Name: Date: AP Biology
LAB : FACTORS INFLUENCING ENZYME ACTIVITY
Background
Enzymes are biological catalysts capable of speeding up chemical reactions by lowering activation energy.
One benefit of enzyme catalysts is that the cell can carry out complex chemical activities at a relatively low
temperature.
Most enzymes are proteins and their 3-dimensional shape is important to their catalytic activity.
Two specific regions on the enzyme structure play an important role in catalytic activity: the active site and the
allosteric site. The active site is the area of the enzyme which binds to the substance(s) (substrate) and aids
in the chemical reaction. The allosteric site is involved in forming the proper 3-dimensional shape when linked
with specific cofactors. As a result of the unique characteristics of these sites, enzymes are highly specific in
terms of the reactions they will catalyze and the condition under which they work best.
In biochemical reactions the enzyme, combines reversibly with its specific substrate, to form an enzyme-
substrate complex. One result of this temporary union is a reduction in the energy required to activate the
reaction of the substrate molecule so that the products of the reaction, are formed. This can be summarized
in the equation:
Enzyme + Substrate Enzyme-Substrate Complex Enzyme + Product
Note that the enzyme is not consumed in the reaction and can recycle to work with additional substrate
molecules. Each enzyme is specific for a particular reaction because its amino acid sequence is unique which
causes it to have a unique 3-dimensional structure. The active site is the portion of the enzyme that interacts
with the substrate, so that any substance that blocks or changes the shape of the active site affects the activity
of the enzyme.
A description of several ways enzyme action may be affected follows:
1. Salt Concentration: If the salt concentration is close to zero, the charged amino acid side chains of
the enzyme molecules will attract each other. The enzyme will denature and form an inactive
precipitate. If, on the other hand, the salt concentration is very high, normal interaction of charged
groups will be blocked, new interactions will occur, and again the enzyme will precipitate. An
intermediate salt concentration such as that of human blood (0.9%) or cytoplasm is optimum for many
enzymes.
2. pH: Amino acid side chains contain groups such as –COOH and –NH2 that readily gain or lose H+ ions.
As the pH is lowered an enzyme will tend to gain H+ ions, and eventually enough side chains will be
affected so that the enzyme’s shape is disrupted. Likewise, as the pH is raised, the enzyme will lose H+
ions and eventually lose its active shape. Many enzymes perform optimally in the neutral pH range and
are denatured at either an extremely high or low pH. Some enzymes, such as pepsin, which acts in the
human stomach where the pH is very low, work best at a low pH.
3. Temperature: Generally, chemical reactions speed up as the temperature is raised. As the
temperature increases, more of the reacting molecules have enough kinetic energy to undergo the
reaction. Since enzymes are catalysts for chemical reactions, enzyme reactions also tent to go faster
with increasing temperature. However, if the temperature of an enzyme – catalyzed reaction is raised
still further, a temperature optimum is reached. Above this value, the kinetic energy of the enzyme and
water molecules is so great that the conformation of the enzyme molecules is disrupted. The positive
effect of speeding up the reaction is now more than off-set by the negative effect of changing the
conformation of more and more enzyme molecules. Temperatures around 40-50 degrees Celsius
denature many proteins, but some are still active at 70-80 degrees Celsius, and a few even withstand
boiling.
4. Activators and Inhibitors: Many molecules other than the substrate may interact with an enzyme. If
such a molecule increases the rate of the reaction it is an activator, and if it decreases the reaction
rate it is an inhibitor. These molecules can regulate how fast the enzyme acts. Any substance that
tends to unfold the enzyme, such as an organic solvent or detergent will act as an inhibitor. Some
inhibitors act by reducing the S-S bridges that stabilize the enzyme’s structure. Many inhibitors act by
reacting with side chains in or near the active site to change its shape or block it. Many well-known
