The enzyme, which is specific for GTP, was characterized from the archaeon Pyrococcus furiosus. The enzyme converts the 3'-terminal phosphate of various RNA substrates into the 2',3'-cyclic phosphodiester in a GTP-dependent reaction. Catalysis occurs by a three-step mechanism, starting with the activation of the enzyme by GTP, forming a phosphoramide bond between guanylate and a histidine residue. The guanylate group is then transferred to the 3'-phosphate terminus of the substrate, forming the capped structure [RNA]-3'-(5'-diphosphoguanosine). Finally, the enzyme catalyses an attack of the vicinal O-2' on the 3'-phosphorus, which results in formation of cyclic phosphate and release of the guanylate. cf. EC 220.127.116.11, RNA-3'-phosphate cyclase (ATP).
the enzyme utilizes GTP approximately 10times more effectively than ATP. UTP and CTP have no effect on activity. Oligoribonucleotide substrates of different lengths (4-, 8- and 12-mers) are tested, the 4-mer is the most efficient substrate