Cytochrome P450 monooxygenase; part of the gene cluster that mediates the biosynthesis of the tetrahydroxanthone dimer secalonic acid D (PubMed:30996871, PubMed:33891392). The pathway begins with the synthesis of atrochrysone thioester by the polyketide synthase AacuL (Probable). The atrochrysone carboxyl ACP thioesterase AacuM then breaks the thioester bond and releases the atrochrysone carboxylic acid from AacuL (Probable). Atrochrysone carboxylic acid is decarboxylated by the decarboxylase AacuI, and oxidized by the anthrone oxygenase AacuG to yield emodin (Probable). Emodin is then reduced to emodin hydroquinone by a yet unidentified oxidoreductase (Probable). A-ring reduction by the short chain dehydrogenase AacuN, dehydration by the scytalone dehydratase-like protein AacuK and probable spontaneous re-oxidation, results in overall deoxygenation to chrysophanol (PubMed:33891392). Baeyer-Villiger oxidation by the Baeyer-Villiger monooxygenase (BVMO) AacuH then yields monodictyphenone (PubMed:33891392). Monodictyphenone is transformed into compounds with the tetrahydroxanthone skeleton via methylesterification by the methyltransferase AacuQ, followed by the action of the flavin-dependent monooxygenase AacuC, the isomerase AacuP, and the short chain dehydrogenase/reductase AacuF or AacuD (PubMed:33891392). AacuF and AacuD should accept the same compound as a substrate but perform the ketoreduction with a different stereoselectivity, thus yielding blennolides B and A, respectively (PubMed:33891392). In the final step of the biosynthesis, the cytochrome P450 monooxygenase AacuE accepts blennolide B and/or blennolide A to conduct the dimerization reaction to furnish the tetrahydroxanthone dimers, secalonic acids D, B, and F (PubMed:33891392). Secondary metabolite biosynthesis. Secalonic acids show unprecedented anticancer activities against various human cancer cells and might be interesting for further derivatization, targeting diseases such as cancer. Belongs to the cytochrome P450 family.