Chapter 5- Enzymes
State Standard
Standard 1.b. – Enzymes are proteins that catalyze
biochemical reactions without altering the reaction
equilibrium and the activities of enzymes depend on
the temperature, ionic conditions, and the pH of the
surroundings
*
http://www.youtube.com/watch?v=AFbPHlhI13g&safety_mode=true&persist_sa
fety_mode=1&safe=active
Enzymes Speed Up Chemical Reactions
• Most of the essential chemical reactions in cells must
occur quickly and precisely for the cell to survive
• For a chemical reaction to begin, reactants must
absorb some energy
- This energy is called the energy of activation
(EA)
http://www.sumanasinc.com/webcontent/animations/content/enzymes/enzymes.html
Enzymes Speed Up Chemical Reactions
 Enzymes decrease the energy of activation
Enzymes Speed Up Chemical Reactions
http://www.ryancshaw.com/Files/micro/Animations/Enzyme-Substrate/micro_enzymesubstrate.swf
Reactants
EA
with
enzyme
EA
without
enzyme
Net
change
in energy
Products
Figure 5.5B
Characteristics of Enzymes
Enzymes
• Are proteins
• Act as a biological catalyst
- Increase the rate of a reaction without being
changed into a different molecule
• Increase the rate of a chemical reaction by lowering
the amount of energy of activation needed
• Are not changed or used up in the chemical reaction
• Are specific
•
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_work.html
Lecture Review
1. Which of the following
can be used to start
(initiate) a chemical
reaction?
A. Addition of ATP
B. Heat and addition of
ATP
C. Enzymes
D. Heat
E. Heat and enzymes
2. True or False? Enzymes
raise the activation energy
to break the bonds of
reactant molecules.
3. Most of a cell’s enzymes
are
A. Lipids
B. Proteins
C. Amino acids
D. Nucleic acids
Lecture Review
4. The amount of energy that reactants need to start a
chemical reaction is the _______.
5. When an enzyme catalyzes (speeds up) a chemical
reaction
A. It acts as a reactant
B. It acts as a product
C. It raises the activation energy of a reaction
D. It lowers the activation energy of a reaction
Enzymes Are Specific
 Substrate – the substance the enzyme acts on, the
reactant in a chemical reaction
 Active site – the part of the enzyme molecule that
binds to a substrate
Active
site
Substrate
Enzymes Are Specific
Enzymes are specific
• It takes many different kinds
of enzymes to catalyze all the
reactions of the cell
• They have a unique 3D shape
• The active site of an enzyme
recognizes only the
substrate(s) of the reaction it
catalyzes
Enzymes Are Specific
Substrate
Active site
• An enzyme is specific
because the active site fits
only the substrate(s) of the
chemical reaction it
catalyzes
•
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/ani
mation__how_enzymes_work.html
The active site
• Is a small part of the
enzyme molecules that
binds to the substrate
• Is usually a pocket or
groove on the surface of the
enzyme
• Forms temporary bonds
with the substrate
Lecture Review
1. Which of the following is
not true of enzymes?
A. Are proteins
B. Act as a biological
catalyst
C. Supplies energy to start
a chemical reaction
D. Is specific
E. Lowers the energy of
activation
2. An enzyme is specific,
this means
A. It has a certain amino
acid sequence
B. It is found only in a
certain place
C. It speeds up a particular
reaction
D. It occurs in only one
type of cell
Lecture Review
3. Enzyme(s)
A. Function depends on its
3D shape
B. Are not specific
C. Are used up in chemical
reactions
D. All of the above
4. Why can a single
enzyme molecule act on
thousands or millions of
substrate molecules per
second?
5. What is the relationship
between the active site and
the substrate?
How An Enzyme Works
Enzyme available
with empty active site
Substrate bonds to
active site with
induced fit
The enzyme is
unchanged and
can repeat the
process
Products are
released
Substrate is
converted to products
How An Enzyme Works
Induced Fit
• The interaction between the active site and
substrate(s) causes the enzyme to change slightly so
the active site fits the substrate better
• Analogy – baseball and glove
•
http://www.uic.edu/classes/bios/bios100/lectures/enzymes02.html
Lecture Review
In your own words explain how an enzyme works.
Include the following:
substrate
enzyme
active site
product
induced fit
Factors that Affect Enzyme Activity
• Enzyme activity is influenced by
- temperature
- salt concentration
- pH
http://usmanscience.com/12bio/enzyme/e
nzyme_animations.htm
Factors that Affect Enzyme Activity
 Temperature
The optimum temperature occurs when the rate of
Contact between the active site and substrate is
greatest. In humans this is between 35-40oC
As temperature increases the
rate of contact between the
substrate and active site
increases
Higher temperatures denature
the enzyme altering the
shape of the active site
Lecture Review
1. Why does heating interfere with the activity of an
enzyme?
A. It kills the enzyme.
B. It changes the enzyme’s shape.
C. It decreases the energy of the substrate molecules.
D. It causes the enzyme to break up.
E. It decreases the chance that the enzymes will meet a
substrate molecule.
Factors That Affect Enzyme Activity
 The optimal pH for most enzymes is in the range of pH
6-8
 At low pH H+ ions interfere with bonds that maintain the
enzyme’s shape
 At high pH OH- ions interfere with the enzyme’s shape
 If additional Acid is added…
Lecture Review
2. Low pH and extremely salty conditions affect
enzyme activity because
A. They cause the structure of the enzyme to change.
B. The salt and hydrogen ions compete with the
substrate for the active site.
C. The salt and hydrogen ions are irreversible
inhibitors.
D. They reduce the number of collisions between the
enzyme and substrate
Lecture Review
3. The pH at which most enzymes’ activity is the
greatest is
A.
B.
C.
D.
pH 3
pH 5
pH 9
pH 7
Factors that Affect Enzyme Function
Salt concentration
• In extremely salty solutions the salt ions interfere
with some of the chemical bonds that maintain the
enzyme’s shape and the enzyme is denatured.
Cl-
ClNa+
Na+
Na+
Cl-
Lecture Review
Food rots when microbes break down food molecules.
Food preservation methods interfere with the enzyme
activity of microbes and prevent them from surviving.
4. Explain how each of the following would interfere
with enzyme activity
-pickling (soaking in acetic acid)
-salting
-canning (heating)
Factors that Affect Enzyme Function
Cofactors and coenzymes
• Many enzymes will not
work unless they are
accompanied by
cofactors or coenzymes
• Cofactor – nonprotein
helper. Ex: zinc, iron
• Coenzyme – cofactor
that is an organic
molecule. Ex: many
vitamins
•
http://highered.mcgrawhill.com/sites/0070960526/student_view0/cha
pter6/animations.html
Lecture Review
5. Some enzymes involved in hydrolysis cannot
function without the help of sodium ions. Sodium in
this case functions as
A. A substrate
B. A cofactor
C. An active site
D. A noncompetitive inhibitor
E. A coenzyme
Factors that Affect Enzyme Function
Inhibitors
• A competitive inhibitor takes the place of a substrate in the
active site and prevents the substrate from binding to the
active site
•
http://www.google.com/search?q=enzyme+competitive+inhibitor+animations&rls=com.microsoft:en
-us:IE-Address&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7ADFA_enUS426
Factors that Affect Enzyme Activity
 A noncompetitive inhibitor alters an enzyme’s
function by changing its shape. The active site can
no longer bind to the substrate.

