Guanosine-triphosphate guanylyltransferase () is an enzyme that catalyzes the chemical reaction
The enzyme characterised from brine shrimp combines two molecules of guanosine triphosphate to give P(1),P(4)-bis(5'-guanosyl) tetraphosphate, with pyrophosphate (PPi) as a byproduct.
This enzyme is a transferases, specifically one 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.
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