Catalyzes the complex heterocyclic radical-mediated conversion of 6-carboxy-5,6,7,8-tetrahydropterin (CPH4) to 7-carboxy-7-deazaguanine (CDG), a step common to the biosynthetic pathways of all 7-deazapurine-containing compounds. 6-carboxy-5,6,7,8-tetrahydropterin + H(+) = 7-carboxy-7-deazaguanine + NH4(+) Binds 1 [4Fe-4S] cluster. The cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Binds 1 S-adenosyl-L-methionine per subunit. kcat is 5.4 min(-1). Purine metabolism; 7-cyano-7-deazaguanine biosynthesis. Homodimer. Belongs to the radical SAM superfamily. 7-carboxy-7-deazaguanine synthase family.