6fik
ACP2 crosslinked to the KS of the loading/condensing region of the CTB1 PKS
Structural highlights
FunctionCTB1_CERNC Polyketide synthase; part of the gene cluster that mediates the biosynthesis of cercosporin, a light-activated, non-host-selective toxin (PubMed:12937958, PubMed:15915645, PubMed:26938470, PubMed:29610486). The perylenequinone chromophore of cercosporin absorbs light energy to attain an electronically-activated triplet state and produces active oxygen species such as the hydroxyl radical, superoxide, hydrogen peroxide or singlet oxygen upon reaction with oxygen molecules (PubMed:11701851). These reactive oxygen species cause damage to various cellular components including lipids, proteins and nucleic acids (PubMed:11701851). The first step of cercosporin biosynthesis is performed by the polyketide synthase CTB1 which catalyzes the formation of nor-toralactone (PubMed:23108075, PubMed:26938470, PubMed:29610486). The starter unit acyltransferase (SAT) domain of CTB1 initiates polyketide extension by the selective utilization of acetyl-CoA, which is elongated to the heptaketide in the beta-ketoacyl synthase (KS) domain by successive condensations with six malonyl units introduced by the malonyl acyltransferase (MAT) domain. The product template (PT) domain catalyzes C4-C9 and C2-C11 aldol cyclizations and dehydrations to a trihydroxynaphthalene, which is thought to be delivered to the thioesterase (TE) domain for product release (PubMed:23108075, PubMed:29610486). The bifunctional enzyme CTB3 then methylates nor-toralactone to toralactone before conducting an unusual oxidative aromatic ring opening (PubMed:17074519, PubMed:26938470). The O-methyltransferase CTB2 further methylates the nascent OH-6 of the CBT3 product, blocking further oxidation at this site before the reductase CTB6 reduces the 2-oxopropyl ketone at position C7, giving naphthalene (PubMed:17660442, PubMed:26938470). The FAD-dependent monooxygenase CTB5 in concert with the multicopper oxidase CTB12 are responsible for homodimerization of naphthalene with CTB7 installing the dioxepine moiety, finally producing cercosporin (PubMed:17660442, PubMed:26938470, PubMed:30809363). The fasciclin domain-containing protein CTB11 might act with CTB5 and CTB12 whereas the roles of CTB9 and CTB10 have still to be elucidated (By similarity).[UniProtKB:Q0UHZ9][1] [2] [3] [4] [5] [6] [7] [8] [9] Publication Abstract from PubMedPolyketide synthases (PKSs) are microbial multienzymes for the biosynthesis of biologically potent secondary metabolites. Polyketide production is initiated by the loading of a starter unit onto an integral acyl carrier protein (ACP) and its subsequent transfer to the ketosynthase (KS). Initial substrate loading is achieved either by multidomain loading modules or by the integration of designated loading domains, such as starter unit acyltransferases (SAT), whose structural integration into PKS remains unresolved. A crystal structure of the loading/condensing region of the nonreducing PKS CTB1 demonstrates the ordered insertion of a pseudodimeric SAT into the condensing region, which is aided by the SAT-KS linker. Cryo-electron microscopy of the post-loading state trapped by mechanism-based crosslinking of ACP to KS reveals asymmetry across the CTB1 loading/-condensing region, in accord with preferential 1:2 binding stoichiometry. These results are critical for re-engineering the loading step in polyketide biosynthesis and support functional relevance of asymmetric conformations of PKSs. The structural organization of substrate loading in iterative polyketide synthases.,Herbst DA, Huitt-Roehl CR, Jakob RP, Kravetz JM, Storm PA, Alley JR, Townsend CA, Maier T Nat Chem Biol. 2018 Apr 2. pii: 10.1038/s41589-018-0026-3. doi:, 10.1038/s41589-018-0026-3. PMID:29610486[10] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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