FIGURE 13.

The proposed biosynthesis of enniatin, beauvericin and bassianolide. (a) Enniatin, bassianolide and beauvericin are made by di‐modular NRPSs. The C₁ domain is inactive, and the adjacent PCP_2a and PCP_2b domains have redundant functions (b) Free‐standing ketoreductases (KR), unrelated to PKS KRs, generate α‐hydroxy acids from α‐keto acids. (c) Chain expansion proceeds through alternating condensation reactions catalyzed by the C₂ (amide forming) and C_T (ester forming) domains, where all PCP domains can alternate between donor or acceptor roles depending on the context of the catalyzed reaction and the nature of the attached substrate. Amino acid acceptor monomers are attached to PCP_2a/b during C₂ amide bond formation with elongated intermediate donors attached to PCP₁, while hydroxy acid acceptor monomers are always attached to PCP₁ in C_T‐mediated ester bond formation with elongated intermediate donors attached to PCP_2a/b. When the elongated chain reaches a specific length, the C_T domain switches from elongation to cyclization activity through a yet unknown switching mechanism