Guanylyl transferases are enzymes that transfer a guanosine mono phosphate group, usually from GTP to another molecule, releasing pyrophosphate.
In capping enzymes, a highly conserved lysine residue serves as the catalytic residue that forms a covalent enzyme-GMP complex.
[1] The transfer RNA (tRNA) for histidine is unique among eukaryotic tRNAs in requiring the addition of a guanine nucleotide before being aminoacylated by the histidine tRNA synthetase.
The yeast guanylyl transferase specific to tRNAHis is unique in being the only known non-tRNA synthetase enzyme that specifically recognizes the tRNA anticodon.
[2] Guanylyl transferases also exist for transferring guanosine nucleotides to sugar molecules, such as mannose and fucose.