Guanosine monophosphate (GMP) is an important intermediate both in the de novo synthesis and salvage synthesis of GTP. It is also formed as a result of hydrolysis of cGMP, which is an important second messenger in cellular signalling mechanisms, particularly involved in vision.

In nucleotide synthesis, GMP is formed from phosphoribosyl pyrophosphate in the first reaction below catalyzed by a phosphoribosyl transferase:

1. PRPP + Guanine <=> GMP + PPi

2. cGMP + H2O <=> GMP (catalyzed by a phosphodiesterase)

GMP is also made in the de novo biosynthetic pathway of purine nucleotide biosynthesis from xanthosine monophosphate, as shown in Figure 22.6.