Guanylyl transferases are
enzymes that transfer a
guanosine monophosphate (GMP; also known as guanylic acid) group, usually from
GTP to another molecule, releasing
pyrophosphate. Many
eukaryotic guanylyl transferases are
capping enzymes that catalyze the formation of the
5' cap in the co-transcriptional modification of
messenger RNA. Because the
5' end of the
RNA molecule ends in a
phosphate group, the bond formed between the RNA and the GTP molecule is an unusual 5'-5' triphosphate linkage, instead of the
3'-5' linkages between the other nucleotides that form an RNA strand. In capping enzymes, a highly conserved
lysine residue serves as the catalytic residue that forms a
covalent enzyme-GMP complex.