Guanosine triphosphate - another nucleotide like ATP, but with a different base. Like adenosine, guanosine is a purine (that is, it has the same fused ring system, but with different pendant groups). Also like ATP it plays a role in energy metabolism and as part of the energy currency of the cell - notably in Tubulin. Amusingly (for me:) caffeine is very similar to adenosine or guanosine (IE ATP/GTP without the sugar or phosphate) and affects the action of both adenosine receptors in the brain (altering sleep patterns) and cyclic AMP phosphodiesterase (which breaks down cAMP). So the other writup here was close - would have been closer under ATP.

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