The hydrolysis of β-lactam antibiotics by class C β-lactamases proceeds through the acylation and the rate-determining deacylation steps mediated by the nucleophilic serine and the deacylation water, respectively. The pose of poor substrates such as carbapenems in the acylated enzyme is responsible for the low efficient deacylation reaction. Here we present the crystal structures of the Y150F variant of the ACC-1 class C β-lactamase in the apo and acylated states. In the acylated enzyme complexed with two carbapenems, imipenem and meropenem, the lactam carbonyl oxygen is located in the oxyanion hole. However, the five-membered pyrroline ring displays a novel orientation that has not been reported so far. The ring is rotated such that its C3 carboxylate makes salt bridges with Lys67 and Ly315, which is accompanied by the side-chain rotamer change of Phe150. The C3 carboxylate is placed where the deacylation water occupies in the apo-enzyme, which, together with the displacement of the catalytic base residue at position 150, explains why carbapenems are poor substrates of ACC-1.

C类β-内酰胺酶水解β-内酰胺类抗生素的过程分别通过亲核丝氨酸和脱酰水介导的酰化和决定速率的脱酰步骤进行。酰化酶中不良的底物（如碳青霉烯）的构成是导致低效脱酰反应的原因。在这里，我们介绍了ACC-1类Cβ-内酰胺酶的Y150F变体在脱辅基和酰化状态下的晶体结构。在与两种碳青霉烯（亚胺培南和美罗培南）复合的酰化酶中，内酰胺羰基氧位于氧阴离子孔中。然而，五元吡咯啉环显示出新颖的取向，迄今为止尚未报道。旋转环，使其C3羧酸盐与Lys67和Ly315形成盐桥，伴随着Phe150侧链旋转异构体的变化。将C3羧酸盐放置在脱辅酶中脱酰水所处的位置，再加上催化碱基残基在位置150的置换，解释了碳青霉烯为什么是ACC-1的不良底物。