2-4
Chemical Reactions &
Enzymes
A chemical reaction is a change in which one or
more substances are converted into new
substances.
A+B
The substances that
will be reacting
together are called
reactants.
C+D
The substances that
are produced are
called products.
Chemical reactions where the reactants
release energy, often occurs almost instantly.
2H2 + O2 → 2H2O
Burning
hydrogen
gas
Oxygen
gas
Water
Exothermic reaction
Chemical reactions where you have to add
energy to the reactants will not happen unless
you add energy to it.
2H2O → 2H2 + O2
Water
Endothermic reaction
Burning
hydrogen
gas
Oxygen
gas
Energy is used to create chemical reactions no
matter which method is used.
+
B
A
Reactants
Energy
Energy changes
into… or forms.
C
Products
You have to put energy into the equation to get a
reaction to work
So how much ENERGY does it take to start a
chemical reaction?
The amount of ENERGY needed to start a
chemical reaction is called the ACTIVATION
ENERGY
Activation Energy
If not enough energy is added, the ball cannot
reach the Activation Energy level and there is
NO REACTION.
If enough energy
is added, the ball reaches
the Activation energy level and the Chemical
Reaction Occurs
Chemical A
(Reactant)
Chemical B
(Reactant)
Activation
energy
Chemical C
(Product)
When the reaction occurs, ENERGY is
RELEASED because the ENERGY is changing
forms.
Energy
Released
B A
Product
ENZYMES
ENZYMES are proteins that help speed up a
chemical reaction.
Another word for this (speed up reaction) is
called CATALYST
A great example of an enzyme is in your
stomach with the enzyme Pepsin.
(which breaks down proteins like in a steak
that you eat)
When you eat, your food is still too big to
digest properly.
Your stomach can’t absorb the food this way
Your stomach releases an ENZYME called
PEPSIN, which breaks down the food proteins
into small monomers which the body absorbs
for energy.
If you didn’t have this enzyme, the stomach
acids would take days for it to dissolve the
food into smaller pieces.
What happens if you don’t eat for days?
1. You’re sluggish / no energy
2. Body systems start to shut down
3. Leads to illness
So the enzyme PEPSIN, speeds up the
chemical breakdown of proteins in our food.
How does it do it? – It lowers the
ACTIVATION ENERGY
Energy
Released
B A
Product
Remember this?
Pepsin lowers the activation level like this.
Energy
Released
B A
You don’t need as much energy!!!
ENZYMES:
1. Speed up a reaction
2. Lower Activation Energy.
3. Are reusable?
Yes, enzymes can be used over an over and
over like re-chargeable batteries.
Let’s look at the enzyme “Pepsin” up close…
Pepsin attaches one of the reactants (protein)
The reactant it attaches to is called a
*SUBSTRATE*
Where it attaches is called the “Active Site”
Active site
Substrate
(protein)
Pepsin
PEPSIN
Pepsin is very specific for one Substrate only!
(Just like only 1 key can fit a lock) All enzymes
are specific
Note how the
curves fit
perfectly
together…
Substrate
PEPSIN
Pepsin
When the enzyme and substrate join together,
it gets a new name called the:
Enzyme-Substrate Complex
Substrate
Pepsin
PEPSIN
PEPSIN
This is when the chemical reaction occurs!
Notice that the lock & key curves are the
same, yet the chemical reaction makes the
new products in that same space.
Pepsin
After the chemical reaction, the new Products
(proteins) are made and the Pepsin looks for
more Substrate to break down.
Pepsin
The Process Simplified…
Pepsin
+
Enzyme + Substrate
E + S
Pepsin
Enzyme
Substrate
Complex
ESC
Pepsin
+
Enzyme + Product(s)
E + P
Since enzymes are specific for a certain
substrate, there are many enzymes in your
body.
Enzymes were originally named with a “-in”
ending (Pepsin).
Later on, scientist started naming the enzymes
with a “-ase” ending
Since Carbohydrate sugars are named with a “ose” ending, there is a enzyme that is specific
for each one.
SUGAR NAME
Lactose
ENZYME NAME
Lactase
Sucrose
Sucrase
Maltose
Maltase
What I want you to remember from this is:
Sugars – have “ose” endings
Enzymes – have “ase” endings
What could cause enzymes not to work
properly?
When an enzyme doesn’t work properly it is
said to be Denatured - the active sites on an
enzyme change in shape, therefore unabling
the enzyme to lock up with a substrate
Pepsin
Causes of Denaturation are:
1. Too high of Acid or Base (pH) in your
body
2. Temperature
Pepsin differences (hot / cold)
fever, chills
3. Lack of Water / Oxygen
One of the most important reactions in your
body involves the enzyme Catalase.
Catalase turns the cell waste product of
Hydrogen Peroxide (H2O2) into Water and
Oxygen gas.
The reaction is:
2H2O2 ----> 2H2O + O2 (fix)
Hydrogen
Peroxide
---->
Water
Oxygen
Gas