Enzymes:
“Helper” Protein molecules
Enzymes







Catalysts for biological reactions
Most are proteins
Lower the activation energy
Increase the rate of reaction
Activity lost if denatured
May be simple proteins
May contain cofactors such as metal ions
or organic (vitamins)
Flow of energy through life
Life is built on chemical reactions
Chemical reactions of life
 Processes of life

building molecules
 synthesis

+
breaking down molecules
 digestion
+
Nothing works without enzymes!
 How important are enzymes?

all chemical reactions in living
organisms require enzymes to work
 building molecules
 synthesis enzymes
enzyme
+
 breaking down molecules
 digestive enzymes
live
We can’t
without enzymes!

enzymes speed up reactions
 “catalysts”
enzyme
+
CATALYST –
A SUBSTANCE THAT CAUSES A
CHEMICAL REACTION TO HAPPEN MORE QUICKLY.
Enzymes work by
weakening bonds
which lowers
activation energy
6
Enzymes
Without Enzyme
With Enzyme
Free
Energy
Free energy of activation
Reactants
Products
Progress of the reaction
7
Enzymes are proteins
 Each enzyme is the specific helper to
a specific reaction
each enzyme needs to be the right shape
for the job
 enzymes are named for the reaction
they help

 sucrase breaks down sucrose
 proteases breakdown proteins
 lipases breakdown lipids
 DNA polymerase builds DNA
Enzymes aren’t used up
 Enzymes are not changed by the reaction
used only temporarily
 re-used again for the same reaction with
other molecules
 very little enzyme needed to help in many
reactions

substrate
active site
product
enzyme
Enzyme vocabulary
 Enzyme

helper protein molecule; catalyst
 Substrate

molecule that enzymes work on
 Products

what the enzyme helps produce from
the reaction
 Active site

part of enzyme
that substrate
molecule fits into
It’s shape that matters!
 Lock & Key model

shape of protein allows enzyme &
substrate to fit

specific enzyme for each specific
reaction
Lock and Key Model
P
S
+
+
S
P
E
+
S
ES complex
E
+
12
P
2
1
3
Enzyme Action:
Induced Fit Model
 Enzyme structure flexible, not rigid
 Enzyme and active site adjust shape to


bind substrate
Increases range of substrate specificity
Shape changes also improve catalysis
during reaction
Enzyme Action:
Induced Fit Model
P
S
S
P
E
+
S
ES complex
E
+
P
Factors Affecting Enzyme Action:
Temperature
 Little activity at low temperature
 Rate increases with temperature
 Most active at optimum temperatures

(usually 37°C in humans)
Activity lost with denaturation at high
temperatures
Factors Affecting Enzyme Action
Optimum temperature
Reaction
Rate
Low
High
Temperature
What affects enzyme action
 Correct protein structure
correct order of amino acids
 why? enzyme has to be right shape

 Temperature

why? enzyme has to be right shape
 pH (acids & bases)

why? enzyme has to be right shape
Order of amino acids
 Wrong order = wrong shape = can’t do its
job!
chain of
amino acids
DNA
folded
protein
right shape!
folded
protein
chain of
amino acids
DNA
wrong shape!
Factors Affecting Enzyme Action:
Substrate Concentration
 Increasing substrate concentration

increases the rate of reaction (enzyme
concentration is constant)
Maximum activity reached when all of
enzyme combines with substrate
Factors Affecting Enzyme
Action
Maximum activity
Reaction
Rate
substrate concentration
Temperature
 Effect on rates of enzyme activity

Optimum temperature
 greatest number of collisions between
enzyme & substrate
 human enzymes
 35°- 40°C (body temp = 37°C)
Raise temperature (boiling)
 denature protein = unfold = lose shape
 Lower temperature T°
 molecules move slower
 fewer collisions between enzyme &
substrate

Temperature
reaction rate
human
enzymes
37°
temperature
How do cold-blooded creatures do it?
pH
 Effect on rates of enzyme activity

changes in pH changes protein shape

most human enzymes = pH 6-8
 depends on where in body
 pepsin (stomach) = pH 3
 trypsin (small intestines) = pH 8
pH
intestines
trypsin
What’s
happening
here?!
reaction rate
stomach
pepsin
0
1
2
3
4
5
6
pH
7
8
9
10 11
12 13 14
Enzyme Inhibition
Inhibitors
 cause a loss of catalytic activity
 Change the protein structure of an enzyme
 May be competitive or noncompetitive
 Some effects are irreversible
27
Competitive Inhibition
A competitive inhibitor
 Has a structure similar to
substrate
 Occupies active site
 Competes with substrate for
active site
 Has effect reversed by increasing
substrate concentration
28
Noncompetitive Inhibition
A noncompetitive inhibitor
 Does not have a structure like substrate
 Binds to the enzyme but not active site
 Changes the shape of enzyme and active
site
 Substrate cannot fit altered active site
 No reaction occurs
 Effect is not reversed by adding substrate
 Allosteric Site – undergoing a
change in the shape and activity of a
protein (as an enzyme) that results from
the combination with another
substance at a point other than the
chemically active site.
For enzymes…
What matters?
SHAPE!