Action mechanism of many hormones: involves cascade, second messenger, amplification
Let's suppose that the purpose of a particular hormone is to stimulate a particular type of cell
(target cell) to synthesize a particular molecule, Z, and release it into the bloodstream. That
molecule Z is made from molecule X by a specific enzyme in the target cell. Somehow, then, the
hormone has to stimulate that enzymatic conversion of X to Z. Refer to the drawing below to
see how this can happen in the target cell.
Each hormone's unique structure (shape) will match the binding site on one type of receptor
protein, embedded in the cytoplasmic membrane of the target cell. The receptor's binding site
faces the target cell's extracellular environment, which might be the bloodstream, e.g. Adjacent
to that receptor protein is an enzyme, adenylate cyclase, also embedded in the membrane. This
adenylate cyclase is inactive before the hormone arrives at the target cell and binds to its
receptor. Note the shapes of receptor and enzyme, and how close together they are, prior to
hormone binding, before STEP 1.
In STEP 1 the hormone binds to the receptor, which causes the receptor to change shape slightly.
That shape change, in turn, causes the adenylate cyclase to change shape slightly. That change
in adenylate cyclase causes it to become catalytically active. That is, the hormone has indirectly
activated this enzyme.
In STEP 2 adenylate cyclase catalyzes its reaction, which is the conversion of ATP to cyclic
AMP (cAMP). This occurs inside the cell, in the cytosol. Note that the hormone does not enter
the cell. The cAMP is called a second messenger. Since a hormone may be though of as
messenger molecule sent by a gland cell to a target cell, the hormone itself is the "first
messenger."
In STEP 3 the cAMP binds to an enzyme known as protein kinase, which then becomes active
in STEP 4. In STEP 5, the kinase molecule causes the conversion of yet another enzyme from
an inactive state to an active state.
And finally, that last enzyme catalyzes a reaction to produce a specific metabolic product.
This sequence of hormone-triggered events is called a cascade, which is something like a
"domino effect." One event triggers a second, which triggers a third, and so on.
The response of the target cell to the hormone, as summarized here, is to begin synthesis of some
organic molecule. However, since the hormone itself can't enter the cell (too large or too
hydrophilic, perhaps) its stimulatory effect on the target cell must somehow be transmitted to the
cell interior where the metabolic machinery is located. Getting the hormonal "signal" across the
plasma membrane is accomplished by the interaction of the receptor and adenylate cyclase to
produce cAMP as a second messenger inside the cell.
This cascade of events involves amplification of the hormonal signal. To amplify means to
increase the magnitude. Amplification is what makes it possible for a very small amount of
hormone to have a large effect. One hormone molecule could bind to one receptor molecule,
which then activates one adenylate cyclase molecule. Since adenylate cyclase is an enzyme, it
can make many copies of cAMP, each of which can activate a protein kinase molecule (STEP 3,
4). And since protein kinase also is an enzyme, each active molecule of that can activate many
copies of the final inactive enzyme (STEP 5). And, of course, that enzyme can catalyze the
conversion of X to Z many times (STEP 6). Therefore, several steps in the cascade multiply the
effect of that one hormone molecule, so that it may cause the ultimate production of millions of
molecules of the final product, Z.