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首页> 外文OA文献 >Catalytic Activity of the Anaerobic Tyrosine Lyase Required for Thiamine Biosynthesis in Escherichia coli*
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Catalytic Activity of the Anaerobic Tyrosine Lyase Required for Thiamine Biosynthesis in Escherichia coli*

机译：大肠杆菌中硫胺素生物合成所需的厌氧酪氨酸裂解酶的催化活性*

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摘要

Thiazole synthase in Escherichia coli is an αβ heterodimer of ThiG and ThiH. ThiH is a tyrosine lyase that cleaves the Cα–Cβ bond of tyrosine, generating p-cresol as a by-product, to form dehydroglycine. This reactive intermediate acts as one of three substrates for the thiazole cyclization reaction catalyzed by ThiG. ThiH is a radical S-adenosylmethionine (AdoMet) enzyme that utilizes a [4Fe-4S]+ cluster to reductively cleave AdoMet, forming methionine and a 5′-deoxyadenosyl radical. Analysis of the time-dependent formation of the reaction products 5′-deoxyadenosine (DOA) and p-cresol has demonstrated catalytic behavior of the tyrosine lyase. The kinetics of product formation showed a pre-steady state burst phase, and the involvement of DOA in product inhibition was identified by the addition of 5′-methylthioadenosine/S-adenosylhomocysteine nucleosidase to activity assays. This hydrolyzed the DOA and changed the rate-determining step but, in addition, substantially increased the uncoupled turnover of AdoMet. Addition of glyoxylate and ammonium inhibited the tyrosine cleavage reaction, but the reductive cleavage of AdoMet continued in an uncoupled manner. Tyrosine analogues were incubated with ThiGH, which showed a strong preference for phenolic substrates. 4-Hydroxyphenylpropionic acid analogues allowed uncoupled AdoMet cleavage but did not result in further reaction (Cα–Cβ bond cleavage). The results of the substrate analogue studies and the product inhibition can be explained by a mechanistic hypothesis involving two reaction pathways, a product-forming pathway and a futile cycle.
机译：大肠杆菌中的噻唑合酶是ThiG和ThiH的αβ异二聚体。 ThiH是一种酪氨酸裂解酶，可裂解酪氨酸的Cα–Cβ键，生成对甲酚作为副产物，形成脱氢甘氨酸。该反应性中间体充当由ThiG催化的噻唑环化反应的三种底物之一。 ThiH是一种自由基S-腺苷甲硫氨酸（AdoMet）酶，它利用[4Fe-4S] +簇还原性裂解AdoMet，形成甲硫氨酸和5'-脱氧腺苷基。对反应产物5'-脱氧腺苷（DOA）和对甲酚的时间依赖性形成的分析表明酪氨酸裂解酶的催化行为。产物形成的动力学显示出预稳态爆发阶段，并且通过向活性测定中添加5'-甲基硫代腺苷/ S-腺苷同型半胱氨酸核苷酶来确定DOA参与产物抑制。这水解了DOA并改变了速率确定步骤，但此外，还大大增加了AdoMet的解耦营业额。乙醛酸和铵的加入抑制了酪氨酸的裂解反应，但AdoMet的还原裂解仍以未偶联的方式继续进行。将酪氨酸类似物与ThiGH孵育，后者对酚类底物表现出强烈的偏好。 4-羟基苯基丙酸类似物允许未偶联的AdoMet裂解，但不会导致进一步的反应（Cα–Cβ键裂解）。底物类似物研究和产物抑制的结果可以通过涉及两个反应途径，产物形成途径和无效循环的机理假说来解释。

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