Enzymes
A.P. Biology
Chapter 8 Supplement
Status Check
• What is the definition of an “enzyme?”
• All enzymes are considered what type of
macromolecules?
Enzyme Kinetics
• Enzymes work by lowering the activation energy of a reaction
– Activation energy is the amt. of energy needed to drive a
reaction forward
Does an enzyme
alter the amount
of ΔG released?
Enzyme Kinetics
• Enzymes lower the activation energy by forcing an induced fit
with the substrates (thus de-stabilizing the substrates)
Enzyme is hexokinase, substrate is glucose Both enzyme and substrate alter their
shape to fit together
Regulating Enzyme Activity
• Enzymes can be affected by several factors:
– pH
– Temperature
– Inhibitors
– Mutations
• All of these factors affect the shape of an
enzyme (altered shape means altered binding
with substrate)
Effect of pH
• Changes in pH changes [H+] in environment
– [H+] will alter the charges in the active site.
– Altered charges in active site affects bonding
between substrate and active site.
– Changes in pH can also denature protein.
Effect of Temperature
• Changes in temperature in the environment
– Too low temperatures do not promote movement and
bonding between enzyme and substrate
– Too high temperatures will cause proteins to denature
(lose secondary, tertiary, and quaternary structures)
Effect of Inhibitors
• Two types of inhibitors
1. Competitive Inhibitors: molecules that bind
directly to active site
1. Noncompetitive Inhibitors: molecules that bind
to somewhere else on enzyme
Competitive Inhibitors
• Directly binds to active site, therefore blocking
the substrate from attaching to the enzyme
Substrate
Active site
Enzyme
Toothpickase Example
• Active Site: Your Hands
• Substrate: Toothpicks
• Competitive Inhibitor: mixing
needles in with toothpicks
NEEDLES DIRECTLY ATTACK
YOUR HAND, PREVENTING YOU
FROM GRABBING TOOTHPICKS!
Noncompetitive Inhibitors
• Binds to a place away from active site
(allosteric site), but causes a change in the
shape of enzyme preventing substrate binding
Substrate
Active Site
Enzyme
Allosteric site
Toothpickase Example
• Active Site: Your Hands
• Substrate: Toothpicks
• Allosteric Site: Your eyes
• Noncompetitive
Inhibitor: Blindfold
PUTTING A BLINDFOLD
ON YOUR EYES WILL
AFFECT HOW FAST YOU
CAN GRAB THE
TOOTHPICKS!
Effect of Mutations
• Changes in DNA
sequence causes
change in amino acid
sequence
• Changes in 1°
structure changes 2°-
4° structure
• Ex. Lactose intolerant:
– Those who are lactose
intolerant produce an
ineffective lactase
enzyme
– This is caused by
genetics
Status Check!
• What is the difference between ΔG and activation energy?
• How does an enzyme lower the activation energy of a
reaction?
• Name the four ways enzyme activity can be changed.
• What is the difference between a competitive and
noncompetitive inhibitor?
Feedback Inhibition
• Important for biological systems
– Prevents overproduction of chemicals and
molecules (prevents waste)
– Allows a system (ex. cellular respiration) to be
slowed down (or sped up) if needed.
– Systems can be influenced by the environment,
either inside or outside the cell.
Classic Model Feedback Inhibition
A B C D
X Y Z
• A, B, C, D are substrates and/or products
• X, Y, Z are the enzymes needed for each reaction
• The final product D acts as an inhibitor for enzyme X
• As the reaction moves forward:
• [D] increases,
• More D binds to X,
• Activity of X shuts down
• Everything that follows shuts down
Cellular Respiration Example
• Phosphofructokinase is
an enzyme used in
glycolysis
• AMP stimulates
phosphofructokinase
• ATP and citrate inhibits
phosphofructokinase
• When will ATP be high?
• When will AMP be
high?
Status Check!
A B CD E
W X Y Z
• Identify the substrate/products
• Identify the enzymes
• If E is an inhibitor for X, which products will eventually decrease in
concentration?
• Is it possible for C to be an inhibitor for Y? Why or why not?