Tutar Yusuf
Department of Chemistry, Cumhuriyet University, Sivas, Turkey.
Cell Biochem Funct. 2008 Jun;26(4):399-405. doi: 10.1002/cbf.1462.
The cyclic AMP receptor protein (CRP) of Escherichia coli regulates the activity of more than 150 genes. Allosteric changes in CRP structure accompanied by cAMP binding, initiate transcription through protein binding to specific DNA sequences. Initially, researchers proposed a two-site cAMP-binding model for CRP-dependent transcription activation since biophysical methods showed two transitions during titration experiments. Three conformational states were considered; apo-CRP, CRP:(cAMP)(1) and CRP:(cAMP)(2), and CRP:(cAMP)(1) was proposed as the active form in this initial model. X-ray data indicated an anti conformation and in contrast NMR experiments suggested a syn conformation for bound cAMPs. For years this paradigm about ligand conformation has been ambiguous. When CRP was crystallized with four bound cAMP in the last decade, two cAMPs were assigned to syn and the other two to anti conformations. Again three conformational states were suggested; apo-CRP, CRP:(cAMP)(2), and CRP:(cAMP)(4). This new structure changed the view of CRP allosteric activation from a two-site model to a four-site model in the literature and the new model has been supported by biochemical and genetic data so far. According to the accepted model, binding of the first two cAMP molecules displays positive cooperativity, however, binding of the last two cAMP molecules shows negative cooperativity. This resolved the conflict between dynamic and static experimental observations. However, this new model cannot explain the initiation mechanism as previously proposed because functionally active CRP has only one cAMP equivalent. Gene regulation and transcription factors are involved in regulating both prokaryotic and eukaryotic metabolism. Although gene regulation and expression are much more complex in eukaryotes, CRP-mediated transcription initiation is a model of general interest to life sciences and medicine. Therefore, the aim of this review is to summarize recent works and developments on the cAMP-dependent CRP activation mechanism in E. coli.
大肠杆菌的环磷酸腺苷受体蛋白（CRP）可调节150多个基因的活性。CRP结构的变构变化伴随着cAMP结合，通过蛋白质与特定DNA序列的结合启动转录。最初，研究人员提出了一种用于CRP依赖性转录激活的双位点cAMP结合模型，因为生物物理方法在滴定实验中显示出两个转变。当时考虑了三种构象状态：无配体CRP、CRP:(cAMP)₁和CRP:(cAMP)₂，并且在这个初始模型中，CRP:(cAMP)₁被认为是活性形式。X射线数据表明是反式构象，而核磁共振实验则表明结合的cAMP是顺式构象。多年来，关于配体构象的这种范式一直不明确。在过去十年中，当CRP与四个结合的cAMP一起结晶时，两个cAMP被指定为顺式构象，另外两个为反式构象。同样提出了三种构象状态：无配体CRP、CRP:(cAMP)₂和CRP:(cAMP)₄。这种新结构改变了文献中关于CRP变构激活从双位点模型到四位点模型的观点，并且到目前为止，新模型已得到生化和遗传数据的支持。根据公认的模型，前两个cAMP分子的结合显示正协同性，然而，最后两个cAMP分子的结合显示负协同性。这解决了动态和静态实验观察之间的冲突。然而，这个新模型无法解释先前提出的起始机制，因为功能活性CRP只有一个cAMP当量。基因调控和转录因子参与调节原核生物和真核生物的代谢。尽管基因调控和表达在真核生物中要复杂得多，但CRP介导的转录起始是生命科学和医学普遍感兴趣的一个模型。因此，本综述的目的是总结大肠杆菌中cAMP依赖性CRP激活机制的最新研究工作和进展。
