Epinephrine, starts the reactions by stimulating effectors, called adenlyl cyclase, which in turn produces a second messanger, cyclic AMP. The cyclic AMP triggers the activation of adrenylyl cyclase. This communication would not be possible if not for the guanine nucleatides that allow the binding of the enzymes to occur. Not only does epinephrine trigger the chemical reactions by bonding to the guanine nucleotides, but they also trigger the production GDP by reducing GTP to its subserviant level.
Not all effectors are triggered by simply hormones. Cells in the retinal area of the eye are triggered by photons that strike rhodopsin. When the rhodospin it activates the G protein, transductin, to travel to the effector enzyme, and thus regulates the levels of soduim ions (Na+). The eye cells become hyperpolarized, thus producing a negative charge, signaling visual data to the brain.
The two afore mentioned receptors are just hundreds that exist in the human body. All the receptors employ the help of G proteins to produce such signals. G proteins regulate the transfer of signals by transfering and connecting chains of proteins, known as alpha, beta, and gamma. The combinations of the protein chains leads to an abundance of codes, with over one thousand combinations possible.
Before encoding the alpha, beta, and gamma subuntis, the three chains are bound together with the GDP unit. When the G protein enables the coding, the GDP detaches and sets out to work. The coding proteins then untangle, and code into their various functions. The G proteins therefore are simply timing and switching units, that trigger reactions to assume various functions.