Figure 5.3 The cyclic AMP second messenger mechanism by which hormones act on target cells. A hormone binds to a G protein coupled receptor (GPCR) on the plasma membrane, which activates a G protein. The G protein activates adenylate cyclase, which causes ATP to be converted to cyclic AMP. The cyclic AMP activates protein kinase A (PKA) that, in turn, activates response proteins to alter cellular activity.