Nonsteroid Hormones

A nonsteroid hormone, such as an amine, peptide, or protein, combines with specific receptor molecules on the target cell membrane. Each receptor molecule is a protein that has a binding site and an activity site. The hormone combines with the binding site of the receptor. This causes the receptor's activity site to interact with other membrane proteins. Receptor binding may alter the function of enzymes or membrane transport mechanisms, changing the concentrations of still other cellular components. The hormone that triggers this cascade of biochemical activity is considered a first messenger. The biochemicals in the cell that induce the changes that are recognized as responses to the hormone are called second messengers.

Many hormones use cyclic adenosine monophosphate (cyclic AMP, or cAMP) as a second messenger. In this mechanism, a hormone binds to its receptor, and the resulting hormone-receptor complex activates a protein called a G protein, which then activates an enzyme called adenylate cyclase (ah-den'i-lat si'klas), an integral membrane protein with its active site facing the inside of the cell. The activated enzyme removes two phosphates from ATP and circularizes it, forming cyclic AMP (fig. 13.5). Cyclic AMP, in turn, activates another set of enzymes called protein kinases (ki'nas-ez). Protein kinases transfer phosphate groups from ATP molecules to protein substrate molecules. This phosphoryla-tion alters the shapes of the substrate molecules and converts some of them from inactive forms into active ones.

The activated protein molecules then induce changes in various cellular processes (fig. 13.6). Thus, the response of any particular cell to such a hormone is determined not only by the type of membrane receptors present, but also by the kinds of protein substrate molecules in the cell. Table 13.4 summarizes these actions. Cellular responses to second messenger activation include altering membrane permeabilities, activating enzymes, promoting synthesis of certain proteins, stimulating or inhibiting specific metabolic pathways, promoting cellular movements, and initiating secretion of hormones and other substances. A specific example is the action of epinephrine to raise blood sugar during periods of physical stress. Epinephrine acts through the second messenger cAMP to increase the activity of the enzyme that breaks down liver glycogen, leading to increased glucose that can diffuse out of liver cells and enter the bloodstream.

Shier-Butler-Lewis: I III. Integration and I 13. Endocrine System I I © The McGraw-Hill

Human Anatomy and Coordination Companies, 2001

Physiology, Ninth Edition

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