Compared to protein enzymes, our knowledge about how RNA accelerates chemical reactions is rather limited. The crystal structures of a ribozyme that catalyzes Diels–Alder reactions suggest a rich tertiary architecture responsible for catalysis. In this study, we systematically probe the relevance of crystallographically observed ground-state interactions for catalytic function using atomic mutagenesis in combination with various analytical techniques. The largest energetic contribution apparently arises from the precise shape complementarity between transition state and catalytic pocket: A single point mutant that folds correctly into the tertiary structure but lacks one H-bond that normally stabilizes the pocket is completely inactive. In the rate-limiting chemical step, the dienophile is furthermore activated by two weak H-bonds that contribute ∼7–8 kJ/mol to transition state stabilization, as indicated by the 25-fold slower reaction rates of deletion mutants. These H-bonds are also responsible for the tight binding of the Diels–Alder product by the ribozyme that causes product inhibition. For high catalytic activity, the ribozyme requires a fine-tuned balance between rigidity and flexibility that is determined by the combined action of one inter-strand H-bond and one magnesium ion. A sharp 360° turn reminiscent of the T-loop motif observed in tRNA is found to be important for catalytic function.
与蛋白质酶相比,我们对 RNA 如何加速
化学反应的了解相当有限。一种能催化 Diels-Alder 反应的
核糖酶的晶体结构表明,其丰富的三级结构对催化起着重要作用。在本研究中,我们利用原子诱变技术结合各种分析技术,系统地探究了晶体学观察到的基态相互作用与催化功能的相关性。最大的能量贡献显然来自于过渡态和催化袋之间精确的形状互补:单点突变体能正确折叠成三级结构,但缺少一个通常能稳定口袋的氢键,因此完全失去活性。在限速
化学步骤中,亲二烯进一步被两个弱氢键激活,这两个氢键对稳定过渡态的作用为 7-8 kJ/mol,缺失突变体的反应速率要慢 25 倍。这些 H 键也是
核糖酶与 Diels-Alder 产物紧密结合并导致产物抑制的原因。为了获得高催化活性,
核糖酶需要在刚性和柔性之间取得微妙的平衡,这种平衡由一个链间 H 键和一个
镁离子的共同作用决定。与 tRNA 中的 T 环图案相似的 360°急转弯对催化功能非常重要。