Functional Role of AveC Residues Ser138 and Ala139 for Avermectin and Doramectin Biosynthesis in Streptomyces avermitilis

Background: Doramectin (CHC-B1) is an excellent antiparasitic drug produced by feeding cyclohexanecarboxylic acid (CHC) to Streptomycesavermitilis bkd− mutants. AveC, a bifunctional enzyme encoded by aveC (sav_0940), catalyzes the stereospecific spiroketalization and selective dehydration of dihydroxy ketone polyketide intermediates and modulates both the yield and the proportion of avermectin/doramectin in Streptomyces avermitilis. In our previous work, we constructed a strain harboring a synthetic aveC* gene encoding ten amino acid mutations, which produced nearly pure doramectin. However, the doramectin yield achieved only approximately 60% of the total doramectin and CHC-B2 output observed in the parental strain. Methods: To investigate the roles of Ser138 and Ala139 of AveC in the biosynthesis of doramectin and avermectin, site-directed mutagenesis was performed at both sites. The production and proportion of avermectin and doramectin were determined using high-performance liquid chromatography (HPLC). AlphaFold2-based molecular docking simulations were used to interpret the results. Results: Among the tested mutants, S138G, S138T, and A139H exhibited the highest doramectin production, achieving 143.87%, 151.22%, and 153.36% of the control level, respectively. Unfortunately, almost none of the tested mutants showed a positive effect on avermectin production. Molecular docking simulations revealed distinct affinities of these mutants for the dihydroxy ketone polyketide intermediate, both with and without a cyclohexyl group. Notably, all three mutants displayed larger substrate-binding cavity volumes compared with the wild-type enzyme, which likely facilitates doramectin synthesis by effectively accommodating the cyclohexyl moiety. Docking results further indicated that Ser138 and Ala139 are positioned within the binding cavity but probably do not directly participate in the dehydration activity. Conclusions: These findings suggest that optimizing cavity size through residue substitutions can enhance substrate specificity for doramectin production while preserving catalytic functionality.