Elucidation of an alternate isoleucine biosynthesis pathway in Geobacter sulfurreducens
Elucidation of an alternate isoleucine biosynthesis pathway in Geobacter sulfurreducens
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DOI：
10.1128/jb.01841-07
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发表时间：
2008-04-01
影响因子：
3.2
通讯作者：
Coppi, Maddalena V.
中科院分区：
文献类型：
--
作者：
Risso, Carla;Van Dien, Stephen J.;Coppi, Maddalena V.
吉西他滨硫还原菌的中心代谢模型包括异亮氨酸生物合成的单一途径，其类似于大肠杆菌的途径，其中异亮氨酸前体2-氧代丁酸由苏氨酸产生。(13)C标记研究在G. sulfurreducens的研究表明，该途径占异亮氨酸生物合成的一小部分，而大部分异亮氨酸可能通过柠檬酸途径来源于乙酰辅酶A和丙酮酸。鉴定并敲除编码柠檬酸苹果酸合酶（GSU 1798）和苏氨酸解氨酶（GSU 0486）的基因，所述柠檬酸苹果酸合酶催化柠檬酸苹果酸途径中的第一个专用步骤，所述苏氨酸解氨酶催化苏氨酸转化为2-氧代丁酸。缺乏这两种酶的突变体是异亮氨酸的营养缺陷型，而单个突变体能够在没有异亮氨酸的情况下生长。单一突变体的生化特性表明，在柠檬酸苹果酸合酶和苏氨酸氨裂解酶活性的缺陷。因此，在G.硫还原菌2-氧代丁酸酯可以由柠檬酸或苏氨酸合成，前者是异亮氨酸生物合成的主要途径。对G.硫还原菌构成了柠檬酸苹果酸酯酶的遗传学上不同的进化枝的第一个特征成员，其包含来自多种微生物的代表。
The central metabolic model for Geobacter sulfurreducens included a single pathway for the biosynthesis of isoleucine that was analogous to that of Escherichia coli, in which the isoleucine precursor 2-oxobutanoate is generated from threonine. (13)C labeling studies performed in G. sulfurreducens indicated that this pathway accounted for a minor fraction of isoleucine biosynthesis and that the majority of isoleucine was instead derived from acetyl-coenzyme A and pyruvate, possibly via the citramalate pathway. Genes encoding citramalate synthase (GSU1798), which catalyzes the first dedicated step in the citramalate pathway, and threonine ammonia-lyase (GSU0486), which catalyzes the conversion of threonine to 2-oxobutanoate, were identified and knocked out. Mutants lacking both of these enzymes were auxotrophs for isoleucine, whereas single mutants were capable of growth in the absence of isoleucine. Biochemical characterization of the single mutants revealed deficiencies in citramalate synthase and threonine ammonia-lyase activity. Thus, in G. sulfurreducens, 2-oxobutanoate can be synthesized either from citramalate or threonine, with the former being the main pathway for isoleucine biosynthesis. The citramalate synthase of G. sulfurreducens constitutes the first characterized member of a phylogenetically distinct clade of citramalate synthases, which contains representatives from a wide variety of microorganisms.