Formation and Loading of a (2S)-2-Ethylmalonamyl Starter Unit in the Assembly Line of Polyketide-Nonribosomal Peptide Hybrid Sanglifehrin A

Sanglifehrin A (SFA) is a spirolactam-conjugated, 22-membered macrolide with remarkable immunosuppressive and antiviral activities. This macrolide is a result of a hybrid polyketide synthase (PKS)-nonribosomal peptide synthetase (NRPS) assembly line that utilizes (2S)-2-ethylmalonamyl as a starter unit. Here, we report that the formation and loading of this starter unit in the SFA assembly line involve two unusual enzymatic reactions that occur on a discrete acyl carrier protein (ACP), SfaO. An amide synthetase, SfaP, catalyzes the amidation of (2S)-2-ethylmalonyl in a SfaO-dependent manner. Then, a beta-ketoacyl-ACP synthase III-like protein, SfaN, transfers resultant (2S)-2-ethylmalonamyl from SfaO onto the loading ACP domain of the hybrid PKS-NRPS assembly line to prime SFA biosynthesis. Both SfaP and SfaN display promiscuous activities. This study furthers the appreciation of assembly line chemistry, as a new paradigm for unusual building block formation and incorporation is provided.