Zhang Fang, Tsunoda Masaru, Kikuchi Yuji, Wilkinson Oliver, Millington Christopher L, Margison Geoffrey P, Williams David M, Takénaka Akio
Graduate School of Science and Engineering, Iwaki-Meisei University, Iwaki 970-8551, Japan.
Faculty of Pharmacy, Iwaki-Meisei University, Iwaki 970-8551, Japan.
Acta Crystallogr D Biol Crystallogr. 2014 Jun;70(Pt 6):1669-79. doi: 10.1107/S1399004714006178. Epub 2014 May 30.
N-Nitrosation of glycine and its derivatives generates potent alkylating agents that can lead to the formation of O(6)-carboxymethylguanine (O(6)-CMG) in DNA. O(6)-CMG has been identified in DNA derived from human colon tissue and its occurrence has been linked to diets high in red and processed meats, implying an association with the induction of colorectal cancer. By analogy to O(6)-methylguanine, O(6)-CMG is expected to be mutagenic, inducing G-to-A mutations that may be the molecular basis of increased cancer risk. Previously, the crystal structure of the DNA dodecamer d(CGCG[O(6)-CMG]ATTCGCG) has been reported, in which O(6)-CMG forms a Watson-Crick-type pair with thymine similar to the canonical A:T pair. In order to further investigate the versatility of O(6)-CMG in base-pair formation, the structure of the DNA dodecamer d(CGC[O(6)-CMG]AATTTGCG) containing O(6)-CMG at a different position has been determined by X-ray crystallography using four crystal forms obtained under conditions containing different solvent ions (Sr(2+), Ba(2+), Mg(2+), K(+) or Na(+)) with and without Hoechst 33258. The most striking finding is that the pairing modes of O(6)-CMG with T are quite different from those previously reported. In the present dodecamer, the T bases are displaced (wobbled) into the major groove to form a hydrogen bond between the thymine N(3) N-H and the carboxyl group of O(6)-CMG. In addition, a water molecule is bridged through two hydrogen bonds between the thymine O(2) atom and the 2-amino group of O(6)-CMG to stabilize the pairing. These interaction modes commonly occur in the four crystal forms, regardless of the differences in crystallization conditions. The previous and the present results show that O(6)-CMG can form a base pair with T in two alternative modes: the Watson-Crick type and a high-wobble type, the nature of which may depend on the DNA-sequence context.
甘氨酸及其衍生物的N-亚硝化反应会生成强效烷基化剂，这些烷基化剂可导致DNA中形成O(6)-羧甲基鸟嘌呤（O(6)-CMG）。在源自人类结肠组织的DNA中已鉴定出O(6)-CMG，其出现与富含红肉和加工肉类的饮食有关，这意味着它与结直肠癌的诱发存在关联。与O(6)-甲基鸟嘌呤类似，预计O(6)-CMG具有致突变性，会诱导G到A的突变，这可能是癌症风险增加的分子基础。此前，已报道了DNA十二聚体d(CGCG[O(6)-CMG]ATTCGCG)的晶体结构，其中O(6)-CMG与胸腺嘧啶形成了类似于经典A:T对的沃森-克里克型碱基对。为了进一步研究O(6)-CMG在碱基对形成中的多样性，通过X射线晶体学确定了在不同位置含有O(6)-CMG的DNA十二聚体d(CGC[O(6)-CMG]AATTTGCG)的结构，该结构使用了在含有不同溶剂离子（Sr(2+)、Ba(2+)、Mg(2+)、K(+)或Na(+)）且有或没有Hoechst 33258的条件下获得的四种晶体形式。最显著的发现是，O(6)-CMG与T的配对模式与先前报道的有很大不同。在当前的十二聚体中，T碱基位移（摆动）到了大沟中，在胸腺嘧啶N(3)的N-H与O(6)-CMG的羧基之间形成了氢键。此外，一个水分子通过胸腺嘧啶O(2)原子与O(6)-CMG的2-氨基之间的两个氢键桥连，以稳定这种配对。无论结晶条件有何差异，这些相互作用模式在四种晶体形式中普遍存在。先前和当前的结果表明，O(6)-CMG可以通过两种不同模式与T形成碱基对：沃森-克里克型和高摆动型，其性质可能取决于DNA序列背景。
