Extended Data Fig. 10: Proposed active site mechanism of SalC.
From: Enzymatic assembly of the salinosporamide γ-lactam-β-lactone anticancer warhead
Catalysis is initiated by deprotonation of Cys180 followed by transacylation of the SalB-PCP tethered substrate through a tetrahedral intermediate (not shown). Lys348 deprotonates His353, and hydrogen bonding of the thioamide carbonyl to Tyr284 facilitates deprotonation of the thioester α-proton by His353. An intramolecular aldol reaction forms the γ-lactam; the oxyanion is presumably stabilized by dipole interactions with backbone amides, as is hypothesized for KSs25. Subsequent β-lactonization through a tetrahedral intermediate leads to release of simplisporamide from SalC. Finally, Cys180 is reprotonated by His353.
