48 Studies on the Enzymes Involved in the Biosynthesis of (+)-Geodin and Asterric acid
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(+)-Geodin (7), an anti-fungal antibiotic produced by Aspergillus terreus, is a seco-anthraquinone derived from octaketide anthraquinone emodin (2) as shown in Fig. 1. Asterric acid (9), a main metabolite of Penicillium frequentans, is biosynthesized in a similar pathway as (+)-geodin (7) except a chlorination step. Enzymes involved in these biosyntheses were studied to clarify and compare their molecular properties. Emodinanthrone oxygenase (EAO) catalyzes the fixation of molecular oxygen at C-10 position of emodinanthrone (1) without any requirement of external electron donor. EAO from both A. terreus and P. frequentans were membrane bound proteins and solubilized by Triton X-100. The solubilized enzymes were purified by column chromatographies as shown in Table I and II. The visible spectra of both enzymes showed no apparent absorption such as flavin or heme. The enzyme activity was inhibited by o-phenanthroline in the presence of 2-mercaptoethanol. These facts suggested the presence of non-heme ferric iron in the active center of EAO like lipoxygenase. Hence, the reaction mechanism of EAO was proposed as shown in Fig. 3. Emodin O-methyltransferase from both A. terreus and P. frequentans, desmethylsulochrin O-methyltransferase from P. frequentans were purified to apparent homogeneity. Sulochrin oxidase (SO), a phenol oxidative coupling enzyme of P. frequentans was also purified and revealed to be copper protein like dihydrogeodin oxidase (DHGO) from A. terreus. However, SO and DHGO showed different properties, such as effects of zinc ion and copper chelating inhibitors.
