8cg4

Structural highlights

Function

AFLC_ASPPU Norsolorinic acid synthase; part of the gene cluster that mediates the biosynthesis of aflatoxins, a group of polyketide-derived furanocoumarins, and part of the most toxic and carcinogenic compounds among the known mycotoxins (PubMed:7592391, PubMed:15094053, PubMed:7565588, PubMed:15006741, PubMed:17086560, PubMed:18403714). The four major aflatoxins produced by A.parasiticus are aflatoxin B1 (AFB1), aflatoxin B2 (AFB2), aflatoxin G1 (AFG1) and aflatoxin G2 (AFG2) (PubMed:15006741). Within the aflatoxin pathway, the norsolorinic acid synthase aflC combines a hexanoyl starter unit provided to the acyl-carrier protein (ACP) domain by the fungal fatty acid synthase aflA/aflB, and 7 malonyl-CoA extender units to synthesize the precursor norsolorinic acid (NOR) (PubMed:17086560, PubMed:18403714). The biosynthesis of aflatoxins begins with the norsolorinic acid synthase aflC that combines a hexanoyl starter unit produced by the fatty acid synthase aflA/aflB and 7 malonyl-CoA extender units to synthesize the precursor NOR. The second step is the conversion of NOR to averantin (AVN) and requires the norsolorinic acid ketoreductase aflD, which catalyzes the dehydration of norsolorinic acid to form (1'S)-averantin. The norsolorinic acid reductases aflE and aflF may also play a role in the conversion of NOR to AVN. The cytochrome P450 monooxygenase aflG then catalyzes the hydroxylation of AVN to 5'hydroxyaverantin (HAVN). The next step is performed by the 5'-hydroxyaverantin dehydrogenase aflH that transforms HAVN to 5'-oxoaverantin (OAVN) which is further converted to averufin (AVF) by aflK that plays a dual role in the pathway, as a 5'-oxoaverantin cyclase that mediates conversion of 5'-oxoaverantin, as well as a versicolorin B synthase in a later step in the pathway. The averufin oxidase aflI catalyzes the conversion of AVF to versiconal hemiacetal acetate (VHA). VHA is then the substrate for the versiconal hemiacetal acetate esterase aflJ to yield versiconal (VAL). Versicolorin B synthase aflK then converts VAL to versicolorin B (VERB) by closing the bisfuran ring of aflatoxin which is required for DNA-binding, thus giving to aflatoxin its activity as a mutagen. Then, the activity of the versicolorin B desaturase aflL leads to versicolorin A (VERA). A branch point starts from VERB since it can also be converted to dihydrodemethylsterigmatocystin (DMDHST), probably also by aflL, VERA being a precursor for aflatoxins B1 and G1, and DMDHST for aflatoxins B2 and G2. Next, the versicolorin reductase aflM and the cytochrome P450 monooxygenase aflN are involved in conversion of VERA to demethylsterigmatocystin (DMST). AflX and aflY seem also involved in this step, through probable aflX-mediated epoxide ring-opening step following versicolorin A oxidation and aflY-mediated Baeyer-Villiger oxidation required for the formation of the xanthone ring. The methyltransferase aflO then leads to the modification of DMST to sterigmatocystin (ST), and of DMDHST to dihydrosterigmatocystin (DHST). Both ST and DHST are then substrates of the O-methyltransferase aflP to yield O-methylsterigmatocystin (OMST) and dihydro-O-methylsterigmatocystin (DHOMST), respectively. Finally OMST is converted to aflatoxins B1 and G1, and DHOMST to aflatoxins B2 and G2, via the action of several enzymes including O-methylsterigmatocystin oxidoreductase aflQ, the cytochrome P450 monooxygenase aflU, but also the NADH-dependent flavin oxidoreductase nadA which is specifically required for the synthesis of AFG1 (PubMed:15006741).^{[1] [2] [3] [4] [5] [6]}

References

1. ↑ Ma Y, Smith LH, Cox RJ, Beltran-Alvarez P, Arthur CJ, Simpson F R S TJ. Catalytic relationships between type I and type II iterative polyketide synthases: The Aspergillus parasiticus norsolorinic acid synthase. Chembiochem. 2006 Dec;7(12):1951-8. PMID:17086560 doi:10.1002/cbic.200600341
2. ↑ Crawford JM, Thomas PM, Scheerer JR, Vagstad AL, Kelleher NL, Townsend CA. Deconstruction of iterative multidomain polyketide synthase function. Science. 2008 Apr 11;320(5873):243-6. doi: 10.1126/science.1154711. PMID:18403714 doi:http://dx.doi.org/10.1126/science.1154711
3. ↑ Chang PK, Cary JW, Yu J, Bhatnagar D, Cleveland TE. The Aspergillus parasiticus polyketide synthase gene pksA, a homolog of Aspergillus nidulans wA, is required for aflatoxin B1 biosynthesis. Mol Gen Genet. 1995 Aug 21;248(3):270-7. PMID:7565588 doi:10.1007/BF02191593
4. ↑ Feng GH, Leonard TJ. Characterization of the polyketide synthase gene (pksL1) required for aflatoxin biosynthesis in Aspergillus parasiticus. J Bacteriol. 1995 Nov;177(21):6246-54. PMID:7592391 doi:10.1128/jb.177.21.6246-6254.1995
5. ↑ Yu J, Chang PK, Ehrlich KC, Cary JW, Bhatnagar D, Cleveland TE, Payne GA, Linz JE, Woloshuk CP, Bennett JW. Clustered pathway genes in aflatoxin biosynthesis. Appl Environ Microbiol. 2004 Mar;70(3):1253-62. PMID:15006741
6. ↑ Yu J, Bhatnagar D, Cleveland TE. Completed sequence of aflatoxin pathway gene cluster in Aspergillus parasiticus. FEBS Lett. 2004 Apr 23;564(1-2):126-30. PMID:15094053 doi:10.1016/S0014-5793(04)00327-8