Structural highlights
Publication Abstract from PubMedNatural product biosynthetic pathways contain a plethora of enzymatic tools to carry out difficult biosynthetic transformations. Here, we discover an unusual mononuclear iron-dependent methyltransferase that acts in the initiation steps of apratoxin A biosynthesis (AprA MT1). Fe3+-replete AprA MT1 catalyzes one or two methyl transfer reactions on the substrate malonyl-ACP (acyl carrier protein), whereas Co2+, Fe2+, Mn2+, and Ni2+ support only a single methyl transfer. MT1 homologues exist within the "GNAT" (GCN5-related N-acetyltransferase) loading modules of several modular biosynthetic pathways with propionyl, isobutyryl, or pivaloyl starter units. GNAT domains are thought to catalyze decarboxylation of malonyl-CoA and acetyl transfer to a carrier protein. In AprA, the GNAT domain lacks both decarboxylation and acyl transfer activity. A crystal structure of the AprA MT1-GNAT di-domain with bound Mn2+, malonate, and the methyl donor S-adenosylmethionine (SAM) reveals that the malonyl substrate is a bidentate metal ligand, indicating that the metal acts as a Lewis acid to promote methylation of the malonyl alpha-carbon. The GNAT domain is truncated relative to functional homologues. These results afford an expanded understanding of MT1-GNAT structure and activity and permit the functional annotation of homologous GNAT loading modules both with and without methyltransferases, additionally revealing their rapid evolutionary adaptation in different biosynthetic contexts. A Mononuclear Iron-Dependent Methyltransferase Catalyzes Initial Steps in Assembly of the Apratoxin A Polyketide Starter Unit.,Skiba MA, Sikkema AP, Moss NA, Tran CL, Sturgis RM, Gerwick L, Gerwick WH, Sherman DH, Smith JL ACS Chem Biol. 2017 Nov 14. doi: 10.1021/acschembio.7b00746. PMID:29096064[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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