The activation loop (A-loop) plays a key role in regulating the catalytic activity of protein kinases. Phosphorylation in this region enhances the phosphoryl transfer rate of the kinase domain and increases its affinity for ATP. Furthermore, the A-loop possesses autoinhibitory functions in some kinases, where it collapses onto the protein surface and blocks substrate binding when unphosphorylated. Due to its flexible nature, the A-loop is usually disordered and untraceable in kinase domain crystal structures. The resulting lack of structural information is regrettable as it impedes the design of drug A-loop contacts, which have proven favourable in multiple cases. Here, we characterize the binding with A-loop engagement between type 1.5 kinase inhibitor ‘example 172’ (EX172) and Mer tyrosine kinase (MerTK). With the help of crystal structures and binding kinetics, we portray how the recruitment of the A-loop elicits a two-step binding mechanism which results in a drug-target complex characterized by high affinity and long residence time. In addition, the type 1.5 compound possesses excellent kinome selectivity and a remarkable preference for the phosphorylated over the dephosphorylated form of MerTK. We discuss these unique characteristics in the context of known type 1 and type 2 inhibitors and highlight opportunities for future kinase inhibitor design.

中文翻译：

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Mer酪氨酸激酶中的A环相互作用产生具有两步机理和长结合时间的抑制剂

激活环（A环）在调节蛋白激酶的催化活性中起关键作用。该区域中的磷酸化增强了激酶结构域的磷酸化转移速率，并增加了其对ATP的亲和力。此外，A环在某些激酶中具有自抑制功能，在此激酶折叠到蛋白质表面并在未磷酸化时阻断底物结合。由于其柔韧性，A环在激酶结构域晶体结构中通常是无序的且不可追踪的。令人遗憾的是，由此导致的结构信息不足，因为它阻碍了药物A环触点的设计，事实证明在多种情况下这种设计都是有利的。在这里，我们表征1.5型激酶抑制剂“实例172”（EX172）和酪氨酸激酶（MerTK）之间具有A环参与的结合。借助晶体结构和结合动力学，我们描绘了A环的募集如何引发两步结合机制，从而形成了具有高亲和力和长停留时间的药物-靶标复合物。此外，1.5型化合物具有优异的激酶组选择性，并且相对于去磷酸化形式的MerTK而言，磷酸化具有明显的优势。我们在已知的1型和2型抑制剂的背景下讨论了这些独特的特征，并强调了未来激酶抑制剂设计的机会。5种化合物具有优异的激酶组选择性，并且相对于去磷酸化形式的MerTK而言，磷酸化具有明显的优势。我们在已知的1型和2型抑制剂的背景下讨论了这些独特的特征，并强调了未来激酶抑制剂设计的机会。5种化合物具有优异的激酶组选择性，并且相对于去磷酸化形式的MerTK而言，磷酸化具有明显的优势。我们在已知的1型和2型抑制剂的背景下讨论了这些独特的特征，并强调了未来激酶抑制剂设计的机会。