Enzymes
Enzymes are biological catalysts that lower the activation energy of chemical reactions.
Enzymes allow reactions to occur at rates of thousands of time per second. Without enzymes,
necessary chemical reactions would not occur at the rate required for life.
Biological Catalysts
Substances which lower the amount of energy needed to activate a chemical reaction, without
being consumed in the reaction, are called catalysts. Enzymes are biological catalysts, generally
composed of proteins. By lowering the activation energy, chemical reactions generally occur
more rapidly. Catalysts are substances that speed up chemical reactions, without being changed
or used up.
How does the energy of a reaction change when it is
catalyzed by an enzyme?
a.
b.
c.
d.
the
the
the
the
energy of the produce increases
energy of the products decreases
energy of the intermediates increases
energy of the intermediates decreases
Most enzymes are proteins. Like other proteins, enzymes are produced by a cell's ribosomes.
Ribosomes produce specific enzymes to act on specific substances, called substrates. For
example, the enzyme catalase assists in the breakdown of hydrogen peroxide into water and
oxygen. In this case, hydrogen peroxide is catalase's substrate. (NOTE THE - ASE ending)
Many of the chemical reactions that occur in cells are catalyzed by enzymes. The activation
energy for many reactions is simply too high to overcome without enzymes, and the reaction will
not occur at all in the absence of an enzyme. Without enzymes catalyzing metabolic reactions,
cells would not be able to perform metabolism quickly enough to support life.
Since enzymes are not consumed in a chemical reaction, their concentration will remain constant
unless the cell triggers for re-uptake of the enzymes. Cells can control chemical reactions by
producing or removing enzymes. Reaction rates can be increased by increasing the production of
enzymes in environments highly concentrated with substrate.
The enzyme above lowers the activation energy needed to split the substrate molecule. After
the reaction, the enzyme is free to catalyze the breakdown of another reactant molecule.
Enzymes are also important for the synthesis of new molecules. For example, RNA polymerase is
an enzyme that is essential to the process of transcription. Molecules of mRNA are transcribed
by RNA polymerases and later new protein molecules are synthesized based on the instructions
coded in the mRNA.
Enzyme Specificity -Shape & Function
The shape of an enzyme determines how it works. Most enzymes have a surface with one or
more deep folds. The folds make pockets, which are called active sites. The active sites
match folds in the substrate's surface. Thus, a particular enzyme fits against its substrate like
two adjacent puzzle pieces.
An enzyme's shape is key to how the enzyme functions.
If its shape is changed, the enzyme may not function as well or at all. Changes in temperature
and pH can affect the shape of an enzyme's active sites. Therefore, enzymes are only able to
work properly in a certain temperature and pH range.
Enzyme and substrate concentrations
It seems obvious that adding more substrate to an enzyme will result in more product (increase
in enzyme activity). It is true to a point however; eventually the activity levels off because of
enzyme concentration. The same would be true if we increased the amount of enzyme.
Questions:
1. Which is NOT structural feature of an enzyme?
a. Protein
b. Substrate
c. Active site
d. Amino acid
2. Organisms produce hydrogen peroxide (H2O2), a byproduct of metabolism that is toxic to
cells. The catalase protein catalyzes the reaction shown below.
2 H2O2  2 H2O + O2
What statement best describe the reaction?
a. Water is the substrate
b. Hydrogen peroxide is the enzyme
c. Catalase is consumed by the reaction
d. Oxygen gas is a product of the reaction
3. The enzyme lactase catalyzes the breakdown of lactose (milk sugar) to glucose and
galactose. Students set up a beaker with milk and lactase enzyme. Which describes how
the concentrations of these substances will change?
a. The concentration of lactase will decrease, and the concentration of galactose will
increase
b. The concentration of galactose will decrease, and the concentration of glucose will
increase
c. The concentration of galactose will increase, and the concentration of lactase will
remain the same
d. The concentration of lactose will increase, and the concentration of glucose will
remain the same
4. A reaction tube is set up at 370 C with twice as much substrate as enzyme. The pH level
of the solution is 5. The reaction is measured. Which of the following changes will not
affect the rate of the reaction?
a. Increasing pH level
b. Increasing the temperature
c. Increasing the enzyme concentration
d. Increasing the substrate concentration
5. The graph shows the rate of enzyme activity in relation to pH for two enzymes – pepsin
and pancreatic trypsin. Both enzymes break down proteins in food. Pepsin work within
the stomach and trypsin works in the small intestine.
A. What does the graph indicate about the pH of the stomach and small intestine?
B. The contents of the stomach are released into the small intestine. How does this
affect the function of pepsin that is included with the stomach contents?
C. What is the advantage of having two different protein-digesting enzymes, rather
than just one.