Contrasting Sensitivities of Escherichia coli Aconitases A and B to Oxidation and Iron Depletion
Contrasting Sensitivities of Escherichia coli Aconitases A and B to Oxidation and Iron Depletion
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DOI：
10.1128/jb.185.1.221-230.2003
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发表时间：
2003-01
影响因子：
3.2
通讯作者：
S. Varghese;Yue-qin Tang;J. Imlay
中科院分区：
文献类型：
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作者：
S. Varghese;Yue-qin Tang;J. Imlay
超氧化物损伤含有催化[4Fe-4S]2+簇合物的脱氢酶。乌头酸酶是该酶家族的成员，先前的工作表明，当大肠杆菌暴露于超氧化物胁迫时，大多数乌头酸酶活性丧失。最近确定E.大肠杆菌合成乌头酸酶的至少两种同工酶AcnA和AcnB。AcnA是一种不太丰富的酶，它的合成受到SoxS的积极控制，SoxS是一种在产生超氧化物的化学物质存在下被激活的蛋白质。我们已经确定，这种安排的存在，因为AcnA是耐超氧化物在体内。令人惊讶的是，纯化的AcnA对超氧化物和其他化学氧化剂非常敏感，除非它与细胞提取物中存在的未表征的因子结合。相反，AcnB是高度敏感的各种化学氧化剂在体内，提取物，并在其纯化形式。因此，在氧化应激过程中AcnA的诱导提供了一种规避三羧酸循环阻滞的机制。AcnA似乎与AcnB一样具有催化活性，因此保留后者作为主要管家酶必须提供一些其他优势。我们观察到AcnB的[4Fe-4S]簇与周围的铁池处于动态平衡，因此当细胞内铁池下降时，AcnB迅速脱金属。AcnA和其他的脂肪酶不显示这种特性。已知脱金属化的AcnB结合其同源mRNA。AcnB活性的缺乏也导致柠檬酸盐的积累和排泄，柠檬酸盐是一种铁螯合剂，E.大肠杆菌合成一个运输系统。因此，AcnB可以保留为主要顺乌头酸酶，因为其暴露簇的不稳定性允许E。大肠杆菌对铁缺乏的感知和反应。
ABSTRACT Superoxide damages dehydratases that contain catalytic [4Fe-4S]2+ clusters. Aconitases are members of that enzyme family, and previous work showed that most aconitase activity is lost when Escherichia coli is exposed to superoxide stress. More recently it was determined that E. coli synthesizes at least two isozymes of aconitase, AcnA and AcnB. Synthesis of AcnA, the less-abundant enzyme, is positively controlled by SoxS, a protein that is activated in the presence of superoxide-generating chemicals. We have determined that this arrangement exists because AcnA is resistant to superoxide in vivo. Surprisingly, purified AcnA is extremely sensitive to superoxide and other chemical oxidants unless it is combined with an uncharacterized factor that is present in cell extracts. In contrast, AcnB is highly sensitive to a variety of chemical oxidants in vivo, in extracts, and in its purified form. Thus, the induction of AcnA during oxidative stress provides a mechanism to circumvent a block in the tricarboxylic acid cycle. AcnA appears to be as catalytically competent as AcnB, so the retention of the latter as the primary housekeeping enzyme must provide some other advantage. We observed that the [4Fe-4S] cluster of AcnB is in dynamic equilibrium with the surrounding iron pool, so that AcnB is rapidly demetallated when intracellular iron pools drop. AcnA and other dehydratases do not show this trait. Demetallated AcnB is known to bind its cognate mRNA. The absence of AcnB activity also causes the accumulation and excretion of citrate, an iron chelator for which E. coli synthesizes a transport system. Thus, AcnB may be retained as the primary aconitase because the lability of its exposed cluster allows E. coli to sense and respond to iron depletion.