganathan60][10989420]). This pathway is comprised of the following steps: 1) deamination of the amino acid to the corresponding alpha-keto acid; 2) decarboxylation of the resulting alpha-keto acid to the respective aldehyde; and, 3) reduction of the aldehyde to form the corresponding long chain or complex alcohol, known as a fusel alcohol or fusel oil (CITS:[10989420][Large 86]). Fusel alcohols are important flavor and aroma compounds in yeast-fermented food products and beverages (as reported in (CITS:[9546164]) Each of the three steps in branched-chain amino acid degradation can be catalyzed by more than one isozyme; which enzyme is used appears to depend on the amino acid, the carbon source and the stage of growth of the culture (CITS:[12499363]). The initial transamination step in iso-leucine degradation can be catalyzed by either of the branched-chain amino acid transaminases BAT1 (mitochondrial) or BAT2 (cytosolic) (CITS:[10989445][8798704][8702755]). The subsequent decarboxylation step can be catalyzed by any one of the five decarboxylases (Pdc1p, Pdc5p, Pdc6p, Thi3p, and Aro10p) (CITS:[9546164][10753893]) and the final step can be catalyzed by any one of six alcohol dehydrogenases (Adh1p, Adh2p, Adh3p, Adh4p, Adh5p, and Sfa1p) (CITS:[12499363]). SOURCE: SGD pathways, http://pathway.yeastgenome.org/server.html

Participants