Recently, significant progress was made on polyketide synthase (PKS) biosynthesis by Professor Sun Yuhui’s research group from the team of CAS member Deng Zixin at the School of Pharmaceutical Sciences, Wuhan University (WHU). The research revealed that double bond reduction of the polyketide chain in PKS domain could be achieved by a novel cross-module “borrowing” method, which had been published on Angewandte Chemie International Edition.
The two co-first authors are Zhai Guifa, a doctoral candidate from the School of Pharmaceutical Sciences, and Wang Wenyan, a doctoral candidate from the College of Chemistry and Molecular Sciences. The corresponding author is Professor Sun Yuhui. The research was supported by National Key R&D Program of China and the Open Funding Project from State Key Laboratory of Microbial Metabolism.
Polyketides are a kind of important natural products produced by continuous condensation reactions of bacteria, fungi and plants in nature. Due to the diversity in their structures and biological activities, they have been widely applied in medicine, animal husbandry and agriculture and provided support for human health and economic industries. PKS plays a core role in catalyzing the diverse polyketides biosynthesis. A set of strict, delicate and effective structures and processes has been established in the long history of natural evolution. Type I PKS, which is understood relatively thoroughly, consists of one or more modules catalyzing the first round of carbon chain elongation, while in each module, there are several domains to carry out the distinct catalysis process.
Generally, in the process of synthesizing the polyketide carbon chain, there is strict collinearity between the number and function of PKS modules and domains and the corresponding chemical structure catalyzed by them. Namely, the selection of the substrate for polyketide chain synthesis, the degree of reduction, and the stereochemical configuration of the product are all determined by the structural domains in the corresponding module on the PKS. By selectively reducing and dehydrating the ketone, the functional groups such as ketone group, hydroxyl group, and double bond are formed at the corresponding position of the final products. The stereochemical configuration of the chiral center of the product is also determined in this process. This phenomenon was found in erythromycin PKS firstly in the early 1990s, which manifested great significance. It suggested great plasticity in the structure of polyketides compound which can be artificially designed through PKS modules or domains to reorganize new biosynthetic pathways and non-natural “natural” products that meet people’s expectations. This is exactly why PKS attracts people to conduct in-depth research of it.
Azalomycin F produced by the Streptomyces sp.211726 in the mangroves of Hainan Province belongs to one type of natural polyketide products, which presents excellent antifungal activity. The discovery of anti-tumor activity further stimulates people’s strong interest in its biosynthesis research. In the early stage of Sun Yuhui’s group’s research, it was discovered that there was an abnormal phenomenon in the seemingly ordinary molecular structure of azamycin, which was contrary to the above-mentioned canonical collinearity in structure and function. They seized this opportunity and discovered an auto-switchable Enoylreductase (ER) domains hidden in PKS by the systematical use of molecular genetics and biochemical research methods for the first time. Namely, in an iterative PKS module, the same ER domain could be turned off or on during the two polyketide carbon chain elongation processes as needed, like a switch, thereby catalyzing different structures. (Angew. Chem.Int.Ed.2017,56:5503-5506). Nevertheless, the absence of the ER domain in the corresponding structure in the adjacent module is still a mystery. To solve this problem, the authors first proved that the ER1/2 domain in Module 1/2 got involved in the reduction process of the intermediate double bond of Module 3 polyketide chain by in vivo genetic methods. In order to further verify the unique catalytic phenomenon of the ER1/2 domain, the authors reconstructed the extension of the Module 3 polyketide chain and the reduction process of the intermediate double bond in vitro. Finally, it was proved that the ER1/2 domain not only selectively catalyzed the double bond but also catalyzed the reduction of the intermediate double bond of the polyketide chain on the adjacent Module 3 in trans.
In addition, Sun Yuhui’s research group also conducted domain splitting and in vitro activity tests on Module 1/2 to have a closer examination of the role of other domains in Module 1/2 in the “borrowing” of the ER1/2 domain. By comparing and analyzing the catalytic activity of different combinations of ER1/2 domains, it was proved that ER1/2 and Ketoreductase (KR) 1/2 domains are the minimum functional units catalyzing the double intermediate bond of Module 3 polyketide chain reduction in trans. It also implied that the KR1/2 domain might exert certain auxiliary influences on the spatial structure of the “borrowing” function of the ER1/2 domain.
Although the non-collinearity between the function of PKS and the product structure is rarely discovered, this novel and unique cross-module “borrowing” method between adjacent catalytic moduless still surprised us at the uncanny craftsmanship of nature. This discovery not only further expands people’s understanding of canonical collinearity modular of PKS, but also enriches the tool box of polyketide combinatorial biosynthesis. It is expected to provide new inspiration for the innovation of polyketide drugs.
Rewritten by: Cao Mi
Edited by: Qin Zichang and Hu Sijia
Source: https://onlinelibrary.wiley.com/doi/10.1002/anie.202011357
