Guanosine triphosphate

Guanosine-5'-triphosphate (GTP) is a purine nucleotide. One role is as substrate for the synthesis of RNA during transcription. Its structure is similar to that of the guanine nucleoside, the only difference being that there are three extra phosphate groups added on to the 5' carbon.

It also has the role of a source of energy or an activator of substrates in metabolic reactions, like that of ATP, but more specific. It is used as a source of energy for protein synthesis.

GTP is essential to signal transduction, particularly with G-proteins, where it is converted to GDP (guanosine diphosphate) through the action of GTPases.

Energy transfer
GTP is involved in energy transfer within the cell. For instance, a GTP molecule is generated by one of the enzymes in the citric acid cycle. This is tantamount to the generation of one molecule of ATP, since GTP is readily converted to ATP.

cGTP
Cyclic guanosine triphosphate (cGTP) helps cyclic adenosine monophosphate (cAMP) activate cyclic nucleotide-gated ion channels in the olfactory system.