Some chemical reactions simply happen when the two reactants
come into contact. For example, you may be familiar with the bubbly
“volcano” that forms when baking soda and vinegar are placed together
in a glass. This reaction is spontaneous because it does not require
outside energy to force it to occur.
Most reactions, however, require energy. For example,
the chemical reactions that produce a cake do not take place when
baking soda, flour, and the other ingredients of a cake are simply
left in a pan on the kitchen counter. Heat is required to break
the existing chemical bonds in the ingredients so that they can
undergo chemical reactions and combine with each other in new ways.
In the laboratory, chemists use heat to create the activation
energy needed to get nonspontaneous reactions started. Animals,
however, can’t rely on internal Bunsen burners to get their chemical
reactions cooking. In order to perform chemical reactions at low
temperatures, the body uses special proteins called enzymes, which
lower the activation energy necessary for chemical reactions to
achievable levels. Enzymes lower the activation energy by interacting
with the substrates, the primary molecules or compounds
involved in the reaction. If you think of the activation energy
needed for a chemical reaction as a mountain that the reactants
have to climb, think of an enzyme as opening up a tunnel through
the mountain. Less energy is required to go through the tunnel than
to climb all the way up the mountain.
Enzymes are not themselves altered when they
help reactions along. Consequently, a single enzyme can be used
repeatedly in many reactions. Because enzymes can be used over and
over again and because they can act very quickly, a relatively small
amount of enzyme is needed to facilitate reactions involving relatively
large amounts of material.
Each enzyme is designed to fit only the substrates in
the reaction that the enzyme is meant to control. The one-to-one
correspondence between enzyme and substrate is referred to as specificity.
An analogy to a lock and key is useful for understanding
the specificity of enzymes. Each enzyme can be thought of as a lock
that can interact only with the appropriate key, or substrate. The
region of the enzyme that interacts with the substrate is known
as the active site.
Enzymes help form bonds by holding two substrates near
each other in the active site. Compounds can form bonds with each
other more easily when they are adjacent than when they are floating
around the cell randomly.
Often, enzymes are named for their substrate. The name
of the enzyme is the name of the starting material followed by the
“-ase.” For example, maltase is an enzyme that breaks down maltose,
a common sugar. (Be careful not to confuse sugars, which end in
“-ose,” with enzymes, which end in “-ase.”)
Factors Affecting Enzymes
Like all proteins, enzymes have a unique three-dimensional
structure that changes under unusual environmental conditions. Enzymes
do not function well when their structure is altered.
Temperature and pH
Depending on where it is normally located in the body,
an enzyme will have different temperature and pH values at which
its structure is most stable. As conditions deviate from this point,
the enzyme’s ability to help along reactions decreases.
Most enzymes work best near a pH of 7, but some enzymes
operate most effectively in a particularly acidic environment,
such as the stomach; a neutral environment impairs their function.
Likewise, the enzymes of creatures that live at high temperatures,
such as bacteria that live in hot springs, do not function properly
at human body temperature.
Cofactors and Inhibitors
In order to control enzyme activity more precisely, the
body has developed a number of compounds that turn enzymes on or
off and make them work faster or slower. Sometimes these compounds
attach to the active site along with the substrate, and sometimes
they bind to another site on the enzyme. Activators of enzymes are
known as cofactors or coenzymes. Many vitamins are
coenzymes. Molecules that prevent enzymes from functioning properly
are known as inhibitors.