title: Starter unit flexibility for engineered product synthesis by the nonreducing polyketide synthase PksA.
abstract: Nonreducing polyketide synthases (NR-PKSs) are unique among PKSs in their domain structure, notably including a starter unit:acyl-carrier protein (ACP) transacylase (SAT) domain that selects an acyl group as the primer for biosynthesis, most commonly acetyl-CoA from central metabolism. This clan of mega-enzymes resembles fatty acid synthases (FASs) by sharing both their central chain elongation steps and their capacity for iterative catalysis. In this mode of synthesis, catalytic domains involved in chain extension exhibit substrate plasticity to accommodate growing chains as small as two carbons to 20 or more. PksA is the NR-PKS central to the biosynthesis of the mycotoxin aflatoxin B1 whose SAT domain accepts an unusual hexanoyl starter from a dedicated yeast-like FAS. Explored in this paper is the ability of PksA to utilize a selection of potential starter units as substrates to initiate and sustain extension and cyclization to on-target, programmed polyketide synthesis. Most of these starter units were successfully accepted and properly processed by PksA to achieve biosynthesis of the predicted naphthopyrone product. Analysis of the on-target and derailment products revealed trends of tolerance by individual PksA domains to alternative starter units. In addition, natural and un-natural variants of the active site cysteine were examined and found to be capable of biosynthesis, suggesting possible direct loading of starter units onto the β-ketoacyl synthase (KS) domain. In light of the data assembled here, the predictable synthesis of unnatural products by NR-PKSs is more fully defined.
authors.affiliation: Department of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, United States.
authors.forename: Callie R
authors.initials: CR
authors.lastname: Huitt-Roehl
authors.affiliation: Department of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, United States.
authors.forename: Eric A
authors.initials: EA
authors.lastname: Hill
authors.affiliation: Department of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, United States.
authors.forename: Martina M
authors.initials: MM
authors.lastname: Adams
authors.affiliation: Department of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, United States.
authors.forename: Anna L
authors.initials: AL
authors.lastname: Vagstad
authors.affiliation: Department of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, United States.
authors.forename: Jesse W
authors.initials: JW
authors.lastname: Li
authors.affiliation: Department of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, United States.
authors.forename: Craig A
authors.initials: CA
authors.lastname: Townsend
mesh_terms: D000105:Acetyl Coenzyme A / Q000737:chemistry / Q000378:metabolism; D016604:Aflatoxin B1 / Q000096:biosynthesis / Q000737:chemistry; D001230:Aspergillus / Q000737:chemistry / Q000201:enzymology* / Q000235:genetics; D020134:Catalytic Domain; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D005656:Fungal Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D007700:Kinetics; D060847:Metabolic Engineering; D009281:Naphthalenes / Q000737:chemistry / Q000378:metabolism; D048630:Polyketide Synthases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D061065:Polyketides / Q000737:chemistry* / Q000378:metabolism; D011753:Pyrones / Q000737:chemistry / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D005656:Fungal Proteins; D009281:Naphthalenes; D061065:Polyketides; D011753:Pyrones; D011994:Recombinant Proteins; D000105:Acetyl Coenzyme A; D048630:Polyketide Synthases; D016604:Aflatoxin B1