Structural rearrangements of a polyketide synthase module during its catalytic cycle

摘要

聚酮合酶(PKSs)是生成聚酮（一大类次级代谢产物——换句话说就是天然产物）的多域酶复合物。来自Georgios Skiniotis及同事的两篇论文用低温电子显微镜来研究"委内瑞拉链霉菌"的苦霉素生物合成中所涉及的一个完好无损的全长度多酶PKS模块在不同功能状态下的结构。这些结构显示，酮基合酶、酰基转移酶、酮还原酶和酰基载体蛋白（ACP)域在催化周期中相互作用。在每一种状态，ACP处于不同位置，来促进中间体向下一个催化步骤和下一个模块转移。%The polyketide synthase (PKS) mega-enzyme assembly line uses a modular architecture to synthesize diverse and bioactive natural products that often constitute the core structures or complete chemical entities for many clinically approved therapeutic agents. The architecture of a full-length PKS module from the pikromycin pathway of Streptomyces venezuelae creates a reaction chamber for the intra-module acyl carrier protein (ACP) domain that carries building blocks and intermediates between acyltransferase, ketosynthase and keto-reductase active sites (see accompanying paper). Here we determine electron cryo-microscopy structures of a full-length pikromycin PKS module in three key biochemical states of its catalytic cycle. Each biochemical state was confirmed by bottom-up liquid chromato-graphy/Fourier transform ion cyclotron resonance mass spectrometry. The ACP domain is differentially and precisely positioned after polyketide chain substrate loading on the active site of the ketosynthase, after extension to the β-keto intermediate, and after β-hydroxy product generation. The structures reveal the ACP dynamics for sequential interactions with catalytic domains within the reaction chamber, and for transferring the elongated and processed polyketide substrate to the next module in the PKS pathway. During the enzymatic cycle the ketoreductase domain undergoes dramatic conformational rearrangements that enable optimal positioning for reductive processing of the ACP-bound polyketide chain elongation intermediate. These findings have crucial implications for the design of functional PKS modules, and for the engineering of pathways to generate pharmacologically relevant molecules.