Second Messengers Many hormones, because of their size or highly charged structure, cannot easily cross cell membranes. These hormones exert their effects by binding to a receptor on the membrane surface and activating a G protein located in the membrane of the cell. The G protein is the link between the hormone receptor interaction on the surface of the membrane and the subsequent events inside the cell. The G protein may open an ion channel to allow Ca to enter the cell, or it may activate an enzyme in the membrane. If the G protein adenylate cyclase , then cyclic AMP activates (cyclic 3',5'-adenosine mono-phosphate) is formed from ATP (see Fig. 5.3). The cyclic AMP concentration increases in the cell and activates protein kinase A that, in turn, activates response proteins to alter cellular activity. For example, this mechanism is used to break down glycogen to glucose (by activating phosphorylase) and break down triglyceride molecules to free fatty acids (by activating hormone sensitive lipase [HSL]). The cyclic AMP is inactivated