Polyketides form many clinically valuable compounds. However, manipulation of their biosynthesis remains highly challenging. An understanding of gene cluster evolution provides a rationale for reprogramming of the biosynthetic machinery. Herein, we report characterization of giant modular polyketide synthases (PKSs) responsible for the production of aminopolyol polyketides. Heterologous expression of over 150 kbp polyketide gene clusters successfully afforded their products, whose stereochemistry was established by taking advantage of bioinformatic analysis. Furthermore, phylogenetic analysis of highly homologous but functionally diverse domains from the giant PKSs demonstrated the evolutionary mechanism for structural diversification of polyketides. The gene clusters characterized herein, together with their evolutionary insights, are promising genetic building blocks for de novo production of unnatural polyketides.

Di- and sesterterpene synthases produce C20 and C25 isoprenoid scaffolds from geranylgeranyl pyrophosphate (GGPP) and geranylfarnesyl pyrophosphate (GFPP), respectively. By genome mining of the fungus Emericella variecolor, we identified a multitasking chimeric terpene synthase, EvVS, which has terpene cyclase (TC) and prenyltransferase (PT) domains. Heterologous gene expression in Aspergillus oryzae led to the isolation of variediene (1), a novel tricyclic diterpene hydrocarbon. Intriguingly, in vitro reaction with the enzyme afforded the new macrocyclic sesterterpene 2 as a minor product from dimethylallyl pyrophosphate (DMAPP) and isopentenyl pyrophosphate (IPP). The TC domain thus produces the diterpene 1 and the sesterterpene 2 from GGPP and GFPP, respectively. Notably, a domain swap of the PT domain of EvVS with that of another chimeric sesterterpene synthase, EvSS, successfully resulted in the production of 2 in vivo as well. Cyclization mechanisms for the production of these two compounds are proposed.

Sesterterpenoids are a group of terpenoid natural products that are primarily biosynthesized via cyclization of the C25 linear substrate geranylfarnesyl pyrophosphate (GFPP). Although the long carbon chain of GFPP in theory allows for many different cyclization patterns, sesterterpenoids are relatively rare species among terpenoids, suggesting that many intriguing sesterterpenoid scaffolds have been overlooked. Meanwhile, the recent identification of the first sesterterpene synthase has allowed the discovery of new sesterterpenoids by the genome mining approach. In this study, we characterized the unusual fungal sesterterpene synthase EvQS and successfully obtained the sesterterpene quiannulatene (1) with a novel and unique highly congested carbon skeleton, which is further oxidized to quiannulatic acid (2) by the cytochrome P450 Qnn-P450. A mechanistic study of its cyclization from GFPP indicated that the biosynthesis employs an unprecedented cyclization mode, which involves three rounds of hydride shifts and two successive C-C bond migrations to construct the 5-6-5-5-5 fused ring system of 1.

The application of ketoreductase-based biocatalytic reduction to access optically pure Prelog or anti-Prelog alcohols offers a valuable approach for asymmetric synthesis. Despite this, control of the stereopreferences of ketoreductases as desired remains challenging, since natural ketoreductases usually display Prelog preference and it is difficult to transfer the knowledge from engineered anti-Prelog ketoreductases to the others. Here, we present the discovery of a switch between Prelog and anti-Prelog reduction toward halogen-substituted acetophenones in six short-chain dehydrogenase/reductases (SDRs). Through carefully analysis of the structural information and multiple-sequence alignment of several reported SDRs with Prelog or anti-Prelog stereopreference, the key residues that might control their stereopreferences were identified using Lactobacillus fermentum short-chain dehydrogenase/reductase 1 (LfSDR1) as the starting enzyme. Protein engineering at these positions of LfSDR1 could improve its anti-Prelog stereoselectivity or switch its stereopreference to Prelog. Moreover, the knowledge obtained from LfSDR1 could be further transferred to the five other SDRs (four mined SDRs and one reported SDR) that have 21−48% sequence identities with LfSDR1. The stereopreferences of these SDRs were able to be switched either from anti-Prelog to Prelog or from Prelog to anti-Prelog by mutagenesis at related positions. In addition, further optimization of LfSDR1 can access stereocomplementary reduction of several halogen-substituted acetophenones with high stereoselectivity (up to >99%), resulting in some valuable chiral alcohols for the synthesis of pharmaceutical agents.

Enantiopure halohydrins, which are important building blocks for pharmaceutical agents, could be synthesized by biocatalytic reduction of α-halo ketones using ketoreductases. In this study, Candida glabrata ketoreductase 1 (CgKR1) variants with >99% stereoselectivity toward α-halo ketones, such as 2-chloroacetophenone, 2-chloro-4′-fluoroacetophenone, and 2-bromoacetophenone, were obtained through engineering of CgKR1 at residues Phe92 and Tyr208. Interestingly, asymmetric reduction of these α-halo ketones by all the variants of CgKR1 followed anti-Prelog's rule, which is rarely found in natural ketoreductases. Moreover, the biocatalytic processes for reduction of these aromatic α-halo ketones with high substrate loading were achieved by coexpression of glucose dehydrogenase (GDH) for NADPH regeneration, indicating the potential of practical applications of these variants.