Guanosine-triphosphate guanylyltransferase

In enzymology, a guanosine-triphosphate guanylyltransferase () is an enzyme that catalyzes the chemical reaction
Hence, this enzyme has one substrate, GTP, and two products, diphosphate and P1,P4-bis(5'-guanosyl) tetraphosphate.
This enzyme belongs to the family of transferases, specifically those transferring phosphorus-containing nucleotide groups (nucleotidyltransferases). The systematic name of this enzyme class is GTP:GTP guanylyltransferase. Other names in common use include diguanosine tetraphosphate synthetase, GTP-GTP guanylyltransferase, Gp4G synthetase, and guanosine triphosphate-guanose triphosphate guanylyltransferase.