3ETR:
reference: Substrate Orientation and Catalysis at the Molybdenum Site in Xanthine Oxidase: CRYSTAL STRUCTURES IN COMPLEX WITH XANTHINE AND LUMAZINE., Pauff JM, Cao H, Hille R, J Biol Chem. 2009 Mar 27;284(13):8760-7. Epub 2008 Dec 24. PMID: 19109252

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