Structures of free and complexed forms of Escherichia coli xanthine-guanine phosphoribosyltransferase
Structures of free and complexed forms of Escherichia coli xanthine-guanine phosphoribosyltransferase
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DOI：
10.1006/jmbi.1998.2051
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发表时间：
1998-10-02
影响因子：
5.6
通讯作者：
Martin, JL
中科院分区：
文献类型：
--
作者：
Vos, S;Parry, RJ;Martin, JL
大肠杆菌黄嘌呤鸟嘌呤磷酸核糖转移酶 (XGPRT) 的游离形式、底物结合形式和产物结合形式的结构已通过 X 射线晶体学确定。将这些结果与先前确定的镁和硫酸盐结合 XPRT 的结构进行比较。分辨率为 2.25 埃的游离 XGPRT 结构证实了其他 PRT 酶中鉴定的移动环内部和周围残基的灵活性，并表明在不存在结合配体的情况下，活性位点 Arg37 的顺式肽构象得以维持。 XGPRT 与嘌呤碱基底物鸟嘌呤或黄嘌呤以及 cPRib-PP（第二底物 PRib-PP 的类似物）复合的结构已解析至 2.0 埃分辨率。在这两种结构中，未复合的 XGPRT 的无序磷酸盐结合环通过与 cPRib-PP 的 5'-磷酸基团相互作用而变得有序。 cPRib-PP 的环戊烷环具有 C3 外褶皱构象，由 cPRib-PP 结合的 Mg2+ 稳定。 XGPRT 的嘌呤碱基特异性似乎是由于鸟嘌呤或黄嘌呤的 2-环外基团与 Glu136 和 Asp140 的侧链以及 Ile135 的主链氧原子之间的水介导的相互作用。 Asp92 与 Lys115 一起可以帮助稳定引入碱基的 N7 质子化互变异构体，并且可以充当通用碱基，以在形成核苷酸产物时从 N7 中除去质子。 XGPRT 与产物 GMP 复合的 2.6 埃分辨率结构与底物结合复合物相似。然而，与cPRib-PP中的同等环相比，GMP的核糖环旋转了大约24度。这种旋转导致产物复合物中核糖基和酶之间的所有相互作用丧失。 (C) 1998 年学术出版社。
Structures of free, substrate-bound and product-bound forms of Escherichia coli xanthine-guanine phosphoribosyltransferase (XGPRT) have been determined by X-ray crystallography. These are compared with the previously determined structure of magnesium and sulphate-bound XPRT. The structure of free XGPRT at 2.25 Angstrom resolution confirms the flexibility of residues in and around a mobile loop identified in other PRTases and shows that the cis-peptide conformation of Arg37 at the active site is maintained in the absence of bound ligands. The structures of XGPRT complexed with the purine base substrates guanine or xanthine in combination with cPRib-PP, an analog of the second substrate PRib-PP, have been solved to 2.0 Angstrom resolution. In these two structures the disordered phosphate-binding loop of uncomplexed XGPRT becomes ordered through interactions with the 5'-phosphate group of cPRib-PP. The cyclopentane ring of cPRib-PP has the C3 exo pucker conformation, stabilised by the cPRib-PP-bound Mg2+. The purine base specificity of XGPRT appears to be due to water-mediated interactions between the 2-exocyclic groups of guanine or xanthine and side-chains of Glu136 and Asp140, as well as the main-chain oxygen atom of Ile135. Asp92, together with Lys115, could help stabilise the N7-protonated tautomer of the incoming base and could act as a general base to remove the proton from N7 .when the nucleotide product is formed. The 2.6 Angstrom resolution structure of XGPRT complexed with product GMP is similar to the substrate-bound complexes. However, the ribose ring of GMP is rotated by similar to 24 degrees compared with the equivalent ring in cPRib-PP. This rotation results in the loss of all interactions between the ribosyl group and the enzyme in the product complex. (C) 1998 Academic Press.