Abstract
7-Carboxy-7-deazaguanine (CDG) is a common intermediate in the biosynthesis of 7-deazapurine-containing natural products. The biosynthesis of CDG from GTP requires three enzymes: GTP cyclohydrolase I, 6-carboxy-5,6,7,8-tetrahydropterin (CPH(4)) synthase, and CDG synthase (QueE). QueE is a member of the radical S-adenosyl-l-methionine (SAM) superfamily and catalyzes the SAM-dependent radical-mediated ring contraction of CPH(4) to generate CDG. This chapter focuses on methods to reconstitute the activity of QueE in vitro.