How lovastatin, the multi-billion sold cholesterol-lowing drug, is formed by the LovBC megaenzyme is critical for the understanding of its biosynthetic mechanism. In a recent paper published in Nature Communications, Prof. Zixin DENG’s lab determined the cryo-EM structures of LovB-LovC complex and the core LovB megasynthase, providing structural insights into the catalytic cycles underlying lovastatin biosynthesis.
The cholesterol-lowering drug lovastatin is formed from dihydromonacolin L (DML) catalyzed by lovastatin nonaketide synthase (LovB), with the assistance of a separate trans-acting enoyl reductase (LovC). A full DML synthesis comprises 8 polyketide synthetic cycles with about 35 steps. The structural basis for the iterative and yet permutative functions of the megasynthase has remained a mystery. Particularly the “gate-keeping” LovC plays a key role in ensuring proper programming, yet the site of interaction between LovC and the LovB PKS is unknown.
In the publication, the researchers presented the Cryo-EM structure of the LovB-LovC complex with the core LovB at 2.91 Å resolution. The domain organization of LovB is an “X”-shaped face to face dimer containing eight connected domains. The KS and MAT domains are connected by the linker domain (LD), with the separate ER (LovC) domain interacting with the MAT domain (Fig. 1). The MAT domain is linked to the upper region, which begins with the DH domain, followed by the intact CMeT domain, protruding from the relatively planar body of the whole complex. Then, the truncated ψKR domain is linked to the nonfunctional ψER domain, and the connected KR domain finally completes the upper tailoring region.
Fig. 1 Overall architecture of the LovBC complex.
In the biosynthesis of DML, LovC functions as a “gate-keeper”, and such trans-acting ERs have been reported across various fungal PKS enzymes as a tactic in nature to diversify polyketides. The LovBC interacting residues is precisely described through computational docking analysis, and the mutational analysis of the LovB–C interface confirms the essential role of the catalytic chamber integrity for the production of DML (Fig. 2).
Fig. 2 Interaction between LovB and LovC is essential for the synthesis of DML.
Together, this architecture confirms the dimeric catalytic mode of type I iterative PKS, and the structural details of the megaenzyme provide the basis for the processing of the intermediates by the individual domains in a stereoselective and structural specific way. The comprehensive architectural model enables the re-engineering of the megaenzyme for the generation of new statins.
This work was financially supported by National Key R&D Program of China (2018YFA0900700, 2019YFA0905400), the Ministry of Science and Technology (2015CB554203), the National Science Foundation of China (91753123, 31470830, 21661140002).