CHEMISTRY OF THE BRAIN: MOLECULES, MOOD, AND MENTAL ILLNESS - Chemistry: Notable Research and Discoveries - page 83

Organisms deal with this situation by speeding up reactions with
catalysts called enzymes. A catalyst affects the rate of a reaction but does
not otherwise participate, so it is not chemically altered. Enzymes are
usually proteins that temporarily bind the reactants in such a way as to
bring them together in the correct position. This binding is not done
with strong bonds such as covalent or ionic bonds, but with weaker at-
tractions that are more easily broken. An enzyme usually catalyzes only
one specific reaction since its shape and composition are generally such
that it binds only a specific set of reactants.
Enzymes are also excellent ways for biological organisms to regu-
late the rate of reactions. An organism's metabolism consists of all the
various reactions in that organism. Thousands of reactions take place
containing a few thousand serotonin molecules. Special pro-
teins transport the vesicles down axons to the release sites.
(For serotonin, these release sites are swellings on the axon
known as varicosities.) An action potential causes the vesi-
cle to fuse with the cell's membrane and releases serotonin
into the gap. The molecules diffuse away, and only a few
reach the other side—the random paths take many of them
off target, just as only a few pellets of a shotgun blast may
hit the mark. The serotonin molecules that reach the target
dock to specific receptors, called serotonin receptors, which
are proteins embedded in the membrane. But many of the
other serotonin molecules are not wasted—proteins called
transporters, embedded in the axon's membrane, bind to
serotonin and transport it across the membrane, retrieving
some of those that wandered off target.
Neurons guard against accumulating too much sero-
tonin with the use of enzymes such as monoamine oxidase
(MAO). This enzyme catalyzes a reaction converting se-
rotonin to a different molecule. Some of these serotonin
metabolites —compounds derived from serotonin—are ex-
creted by the body, marking the end of the road for these
molecules.