Leucine degradation

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 (leucine, iso-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]). In leucine degradation, Thi3p is believed to be the major decarboxylase (CITS: [12499363]). SOURCE: SGD pathways, http://pathway.yeastgenome.org/server.html

虽然酿酒酵母可以利用大多数氨基酸作为其唯一的氮源，但它们只能利用少数几种氨基酸作为碳源以支持生长（参见文献：[Large86][Cooper82]）。这与大多数真核生物和某些真菌形成鲜明对比，这些生物可以完全代谢氨基酸，将其作为碳和氮的唯一来源（参见文献：[Stryer88][Large 86]）。酿酒酵母通过Ehrlich途径（参见文献：[Sentheshanmuganathan60][10989420]）降解支链氨基酸（亮氨酸、异亮氨酸和缬氨酸）以及芳香族氨基酸（色氨酸、苯丙氨酸和酪氨酸）。该途径包括以下步骤：1）氨基酸脱氨基生成相应的α-酮酸；2）将生成的α-酮酸脱羧生成相应的醛；3）将醛还原形成相应的长链或复杂醇，即所谓的副醇或副醇油（参见文献：[10989420][Large 86]）。副醇是酵母发酵食品和饮料中重要的风味和香气化合物（参见文献：[9546164]）。支链氨基酸降解的三个步骤可由多种同工酶催化；所使用的酶似乎取决于氨基酸、碳源以及培养物的生长阶段（参见文献：[12499363]）。在亮氨酸降解中，Thi3p被认为是最主要的脱羧酶（参见文献：[12499363]）。资料来源：酵母基因组的通路，http://pathway.yeastgenome.org/server.html