Kleanthous C, Kühlmann U C, Pommer A J, Ferguson N, Radford S E, Moore G R, James R, Hemmings A M
School of Biological Sciences, University of East Anglia, Norwich, UK.
Nat Struct Biol. 1999 Mar;6(3):243-52. doi: 10.1038/6683.
The crystal structure of the cytotoxic endonuclease domain from the bacterial toxin colicin E9 in complex with its cognate immunity protein Im9 reveals that the inhibitor does not bind at the active site, the core of which comprises the HNH motif found in intron-encoded homing endonucleases, but rather at an adjacent position leaving the active site exposed yet unable to bind DNA because of steric and electrostatic clashes with incoming substrate. Although its mode of action is unorthodox, Im9 is a remarkably effective inhibitor since it folds within milliseconds and then associates with its target endonuclease at the rate of diffusion to form an inactive complex with sub-femtomolar binding affinity. This hyperefficient mechanism of inhibition could be well suited to other toxic enzyme systems, particularly where the substrate is a polymer extending beyond the boundaries of the active site.
细菌毒素大肠杆菌素E9的细胞毒性内切核酸酶结构域与其同源免疫蛋白Im9形成的复合物的晶体结构表明，抑制剂并不结合在活性位点，活性位点的核心包含内含子编码的归巢内切核酸酶中发现的HNH基序，而是结合在相邻位置，使活性位点暴露，但由于与进入的底物存在空间位阻和静电冲突而无法结合DNA。尽管其作用方式非传统，但Im9是一种非常有效的抑制剂，因为它在几毫秒内折叠，然后以扩散速率与其靶标内切核酸酶结合，形成具有亚飞摩尔结合亲和力的无活性复合物。这种高效的抑制机制可能非常适合其他有毒酶系统，特别是当底物是延伸到活性位点边界之外的聚合物时。
