The lyngbyatoxin biosynthetic assembly line: chain release by four-electron reduction of a dipeptidyl thioester to the corresponding alcohol.

In comparison with the large number of nonribosomal peptide synthetases (NRPSs) that release their peptide products by hydrolytic cleavage of the peptide carrier protein (PCP) bound thioester, there are relatively few NRPSs that have been shown to use a nicotinamide cofactor to reduce this PCP-peptidyl thioester to an aldehyde or imine moiety. This work describes the first example of a reductase domain within a NRPS scaffold shown to reduce a PCP-peptidyl thioester to the corresponding primary alcohol, via an aldehyde intermediate, using two equivalents of reduced nicotinamide adenine dinucleotide phosphate (NADPH). By employing a ketone mimic of the aldehyde intermediate, as well as a specifically deuterated NADPH, it was further demonstrated that the pro-S hydride of the cofactor is transferred to the re face of the carbonyl group.