Guanylyltransferase

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Guanylyltransferases are enzymes that transfer a GMP group from GTP to another molecule, releasing pyrophosphate. Many are responsible for adding the 5' cap to messenger RNA as it’s being made. The 5' end of RNA forms a unique 5'-5' triphosphate bond with the GTP, unlike the usual RNA backbone links. A conserved lysine in these enzymes acts as the catalyst, forming a temporary enzyme–GMP intermediate during the transfer. In eukaryotes, the tRNA for histidine (tRNAHis) needs an extra guanine before it is charged with histidine, and the yeast enzyme that adds this guanine to tRNAHis is notable for recognizing the tRNA’s anticodon specifically. Guanylyl transferases also exist that attach guanine nucleotides to sugars such as mannose and fucose. This enzyme family is classified as EC 2.7.