G-四联体的小分子配体的研究进展

鲍伟伟

（温州大学，浙江 温州 310015）

Davies[1]于 1962年首次提出G-四联体。G-四联体是由富G序列在一定浓度的Na+、K+条件下所形成一种DNA二级结构[2-4]。G-四联体结构具有多态性，主要是因为链的数量、链的结构、链的取向以及环的结构形态等存在着多样性。G-四联体的基本单元是4个鸟嘌呤通 过氢键围绕形成平面得到G-四分体，其中的氢键包括Hoogste en (N1-O6)(N2-N7)和Watson-Crick，而 G-四分体之间通过π-π堆积得到G-四联体。G-四联体主要有平行结构和反平行结构，主要受到鸟嘌呤中的碱基和糖之间的syn和anti这2种糖苷键的影响。E Gavathiotis等对于平行的G-四联体带 有anti糖苷键的阐述非常明白[5]。其中最影响拓扑结构的应当是一条链中带有对角环、侧边环以及螺旋桨环等多种环形结构的G-四联体，因为这样形成的结构中存在着4种形态的沟槽，比如对角环和对角环所形成的简单环，但存在螺旋桨环就会很复杂。

在20世纪80年代初期，只有生物物理学家对其感兴趣，而到80年代末期时，位于端粒末端的富G序列 能够形成G-四联体结构的这个事实[6-7]以及在1991年时Zahl er[8]与其同伴证实了经K+稳定G-四联体结构具有限制端粒酶活性的功能后，G-四联体就成为了抑制端粒酶的靶点[9-10]。随着对G-四联体研究的不断深入，大量事实证实G-四联体确实存在于活的生物体中[11-14]并且发挥着重要的生物性作用[15]。在真核细胞的基因组中，有很多关键的富G区域有望形成G-四联体，像端粒[16]、免疫球蛋白开关区域[17]、致癌基因启动子区域[18]以及核糖体DNA[19]。G-四联体与小分子配体结合时能够发挥重要作用，如限制端粒酶的活性[20]，干扰端粒的活性，影响端粒的维持[21-22]，进一步影响细胞的生长与增殖[23]，甚至影响癌细胞的发 生与发展。

本文综述了药物及其衍生物作为G-四联体的配体以及显 示其优越的抗肿瘤活性。

1 蒽醌类化合物

蒽醌类化合物广泛存在于蓼科、豆科等众多的植物中。早期因其对肿瘤细胞具有毒性而作为DNA嵌入剂。分子模拟研究表明蒽醌类化合物可能可以通过嵌插 模式与G-四联体相互作用[24]，Sun等[25]于1997首先提出G-四联体的配体BSU-1051，且抑制端粒酶活性的50%的有效浓度（teIIC50）为23μM。随后大量报道经取代反应得到蒽醌类衍生物，其中包括1,4-, 1,5-, 1,8-, 2,6-,和2,7-等，其中的侧链变化趋势是氨基到特异性氨基酸官能团，并且考察了侧链的大小和长度之间的关系以及活性[26-30]。

近年来，研究者发现经肽基取代的蒽醌类衍生物对G-四联体表现出极为卓越的选择性。他们通过加入疏水性残基苯丙氨酸到赖氨酸侧链的2,6 或者 2,7上，表现出的选择性要远远大于双链DNA，并且赖氨酸被苯丙氨酸所取代的蒽醌类衍生物能够在更低的浓度下使得细胞凋亡[31]。

2 喹叨啉类衍生物

喹叨啉于1977年第一次从南非的一种 植株中提取出来[32]，它具有许多优良的性质，包括抗菌、抗疟疾、抗炎症等。SYUIQ-5是一种广为研究的喹叨啉类衍生物，一系列的抗癌研究表明SYUIQ-5具有抑制端粒酶的活性，可缩短端粒的长度，诱导细胞的衰老和生长的停止。另外，这种化合物能够和c-myc启动子区域上的G-四联体相互结合并且在细胞的增殖和衰老的过程中占据重要作用[33]。

双取代以及11号位上取代的喹叨啉衍生物都表现为使得G-四联体更为稳定，以及端粒酶抑制作用，但是它们对G-四联体的选择性与双链DNA相比却显得差强人意。不过11位取代的喹叨啉衍生物表现出了更强的端粒酶活性，其中喹叨啉telIC50＞138μM；11位取代衍生物telIC50= 0.44～12.3μM[34]。另外5-N-甲基化喹叨啉因为正电荷中心与G-四联体的负离子通道之间的相互作用，提高了其与反平行G-四联体结构之间的选择性以及稳定性[35]。

Jixun Dai等[36]通过了解溶液中的喹叨啉与G-四联体（2∶1）复合物的结构，表明药物能够诱导侧翼序列形成一个新的绑定位点，并且强调了堆垛相互作用以及静电相互作用间的重要性。他们首先强调了药物的形状以及2个侧翼碱基在决定药物绑定特异性方面的 重要作用。

3 小檗碱类衍生物

小檗碱是从中药黄连中提取出来的一种药物，在中 医中主要是用来医治湿热症。在早期，小檗碱因为其本身是一种抗生素使得其不是一种良好的抗肿瘤药品，后来因获得了一种混合小檗碱的新药，使其成为 了抗肿瘤药物[37]，引起了广泛关注。小檗碱能够抑制各种肿瘤细胞的生长[38-39]，其中包括乳腺癌细胞[40]、胰腺癌细胞[41]以及胃癌细胞[42-43]。后来，Tsuruo和其同伴发现小檗碱能够抑制端粒酶的活性并且限制端粒的延长，同时也报道能够和G-四联体相互作用。Zhang等[44]发现9-取代小檗碱衍生物对G-四联体结构具有超强的相互结合能力并且能够抑制端粒酶的活性。为了开发更具活性的端粒酶抑制剂，研究者开发了一系列的9-N-取代小檗碱衍生物，研究证实这些衍生物都能够诱导并且稳定c-myc上启动子区域平行G-四联体，还能够抑制c-myc表达[45]。

4 卟啉类衍生物及其相关的类似物

众所周知的卟啉是红细胞中的色素— —亚铁血红素。化合物TMPyP4是最具代 表性的阳离子卟啉类化合物。Hurley课题组对此进行了深入而广泛的研究[46-56]。TMPyP4对G-四联体表现出较强的相互作用（ΔT1/2= 17℃），能够有效抑制端粒酶的活性（IC50=6μM）。但问题是其对于平行以及反平行的G-四联体缺乏明显的选择性[57-58]，甚至它对所有的核酸都具有选择性(包括单链、双链、三链以及四链)[59]。Parkinson等[60]报道了端粒G-四联体与TMPyP4复合物的X单晶衍射结构，表明TMPyP4不直接与G-四分体相互结合。

尽管TMPyP4表现出来的选择性不尽人意， 但是研究者们对其的热情却不见消退，如Gerald等通过ITC、CD以及ESI-MS等方法来考察卟啉经N-甲基-4-吡啶基取代的取代数目(如 5-N-甲基-4-吡啶基卟啉)、有效性以及取代后的结合能的大小，KTMPyP4，KP(5,15)＞ KP(5,10,15)＞ KP(5,10)，KP(5)，其中认为P(5,15)和G-四联体采用的是嵌插模式，而P(5,10,15)和G-四联体采用的是底端堆垛模式[61]。

5 展望

自然界本身存在的药物对新药物的发现以及发展是一笔巨大宝贵的财富。其多态 性和结构的复杂性，使其成为抗肿瘤药物选更具选择性，通过对G-四联体的结构特征以及功能的深入了解，我们期望在未来寻找到更多更好更具针对性的配体。

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