关键词: 天然产物, 聚酮化合物, 聚酮合酶, 延伸单元, 生物合成, 酶工程

Abstract:

Polyketide natural products include over 10,000 molecules with a wide range of bioactivities and are among the most prominent classes of approved clinical agents. Usually, active lead compounds require structural modifications to improve their absorption, distribution, metabolism, and excretion and to facilitate the drug development process. However, due to the large number of stereocenters and inert carbon atoms, it is challenging for chemical synthesis to accurately and efficiently derive the polyketide scaffold, making synthetic biological methods for structural optimization of polyketides a hot topic. In nature, the majority of polyketides are assembled from simple acetate and propionate building blocks by polyketide synthases, while few polyketides with special building blocks provide inspiration for researchers, to set and introduce unnatural building blocks selectively into the polyketides scaffold for structure modification. Polyketides can be built with relatively predictable biosynthetic logic, each module of a modular polyketide synthase elongates the product backbone by two carbons by a concerted action of its three essential domains: ketosynthase, acyltransferase and acyl carrier protein. The acyltransferase domain selects for and loads a carboxyacyl-Coenzyme A extender unit to the phosphopantetheinyl modification of the acyl carrier protein domain, whereas the ketosynthase domain then uses the extender unit to elongate the growing polyketide intermediate, before passing it to the following module. Given the hierarchical domain and module organization of the type I modular PKSs that make these molecules, gene sequence and product structure are directly connected such that changes can be introduced site selectively into the molecule by targeting building blocks and promiscuous acyltransferase domain to the corresponding domain. Besides, the biosynthesis of polyketide scaffolds depends on the assembly of a starter unit and variable extender units, therefore, introducing anticipated structures into the polyketide through the artificial extender units is considered as a powerful breakthrough for precise and efficient modifications of polyketides. This review summarized three important enzymatic synthesis methods for unnatural polyketides extender units reported in the past decade. As results, a large number of unnatural extender units have been obtained through mining novel extender unit synthetase and exploring their substrate scope, or using enzyme engineering methods to modify the substrate range. Also, this review summarized cases of modifying polyketide structure using unnatural extender units, achieving the desired derivatives either through the natural synthetic pathway of polyketides or by utilizing modified synthetic pathways. Finally, we discussed some restrictions existing in this research field and potential directions for better application, including the compatibility issues of polyketides synthase with unnatural extender units, precursor supply for unnatural extender units, and etc. In recent years, interest and enthusiasm for the modification of polyketides using unnatural extender moieties has increased dramatically, and our review draws a concise and clear map for the research of polyketide structure modifications by artificial extender units, hoping to lay a solid foundation for accelerating the efficient development of polyketides drugs.

Key words: Natural products, polyketides, polyketide synthase, extender units, biosynthesis, enzyme engineering