Alpha-humulene synthase asR6

UniProtKB accession:  A0A2U8U2L5

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UniProtKB description:  Alpha-humulene synthase; part of the gene cluster that mediates the biosynthesis of xenovulene A, an unusual meroterpenoid that has potent inhibitory effects on the human gamma-aminobutyrate A (GABAA) benzodiazepine receptor (PubMed:29773797). The first step of xenovulene A biosynthesis is the biosynthesis of 3-methylorcinaldehyde performed by the non-reducing polyketide synthase aspks1 (PubMed:17912413, PubMed:20552126, PubMed:29773797). The salicylate hydroxylase asL1 then catalyzes the oxidative dearomatization of 3-methylorcinaldehyde to yield a dearomatized hydroxycyclohexadione (PubMed:29773797). The 2-oxoglutarate-dependent dioxygenase asL3 further catalyzes the oxidative ring expansion to provide the first tropolone metabolite (PubMed:29773797). The cytochrome P450 monooxygenase asR2 allows the synthesis of tropolone hemiacetal (PubMed:29773797). In parallel, a previously unrecognised class of terpene cyclase, asR6, produces alpha-humulene from farnesylpyrophosphate (FPP) (PubMed:29773797). The putative Diels-Alderase asR5 probably catalyzes the formation of the tropolone-humulene skeleton by linking humulene and the polyketide moiety (PubMed:29773797). Oxidative-ring contractions catalyzed by asL4 and asL6 then processively remove carbon atoms from the polyketide to yield xenovulene A (PubMed:29773797).