Lecture 11
Enzymes
Enzymes
• Biological catalysts
– Lower the activation energy, increase the speed of
a reaction (millions of reactions per minute!)
How Enzymes Work
 Enzymes = large protein molecules that
function as biological catalysts.
 Catalyst = chemical that speeds up a reaction
without being consumed
 NOTE: enzyme names end in –ase and are
often named after their substrates
 The enzyme that hydrolyzes sucrose is sucrase
 Hydrolases, Add water during Hydrolysis
reactions
Energy of activation(Ea)
• There is an energy barrier that must be
overcome before a chemical reaction can
begin. This is called The energy of activation
• Enzymes speed up the reaction by lowering
the Ea barrier
• Energy of Activation
How Enzymes Work
– Enzymes lower the barrier
– Eg.Mexican jumping bean
How Enzymes Work
The effect of an enzyme on the energy of activation
WITHOUT ENZYME
WITH ENZYME
Activation
energy
required
Less activation
energy required
Reactants
Reactants
Product
PLAY
Product
Animation: Enzymes
Figure 2.20
How Enzymes Work
 Enzymes
 Very selective
 3D shape that determines its specificity for a
substrate
 Substrate = the substance that the enzyme works
on
 Substrate binds to the enzyme in the active site
 Pocket or groove on protein surface where binding
occurs
Enzyme action
Three basic steps involves in enzyme action:
1) The enzyme active site binds to the
substrate.
2)The enzyme –substrate complex undergoes
internal rearrangement that form the product.
3)The enzyme releases the the product of the
reaction.
How Enzymes Work
Substrates (S)
e.g., amino acids
+
Product (P)
e.g., dipeptide
Energy is
absorbed;
bond is
formed.
Water is
released.
H2O
Peptide
bond
Active site
Enzyme (E)
Enzyme-substrate
complex (E-S)
1 Substrates bind
2 Internal
at active site.
rearrangements
Enzyme changes
leading to
shape to hold
catalysis occur.
substrates in
proper position.
Enzyme (E)
3 Product is
released. Enzyme
returns to original
shape and is
available to catalyze
another reaction.
Figure 2.21
Substrates (S)
e.g., amino acids
+
Active site
Enzyme (E)
Enzyme-substrate
complex (E-S)
1 Substrates bind
at active site.
Enzyme changes
shape to hold
substrates in
proper position.
Figure 2.21, step 1
Substrates (S)
e.g., amino acids
+
Energy is
absorbed;
bond is
formed.
Water is
released.
H2O
Active site
Enzyme (E)
Enzyme-substrate
complex (E-S)
1 Substrates bind
2 Internal
at active site.
rearrangements
Enzyme changes
leading to
shape to hold
catalysis occur.
substrates in
proper position.
Figure 2.21, step 2
Substrates (S)
e.g., amino acids
+
Product (P)
e.g., dipeptide
Energy is
absorbed;
bond is
formed.
Water is
released.
H2O
Peptide
bond
Active site
Enzyme (E)
Enzyme-substrate
complex (E-S)
1 Substrates bind
2 Internal
at active site.
rearrangements
Enzyme changes
leading to
shape to hold
catalysis occur.
substrates in
proper position.
Enzyme (E)
3 Product is
released. Enzyme
returns to original
shape and is
available to catalyze
another reaction.
Figure 2.21, step 3
Characteristics of Enzymes
• Often named for the reaction they catalyze;
usually end in -ase (e.g., hydrolases, oxidases)
• Some enzymes are purely protein.
• Some functional enzymes (holoenzymes)
consist of two parts:
– Apoenzyme (protein)
– Cofactor (metal ion) or coenzyme (a vitamin)
Characteristics of Enzymes
• Cofactors
– Nonprotein helpers
• May be inorganic substances
– Zinc, iron or copper
– Magnesium is a cofactor essential for proper functioning
of chlorophyll
• Coenzymes
– Organic helpers
• Vitamins
– Vitamin B6 is used to convert one amino acid to another
• Enzyme activity is affected by its
environment
– Temperature affects molecular motion
• Enzyme’s optimal temperature is when there is the
highest rate of contact between the enzyme and
substrate
• Temperature too high – denaturation
– Changes the shape and the function of the enzyme
– Salt concentration
• Salt interferes with some of the chemical bonds
that maintain protein shape
– pH
• Same is true for pH outside of the 6-8
range.eg.digestive enzyme produce in pancreas are
activated in small intestine.
Characteristics of Enzymes
• Sometimes enzymes are inactivated
immediately after they have performed .eg.
blood clot formation.
Thank you