http://highered.mcgrawhill.com/sites/0072943696/student_view0/chapter2/animation__feedback_inhibition_of_bi
ochemical_pathways.html
Factors that Affect Enzyme Activity
 JUST ADDED- not in Outline!!!
 Feedback regulation to maintain
homeostasis

The product of the reaction catalyzed by
an enzyme acts as a reversible inhibitor
and regulates the reaction… this is called
negative feedback
A=sugar

D=ATP
Ex: enzymes involved in the breakdown
of sugars to provide the ENERGY to
recycle ADP +P back to ATP are
inhibited by ATP…prevents cells from
“burning” more sugar than needed!
Factors that Affect Enzyme Function
 Irreversible inhibitors
• Reversible inhibitors
form covalent bonds
between the enzyme and
the inhibitor
 Ex: certain pesticides
are toxic to insects
because they inhibit key
enzymes in the nervous
system
form weak bonds
between the inhibitor
and the active site
• Ex: penicillin, an
antibiotic, inhibits an
enzyme that bacteria use
in making cell walls

http://www.wiley.com/college/pratt/047139
3878/student/animations/enzyme_inhibitio
n/
Lecture Review
A farm worker accidentally was splashed with a
powerful insecticide. A few minutes later, he went into
convulsion, stopped breathing, and died. The
insecticide was a competitive inhibitor of an enzyme
important in the function of the nervous system.
6. Describe the relationship between the structure of
the enzyme, the structure of its substrate, and the
structure of the insecticide.
Factors that Affect Enzyme Function
Negative Feedback
• The product of a reaction
acts as a reversible
inhibitor of the enzyme
that catalyzes the
reaction
• This regulates the
reaction