Adenylyl cyclase cAMP derived from

In the same way as cAMP is formed from ATP by adenylyl cyclase, the guanine analogue, cGMP, can be formed from GTP by guanylyl cyclase. This may be either an integral membrane protein, like adenylyl cyclase, or a cytosolic protein. cGMP is produced in response to a number of neurotransmitters and also nitric oxide, the endothelium-derived relaxation factor that is important in vasodilatation. 

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