7f10
Crystal structure of NsrQ M128I in complex with substrate analogue 3
Structural highlights
FunctionNSRQ_ASPN1 Monooxygenase; part of the gene cluster that mediates the biosynthesis of the tetrahydroxanthone dimer neosartorin, which exhibits antibacterial activity (PubMed:30394754, PubMed:32105084, PubMed:33891392). The two different monomeric units appear to be synthesized by the same set of enzymes, among which the Baeyer-Villiger monooxygenase nsrF is the key enzyme for the divergence of the biosynthetic routes (PubMed:32105084). The pathway begins with the synthesis of atrochrysone thioester by the polyketide synthase nsrB (PubMed:32105084). The atrochrysone carboxyl ACP thioesterase nsrC then breaks the thioester bond and releases the atrochrysone carboxylic acid from AacuL (PubMed:32105084). Atrochrysone carboxylic acid is decarboxylated by the decarboxylase nsrE, and oxidized by the anthrone oxygenase nsrD to yield emodin (PubMed:32105084). Emodin is then reduced to emodin hydroquinone by the oxidoreductase nsrR (PubMed:32105084). A-ring reduction by the short chain dehydrogenase nsrJ, dehydration by the scytalone dehydratase-like protein nsrI and probable spontaneous re-oxidation, results in overall deoxygenation to chrysophanol (PubMed:32105084). The Baeyer-Villiger monooxygenase nsrF accepts chrysophanol as a substrate to insert one oxygen atom at two different positions to yield the precursors of both monomric units (PubMed:30394754, PubMed:32105084, PubMed:33891392). NsrF is promiscuous/flexible in interacting with the 2 (non methylated and methylated) aromatic rings of chrysophanol, thus diverging the biosynthetic pathway at this point (PubMed:30394754, PubMed:32105084, PubMed:33891392). After the hydrolysis of the lactones, methylesterification by the methyltransferase nsrG yields respectively moniliphenone and 2,2',6'-trihydroxy-4-methyl-6-methoxya-cyldiphenylmethanone (PubMed:30394754, PubMed:32105084). The next steps are the hydroxylation by the FAD-dependent monooxygenase nsrK, followed by isomerization by the monooxygenase nsrQ (PubMed:32105084). The short chain dehydrogenase/reductase nsrO then catalyzes the C-5 ketoreduction to give the xanthone skeleton of blennolide C and 5-acetylblennolide A (PubMed:32105084). The acetyltransferase nsrL has a strict substrate specificity and uses only blennolide A but not blennolide C to yield 5-acetylblennolide A as the single-acetylated product (PubMed:30394754). In the final step of the biosynthesis, the heterodimerization of the 2 xanthones, blennolide C and 5-acetylblennolide A, is catalyzed by the cytochrome P450 monooxygenase nsrP (PubMed:30394754). NsrP can utilize at least three different xanthones as its substrates to perform the dimerization reaction (PubMed:30394754).[1] [2] [3] Publication Abstract from PubMedThe novel isomerase NsrQ, from Aspergillus novofumigatus, is a key enzyme in the biosynthesis of fungal tetrahydroxanthones and is responsible for dearomatizing cyclization to provide a tetrahydroxanthone scaffold. NsrQ catalyzes a two-step isomerization reaction, involving the isomerization of allylic alcohol and subsequent inversion of configuration at the methyl group. We report on the biochemical and structural characterizations of NsrQ, and its homologue Dcr3, from Diaporthe longicolla. The crystal structures of NsrQ and Dcr3 revealed their similar overall structures, with a cone-shaped alpha+beta barrel fold, to those of the nuclear transport factor 2-like superfamily enzymes. Furthermore, the structures of Dcr3 and NsrQ variants complexed with substrate analogues and the site-directed mutagenesis studies identified the catalytic residues and the important hydrophobic residues in shaping the active site pocket for substrate binding. These enzymes thus utilize Glu and His residues as acid-base catalysts. Based on these observations, we proposed a detailed reaction mechanism for NsrQ-catalyzed isomerization reactions. Structural Basis for Isomerization Reactions in Fungal Tetrahydroxanthone Biosynthesis and Diversification.,Yang J, Mori T, Wei X, Matsuda Y, Abe I Angew Chem Int Ed Engl. 2021 Aug 23;60(35):19458-19465. doi:, 10.1002/anie.202107884. Epub 2021 Jul 20. PMID:34180120[4] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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