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The life-span of eukaryotic mRNA is believed to be controlled by the so called cap at the 5' end and by a poly(A) tail at the 3' end. The cap is a covalently attached guanidine group which is methylated at N7. The process involves three separate enzymatic reactions, which may be catalyzed by the same or by different polypeptides, depending on the organism:

1) removal of the 5' phosphate group of the mRNA by RNA triphosphatase (EC 3.1.3.33),
2) addition of a GMP moiety to the remaining 5' diphosphate by RNA guanylyltransferase, the capping enzyme (EC 2.7.7.50),
3) methylation at N7 of the guanine base by RNA guanine-7-methyltransferase (EC 2.1.1.56).

pppRNA -> ppRNA + Pi

ppRNA + GTP <--> GpppRNA + PPi

GpppRNA + AdoMet -> m7GpppRNA + AdoHcy

The second reaction, linking the GMP to 5'diphosphate RNA consists of several steps, and involves an intermediate with the GMP moiety covalently attached to the active site lysine of the capping enzyme through a phosporamidate bond.

E + GTP <-> E-pG _+PPi

E-pG + ppRNA <-> GpppRNA +E

A) Capping chemistry
B) Sequence and conserved motifs
C) Structure of mRNA capping enzyme
D) Binding of cap analogue
E) Reaction mechanism
F) Authors and references

These capping enzyme homepages were constructed by Kjell.