1. ENZYMES
A catalyst
Is a chemical agent that speeds up a reaction without being consumed by the reaction
An enzyme is an organic catalyst
Enzymes are proteins

2. 1. An Enzyme Enzyme Active Site Substrate (Reactants) Inhibitor
Allosteric Site

3. 2. Enzyme Specificity Enzymes can only work with certain substrates
The shape of the enzyme must match the shape of its substrate
- The root of the enzyme’s name typically indicates the substrate which it acts upon
e.g. ATPase, Amylase, Sucrase

4. 3. Induced Fit Model
Figure 8.16
Substate
Active site
Enzyme
(a)
Figure 8.16
(b)
Enzyme- substrate
complex
When the substrate binds to the active site, the enzyme changes conformation (shape) to make a better fit.
Interactions between chemical groups on substrate and those of the amino acids as well as the shape of the active site cause the induced fit

6. Sucrose Glucose + Fructose
Sucrase
Sucrose + Sucrase  Glucose + Fructose + Sucrase

7. Progress of the reaction
Activation Energy
The initial amount of energy needed to start a chemical reaction (i.e. break the bonds)
Free energy
Progress of the reaction EA
Figure 8.14 A B C D
Reactants
Transition state
Products
activation energy  EA

8. Progress of the reaction
What do enzymes do?
Enzymes lower the activation energy
How?
Orienting substrates correctly
Putting stress on substrate bonds
Providing a favorable environment
This increases the rate of the reaction.
Progress of the reaction
Products
Course of
reaction
without
enzyme
Reactants
with enzyme EA
EA with
is lower
Free energy
Figure 8.15

9. 5. Increasing the Rate of Reaction
Increase the number of substrate molecules in solution (increase conc’n)
Increase the number of enzymes in solution (increase conc’n)
Increase the temperature of the solution (up to a certain point)

10. 6. Saturation
A reaction is said to be “saturated” when 100% of enzymes have their active sites filled with substrate.
Vmax is maximum
velocity (speed) of rxn

11. Question 7.
If there are left-over reactants (substrates), then you could add more enzymes.
If there are no more left-over reactants (substrates), then adding more enzymes will not increase the rate.

12. 8. Stopping a Reaction E.g. Add H2SO4(aq) (Sulfuric Acid)
Change the pH so it is above or below its optimal value.
This changes the enzyme’s conformation (shape) making it lose it dysfunctional
e.g. add H2SO4(aq) – Sulfuric Acid
E.g. Add H2SO4(aq) (Sulfuric Acid)

13. 9. Effect of Increasing Temperature
Above a certain temperature, enzymes’ activity starts to decline because the enzyme begins to denature (unravel)

14. Metabolic Pathways
Reactions occur in a sequence and specific enzymes catalyze each step

15. 10. Cyclic Metabolic PathwaysZ U Y V W X
Z + Y  U
U  V  W
W  X + Y
(b) Initial Reactant: Z
End product: X
(desired product)

16. 11. Competitive vs. Non-competitive Inhibition
Figure 8.19
(b) Competitive inhibition
A competitive
inhibitor mimics the
substrate, competing
for the active site.
Competitive
inhibitor
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Active site
Enzyme
(a) Normal binding
If the inhibitor attaches by weak bonds, it is usually reversible.
Figure 8.19
A noncompetitive
inhibitor binds to the
enzyme away from
the active site, altering
the conformation of
the enzyme so that its
active site no longer
functions.
Noncompetitive inhibitor
(c) Noncompetitive inhibition
If the inhibitor attaches by covalent bonds, it is usually irreversible (usually (c) is)

17. 12. Allosteric Regulation: Activation and Inhibition
Stabilized inactive form
Allosteric activater stabilizes active from
Allosteric enyzme with four subunits
Active site (one of four)
Regulatory site (one of four)
Active form
Activator
Stabilized active form
Allosteric activater stabilizes active form
Inhibitor
Inactive form
Non- functional active site
(a) Allosteric activators and inhibitors. In the cell, activators and inhibitors dissociate when at low concentrations. The enzyme can then oscillate again.
Oscillation
Figure 8.20
When one activator or inhibitor bind to an allosteric site, and have an effect on all four subunits of an enzyme
Bonds are non-covalent, so they are reversible

18. Feedback Inhibition
Active site available
Isoleucine used up by cell
Feedback inhibition
Isoleucine binds to allosteric site
Active site of enzyme 1 no longer binds threonine; pathway is switched off
Initial substrate (threonine)
Threonine in active site
Enzyme 1 (threonine deaminase)
Intermediate A
Intermediate B
Intermediate C
Intermediate D
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
End product (isoleucine)
Figure 8.21
The end product of a metabolic pathway shuts down the pathway
Prevents the cell from wasting chemical resources