FAD-linked oxidoreductase; part of the gene cluster that mediates the biosynthesis of alternapyrone derivatives (PubMed:16356847). Alternapyrone is a decaketide with octa-methylation from methionine on every C2 unit except the third unit (PubMed:16356847). All the domains in the polyketide synthase alt5 are apparently involved in alternapyrone synthesis, that is, the 8 CMeT, 7 KR, 7 DH, and 4 ER reactions in the 9 KS-mediated condensation steps required for alternapyrone synthesis (PubMed:16356847). the alternapyrone produced by alt5 might be intensively modified by cytochrome P450 monooxygenases alt1, alt2 and alt3 and FAD-dependent oxidoreductase alt4 present in the alt gene cluster (PubMed:16356847). Secondary metabolite biosynthesis. Belongs to the oxygen-dependent FAD-linked oxidoreductase family.