In response to extracellular signals, guanylyl cyclase converts the nucleotide GTP to cyclic GMP. It is an intracellular signalling molecule. For example, in the smooth muscle of blood vessels, NO binds to the iron in the active site of guanlyl cyclase, activating the enzyme and leading to the production of cyclic GMP. In response to this the blood vessel dilates. Cyclic GMP is broken down to GMP by cyclic GMP phosphodiesterase .
Cyclic GMP (cGMP) is a nucleotide which is synthesised from GTP using the enzyme guanylyl cyclase. It is broken down by cyclic GMP phosphodiesterase. It is very important in controlling visual transduction responses. It is especially important for keeping cyclic GMP gated cation channels open in the dark, which are found in the plasma membrane surrounding the outer segments of a rod .