121st General Meeting of the KCS

Type Symposium
Area Current Trends in Organic Chemistry I: New Reactions and Methodology
Room No. Halla Hall B
Time THU 17:35-:
Code ORGN1-5
Subject The Key Reaction Catalyzed by A Cobalamin-Dependent Radical SAM Enzyme in Oxetanocin-A Biosynthesis
Authors Hung-wen Liu
Division of Chemical Biology and Medicinal Chemistry, College of Pharmacy, and Department of Chemistry, The University of Texas at Austin, United States
Abstract Radical S-adenosyl-L-methionine (SAM) enzymes contain [4Fe-4S] clusters that act to reductively cleave SAM and produce highly reactive 5'-deoxyadenosyl radical (5'-dAdo•) species. Abstraction of a hydrogen atom from the substrate by 5'-dAdo• initiates subsequent chemical transformations. With over 7,000 radical SAM enzymes now annotated as cobalamin-dependent, these dual cofactor enzymes are emerging as a new superfamily. Many members are believed to catalyze methylation reactions; however, some others are hypothesized to catalyze complex, radical-mediated rearrangements in which the functions of the SAM and cobalamin (Cbl) cofactors remain enigmatic. Oxetanocin A (OXT-A) is a nucleoside antibiotic produced by Bacillus megaterium NK 84-0218 and possesses an unusual oxetane sugar attached to adenine. The oxetane sugar ring is rare among natural products and is of particular interest for biosynthetic study due to its highly strained, four-membered ring. Through study of the OXT-A biosynthetic pathway, we have identified a Cbl-dependent radical SAM enzyme, OxsB, which catalyzes an oxidative ring contraction resulting in an aldehyde intermediate. Genetic experiments were carried out to show that OxsB and an HD-domain phosphohydrolase OxsA are both indispensable for the production of OXT-A. The enzymatic products of coupled OxsB and OxsA activity were fully characterized. Furthermore, two alcohol dehydrogenases from B. megaterium were also shown to be able to reduce the aldehyde intermediate to the corresponding alcohol. In addition, labeled substrates were prepared to study the stereospecificity of hydrogen atom abstraction. Finally, the crystal structure of OxsB was solved and represents the first structural characterization of a member of this superfamily.
E-mail hakkim@korea.ac.kr