MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 0QH, UK.
Nature. 2011 May 18;474(7352):521-5. doi: 10.1038/nature10136.
Adenosine receptors and β-adrenoceptors are G-protein-coupled receptors (GPCRs) that activate intracellular G proteins on binding the agonists adenosine or noradrenaline, respectively. GPCRs have similar structures consisting of seven transmembrane helices that contain well-conserved sequence motifs, indicating that they are probably activated by a common mechanism. Recent structures of β-adrenoceptors highlight residues in transmembrane region 5 that initially bind specifically to agonists rather than to antagonists, indicating that these residues have an important role in agonist-induced activation of receptors. Here we present two crystal structures of the thermostabilized human adenosine A(2A) receptor (A(2A)R-GL31) bound to its endogenous agonist adenosine and the synthetic agonist NECA. The structures represent an intermediate conformation between the inactive and active states, because they share all the features of GPCRs that are thought to be in a fully activated state, except that the cytoplasmic end of transmembrane helix 6 partially occludes the G-protein-binding site. The adenine substituent of the agonists binds in a similar fashion to the chemically related region of the inverse agonist ZM241385 (ref. 8). Both agonists contain a ribose group, not found in ZM241385, which extends deep into the ligand-binding pocket where it makes polar interactions with conserved residues in H7 (Ser 277(7.42) and His 278(7.43); superscripts refer to Ballesteros-Weinstein numbering) and non-polar interactions with residues in H3. In contrast, the inverse agonist ZM241385 does not interact with any of these residues and comparison with the agonist-bound structures indicates that ZM241385 sterically prevents the conformational change in H5 and therefore it acts as an inverse agonist. Comparison of the agonist-bound structures of A(2A)R with the agonist-bound structures of β-adrenoceptors indicates that the contraction of the ligand-binding pocket caused by the inward motion of helices 3, 5 and 7 may be a common feature in the activation of all GPCRs.
腺苷受体和β-肾上腺素能受体是 G 蛋白偶联受体 (GPCR)，分别在与激动剂腺苷或去甲肾上腺素结合时激活细胞内 G 蛋白。GPCR 具有相似的结构，由七个跨膜螺旋组成，其中包含高度保守的序列基序，表明它们可能通过共同的机制激活。最近β-肾上腺素能受体的结构突出了跨膜区域 5 中的残基，这些残基最初与激动剂而不是拮抗剂特异性结合，表明这些残基在激动剂诱导的受体激活中具有重要作用。在这里，我们展示了两种热稳定的人腺苷 A(2A)受体 (A(2A)R-GL31)与内源性激动剂腺苷和合成激动剂 NECA 结合的晶体结构。这些结构代表了无活性和活性状态之间的中间构象，因为它们共享所有被认为处于完全激活状态的 GPCR 特征，只是跨膜螺旋 6 的细胞质末端部分阻塞了 G 蛋白结合位点。激动剂的腺嘌呤取代基以与化学相关的反向激动剂 ZM241385 的区域相似的方式结合 (参考文献 8)。两种激动剂都含有核糖基，而 ZM241385 中没有，它延伸到配体结合口袋深处，与 H7 中的保守残基 (Ser277(7.42)和 His278(7.43); 上标指的是 Ballesteros-Weinstein 编号)形成极性相互作用，并与 H3 中的残基形成非极性相互作用。相比之下，反向激动剂 ZM241385 与这些残基没有相互作用，与激动剂结合结构的比较表明 ZM241385 从空间上阻止了 H5 的构象变化，因此它作为反向激动剂起作用。与β-肾上腺素能受体的激动剂结合结构的比较表明，由螺旋 3、5 和 7 的内向运动引起的配体结合口袋的收缩可能是所有 GPCR 激活的共同特征。
