2.5 Enzymes
KEY CONCEPT
Enzymes are catalysts for chemical reactions in
living things.
2.5 Enzymes
!   A catalyst lowers activation energy.
•  Catalysts are substances that speed up chemical
reactions.
–  decrease activation energy
–  increase reaction rate
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!   Enzymes allow chemical reactions to occur under tightly
controlled conditions.
•  Enzymes are catalysts in living things.
–  Enzymes are needed for almost all processes.
–  Most enzymes are proteins.
–  Most enzyme names end in –ase
–  Enzymes are specific for what they catalyze:
Ex. Sucrase- breaks down sucrose
Lipase- breaks down lipids
Protease- ?
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•  An enzyme s structure allows only certain reactants to
bind to the enzyme.
–  substrates
–  active site
substrates
(reactants)
enzyme
Substrates bind to an
enzyme at certain places
called active sites.
2.5 Enzymes
1.  An enzyme and a substrate are in the
same area.
2. The enzyme grabs onto the substrate
with a special area called the active site
The active site is a specially shaped area
of the enzyme that fits around the
substrate. The active site is the keyhole
of the lock.
3. Chemical reaction takes place
4. The enzyme lets go. Big idea. When the
enzyme lets go, it returns to normal,
ready to do another reaction. The
substrate is no longer the same. The
substrate is now called the product.
2.5 Enzymes
•  The lock-and-key model helps illustrate how enzymes
function.
–  substrates brought together
–  bonds in substrates weakened
Substrates bind to an
enzyme at certain places
called active sites.
The enzyme brings
substrates together
and weakens their
bonds.
The catalyzed reaction forms
a product that is released
from the enzyme.
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!   Induced Fit Model vs. Lock and Key Model
•  Induced Fit Model claims that substrate binding slightly
changes the shape of the enzyme.
2.5 Enzymes
•  An enzyme s function depends on its structure. If
the shape of the active site is changed, the enzyme
no longer functions.
–  Enzymes function best in a small range of conditions.
–  Changes in temperature and pH can break hydrogen
bonds which changes the enzyme’s shape.
–  Inhibitors also effect the way enzymes work
2.5 Enzymes
!I
  nhibitors also can prevent
enzymes from working properly
Competitive inhibitors:
are chemicals that
resemble an enzyme s
normal substrate and
compete with it for Substrate
the active site.
Competitive inhibitor
Enzyme
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!  Inhibitors continued…
Noncompetitive inhibitors:
Inhibitors that do not enter the
active site, but bind to another part
of the enzyme causing the enzyme to
change its shape, which in turn alters
the active site.
Substrate
active site
altered
Enzyme
Noncompetitive
Inhibitor
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