Biosynthesis of Athmu, a α,γ-hydroxy-β-amino acid of pahayokolides A–B

Pahayokolides A–B are cyanobacteria derived non-ribosomal peptides which exhibit cytotoxicity against a number of cancer cell lines. The biosynthetic origin of the 3-amino-2,5,7,8-tetrahydroxy-10-methylundecanoic acid (Athmu) moiety has been investigated using stable isotope incorporation experiments. While α-ketoisocaproic acid (α-KIC), α-hydroxyisocaproic acid (α-HIC), and leucine all serve as precursors to Athmu, the feeding of [1-13C] α-KIC results in more than threefold greater 13C enrichment than the other precursors. This result suggests that α-KIC is the immediate precursor which is selected and activated by the adenylation domain of the loading NRPS module and subsequently reduced in a fashion similar to that of the recently identified pathways for cryptophycins A–B, cereulide, and valinomycin.