Cell Biology:
Enzymes
Lesson 2 – Factors Affecting Enzyme Activity
(Inquiry into Life pg. 106-109)
Today’s Objectives
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Analyze the roles of enzymes in biochemical
reactions, including:
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Apply knowledge of proteins to explain the effects
on enzyme activity of pH, temperature, substrate
concentration, enzyme concentration, competitive
inhibitors, and non-competitive inhibitors including
heavy metals
Factors Affecting Enzyme Activity
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There are several factors that have an effect on
enzyme activity, such as:
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1) pH
2) Temperature
3) Substrate concentration
4) Enzyme concentration
5) Competitive inhibition
6) Non-competitive inhibition (ex: heavy metals)
1) pH
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Each enzyme operates best at a preferred pH level
Any other pH affects tertiary structure and slows
down reactions
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Tertiary structure: remember, enzymes are proteins!
Too much of a change in pH stops the reaction
Why?
Recall the tertiary structure of proteins:
Tertiary Structure
Bonds between amino acids on
the polypeptide help maintain
the protein’s shape:
Hydrogen bonds + electrostatic
interactions = “Salt bridges”
1) pH
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Since salt bridges depend on ionic charges for their bonding
power, anything which neutralizes such a charge will destroy
the salt bridge and make the folded structure of the enzyme
(protein) less stable
If an enzyme’s normal shape is destroyed, it is said to be
“denatured” and is no longer able to combine efficiently
with its substrate
An increase of pH will take an H+ from an NH3+ group and
neutralize its charge.
A decrease in pH will put an H+ on a COO- and neutralize its
charge
This means that each enzyme has an optimum pH which its
folded (active) structure its most stable.
It has its maximum catalytic power at that pH
Animation
2) Temperature
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Cold temperature slows down enzymatic reactions
Warm temperatures up to around 40°C will speed up
reactions
High temperatures will denature enzymes and cause reactions
to stop
If we increase the temperature of the solution the enzymes
are operating in, we will typically see an increase in the
reaction rate until a point is reached at which the enzyme
starts to unfold
This is a result of breaking hydrophobic bonds and salt
bridges between R-groups of the protein as the increase in
temperature causes the enzyme’s structure to “wiggle” around
Animation
3) Substrate concentration
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The more concentration of substrate, the greater the
rate of reaction
If we do a series of experiments where we maintain the
same enzyme concentration, but increase the substrate
concentration each time, we find that the reaction rate
increases as we increase the substrate concentration…..
But eventually a maximum reaction rate will be
reached
3) Substrate concentration
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Eventually, adding substrate will not increase the reaction
rate any further
At this point, we say that the enzyme is saturated
To increase the rate again, we would need to add more
enzyme
4) Enzyme concentration
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Adding more enzymes
will increase the rate of
reaction
More catalyst means a
faster reaction, so the
reaction rate increases
Reaction rate is basically
“how much substrate
reacts in a particular
amount of time”
(usually per second)
5) Competitive Inhibition
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Some molecules are shaped like a
substrate and compete with the
substrate for the enzyme’s active site
Example: carbon monoxide (CO)
competes with Oxygen (O2)
5) Competitive Inhibition
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Since some of the enzymes get bonded to the “wrong” substrate
(competitive inhibitor), the amount of “correct” product is
reduced
Sometimes these molecules bond temporarily with the enzyme, but
sometimes they bond permanently
When this happens, the enzyme is rendered useless
If too many important enzymes are inactivated, the organism may
die
5) Competitive Inhibition
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A molecule which fits into an enzyme’s active site, but
doesn’t react with anything there, is called a
competitive inhibitor
If this molecule (an inhibitor) is in the active site, a
substrate molecule can’t get in, and that particular
enzyme molecule is inactive until the inhibitor falls off
6) Non-competitive inhibition
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Certain molecules can bond
with parts of enzymes (other
than the active site) and cause
them to change shape
(denature)
This bonding is called noncompetitive inhibition
Example: Heavy Metals such as
Pb2+ (lead) or Hg2+ (mercury)
6) Non-competitive inhibition
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A molecule that bonds to an enzyme (other than in the
active site), causing the enzyme to change it’s shape is
called a non-competitive inhibitor
Once the enzyme changes shape, it is inactive (no
products can form)
When the molecule bonds to the enzyme, the folding of
the enzyme changes a little bit, and the active site is
distorted in a way which makes it a less effective catalyst
Inhibitor
Animation