Two bifunctional enzymes, MTM GIV and MTM C, involved in the biosynthesis of deoxysugar moieties of the antitumor antibiotic mithramycin

Abstract: Mithramycin is an aureolic acid-type anticancer agent consisting of a polyketide core that is decorated with disaccharide and trisaccharide units. Previous studies have shown that two gene products, MtmGIV (a putative glycosyltransferase) and MtmC (a putative C-methyltransferase), are indispensible for the assembly of the complex trisaccharide component. However, their exact functions remained unclear. Using in vitro reconstitution, these two enzymes are now shown to both have bifunctional activities and hence dual roles in the biosynthesis of mithramycin. MtmC is demonstrated to function as both a 4-ketoreductase and 3-C-methyltransferase, with each activity representing a branching point from TDP-4-keto-D-olivose to yield TDP-D-olivose and TDP-D-mycarose, respectively. Both sugars then serve as donor substrates for MtmGIV, a bifunctional glycosyltransferase that incorporates the first sugar (D-olivose) and the last (D-mycarose) into two distinct acceptor substrates to yield the trisaccharide component of mithramycin. The discovery of these dual enzyme functions explains two previously missing activities that are essential for mithramycin biosynthesis that were not readily predicted from bioinformatic analysis of the gene products.

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