Transcript Enzyme Structure and Function11

Enzyme Structure and
Function
Protein catalysts
Enzymes are Catalysts
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This means that enzymes help speed
up chemical reactions.
How?
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Enzymes lower the activation energy of
a reaction. (Energy needed to get it going)
If the Ea is lower, more reactant
molecules will make it over the
“energy hill”, so more products are
produced per second.
Lock and Key Model
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Enzymes are specific
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Each enzyme has a unique “active site”.
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each one helps only one type of reaction to occur.
An enzyme binds a substrate molecule in its active
site.
When the substrate is bound to the enzyme,
the bonds are easier to break, so less energy
is required for the reaction to occur.
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_work.html
“Lock and Key” Model
Enzymes are Organic Catalysts
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Organic compound = made by living things,
carbon based.
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Lipids, carbohydrates, proteins, nucleic acids
Enzymes are proteins.
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Proteins are polypeptides. They are long
chains of amino acids.
The number, type, and sequence of amino
acids determines a protein’s shape.
So, what makes each enzyme different (i.e.: the
active sites different) are the amino acids that
make up each enzyme.
Enzyme (and protein) structure
“Magnified” section of
protein (unfolded)
This is the enzyme
“catalase”, the
enzyme found in liver
Review: Amino Acids
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“Building Blocks” of proteins.
20 different ones
Each one has an amino group, a
carboxyl group, and an “R-group”.
The R-group of each amino acid is
unique.
General
Structure
Specific Examples
More Review of Protein Structure
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An enzyme (and any other protein) is made by
linking amino acids together in the correct order
(sequence).
Reaction = dehydration synthesis
(condensation) reaction.
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Many of these reactions occur until the protein consists of
100+ amino acids linked together.
The amino acid chain (polypeptide)
twists and folds to form a functional
protein
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The sequence of amino
acids making up an
enzyme determines
how that enzyme folds
into the correct 3-D
shape, allowing it to bind
with its substrate.
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So a “real” enzyme looks
something like this.
An enzyme cannot function if it
has an improper shape
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Environmental factors:
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May temporarily change the shape of the
enzyme = denaturation
May “block” the active site, rather than
change the shape
Genetic disorder:
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Enzyme never folds into the correct shape
because the “recipe” is wrong.
Adding the wrong amino acid(s) is like
adding the wrong ingredient(s) = incorrect
product.
The Story so far…
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Substrate Concentration:
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Decreases over time
Less collisions between substrate &
enzyme = decreased reaction rate
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Bubbling slows due to less hydrogen peroxide
molecules colliding with liver enzymes.
(Remember the toothpicks)
When all of the substrate has been used,
the reaction stops.
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Heavy Metal Concentration:
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As heavy metal concentration increases,
reaction rate decreases.
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A heavy metal can bind to an enzyme’s
active site.
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The copper sulfate soaked liver had a reaction rate of 1
vs. the water soaked liver had a reaction rate of 3.
Think of the straws mixed with toothpicks.
Enzyme can’t bind to substrate as often so
the reaction is slower.
Metal Ions and Enzyme Inhibition
Hydrogen peroxide
(substrate) binds to
enzyme and is broken
down (reaction occursbubbling observed.)
Metal ion gets in the way
and hydrogen peroxide
cannot bind to the
enzyme. Reaction either
doesn’t occur or occurs
at a slower rate. (Not all
enzymes are blocked.)
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pH
 For catalase (liver enzyme) a neutral pH
(7) is ideal. Lowering or raising pH
decreases reaction rate.
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Acid (pH of 2): reaction rate of 1
Base (pH of 12): reaction rate of 2
H+ and OH- ions do NOT block the
enzyme’s active site. They DO change the
3-D shape of an enzyme, thereby changing
its active site.
“Lock and Key” no longer match; enzyme
can’t bind to substrate when they collide.
pH and denaturation
Not all enzymes have the same “optimal” pH. Catalase
(liver enzyme) is more like chymotrypsin. However,
pepsin (a stomach enzyme) functions best at a low
(acidic) pH. At pH 1, pepsin is in it’s functional shape; it
would be able to bind to its substrate. At pH 5, the
enzyme’s shape is different and it no longer has an active
site able to bind the substrate. The change in enzyme
activity is observed as a difference in reaction rate.
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Temperature
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The ideal temperature for the functioning of
catalase is around 40°C (human body temp.)
At lower temperatures enzyme and substrate
molecules move less, so fewer collisions occur
and the reaction is slower
At higher temperatures the 3-D shape of the
enzyme changes and the active site takes on
a different shape (denaturation)
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Substrate can’t fit into active site
Temperature and Reaction Rate
Shape of active
site changes
(substrate can’t
bind)
Slower reaction due
to fewer collisions
http://www.kscience.co.uk/animations/model.swf