Catalyzes the conversion of 7,8-dihydroneopterin triphosphate (H2NTP) to 6-carboxy-5,6,7,8-tetrahydropterin (CPH4) and acetaldehyde. 7,8-dihydroneopterin 3'-triphosphate + H2O = 6-carboxy-5,6,7,8-tetrahydropterin + acetaldehyde + 2 H(+) + triphosphate Binds 1 zinc ion per subunit. Purine metabolism; 7-cyano-7-deazaguanine biosynthesis. The active site is at the interface between 2 subunits. The proton acceptor Cys is on one subunit, and the charge relay system is on the other subunit (By similarity). Belongs to the PTPS family. QueD subfamily. Truncated N-terminus. Truncated N-terminus.