Tryptophan biosynthesis

Biosynthesis of the aromatic amino acids tryptophan, tyrosine, and phenylalanine proceeds via a common pathway to chorismate, at which point the pathway branches (CITS:[Jones][1943992]). One branch proceeds to tryptophan, and the other to tyrosine and phenylalanine (CITS:[Jones]). The series of reactions to chorismate, called the shikimate pathway, and the series of reactions from chorismate to tryptophan have been found to be common to all eukaryotes and prokaryotes studied thus far (as reported in (CITS:[1943992])). In contrast, there appears to be two separate routes from chorismate to tyrosine and phenylalanine, only one of which has been found in S. cerevisiae (CITS:[1943992]). Aromatic amino acid biosynthesis in S. cerevisiae is controlled by a combination of feedback inhibition, activation of enzyme activity, and regulation of enzyme synthesis (CITS:[Jones][1943992]). The first step in the tryptophan branch is feedback inhibited by tryptophan, and the first step in the phenylalanine-tyrosine branch is feedback inhibited by tyrosine and activated by tryptophan (CITS:[1943992]). The transcriptional activator GCN4 regulates most of the genes encoding for the aromatic amino acid biosynthetic enzymes; however, no GCN4 regulation was found for TRP1 of the tryptophan branch, TYR1 of the tyrosine branch or ARO7 of the tyrosine and phenylalanine branch (CITS:[1943992]). SOURCE: SGD pathways, http://pathway.yeastgenome.org/server.html Based on http://pathway.yeastgenome.org/biocyc/

芳香族氨基酸如色氨酸、酪氨酸和苯丙氨酸的生物合成途径均通过共同的途径至邻氨基苯甲酸，随后路径分支（参见CITS:[Jones][1943992]）。其中一支路径通向色氨酸，而另一支则通向酪氨酸和苯丙氨酸（参见CITS:[Jones]）。至邻氨基苯甲酸的反应系列，即莽草酸途径，以及从邻氨基苯甲酸至色氨酸的反应系列，已被发现是所有迄今为止研究的真核生物和原核生物共有的（如（CITS:[1943992]）所述）。相反，从邻氨基苯甲酸至酪氨酸和苯丙氨酸似乎存在两条独立的途径，其中仅在酿酒酵母（S. cerevisiae）中发现了其中之一（参见CITS:[1943992]）。酿酒酵母中的芳香族氨基酸生物合成受反馈抑制、酶活性的激活以及酶合成的调控（参见CITS:[Jones][1943992]）。在色氨酸分支中的第一步受色氨酸的反馈抑制，而在苯丙氨酸-酪氨酸分支中的第一步受酪氨酸的反馈抑制并由色氨酸激活（参见CITS:[1943992]）。转录激活因子GCN4调控编码芳香族氨基酸生物合成酶的大部分基因；然而，在色氨酸分支的TRP1、酪氨酸分支的TYR1以及酪氨酸和苯丙氨酸分支的ARO7中均未发现GCN4的调控（参见CITS:[1943992]）。资料来源：SGD途径，http://pathway.yeastgenome.org/server.html，基于http://pathway.yeastgenome.org/biocyc/。