Introduction Structure Biological role

1. ENZYMES (PART I)
• Mechanism of
action
• Structure of the
enzymes
• Classification of
enzymes

2. Definition
• Enzymes are protein catalysts that
speed up the rate of a chemical
reaction.
Model of the surface of an enzyme.

3. Structure of enzymes
• All enzymes are proteins in nature
with large molecular mass.
• However, there are also:
- Catalytically active ribonucleic acids
(ribozyme) and artificially synthesized
antibody with catalytic action (abzyme)

4. Enzymes
• Some enzymes are simple proteins and
some are conjugated proteins.
• Some enzymes are monomeric in nature,
and some are oligomeric.
• All enzymes have active site a small
region at which the substrate binds.
• The active site have a substrate binding
site and catalytic site.

5. • Active site of most
enzymes is lined
with hydrophobic
amino acids.
Substrate binds at
the active site by
weak covalent
bonds.
Hexokinase,
an enzyme
(blue),
binding its
substrate,
glucose
(yellow).

6. Biological role of enzymes
• Chemical reactions have two independent
properties, their energy and their rate.
• Free energy (G)- represents the amount
of energy released or required for
conversion of 1 mole of reactant.
• The rate of reaction is determined by the
energy of activation, which is the energy
required to initiate the reaction.

7. Comparison of Energy and Rate
Not affected
by enzymes
Increased
by enzymes
Energy ( G) Rate (V)
G <0
-termodinamically
spontaneous (energy released
often irreversible)
G >0
-termodinamically
nonspontaneous
(energy required)
G = 0
-reaction at equilibrium
(freely reversible)
Decrease
energy of
activation

9. Uncatalyzed reaction
• The most reactions involving organic
substances have slow rates, because
requires high activation energy.

10. Uncatalyzed reaction
• 1. In solution reactants A and B are
surrounded by hydration shell.
• 2. They can only react with each other if
they collide in a favorable orientation
(formation of collision complex I)
• 3. Collision complex A-B has to pass
through a transition state, the formation
of which usually requires a large amount
of activation energy.

11. Uncatalyzed reaction
• 4. A large portion of activation energy
required for the removal of the hydration
shells.
• 5. Only small amount of A-B complexes
can produce this amount of energy.
• Hence the uncatalyzed organic reaction is
slow.
• 6. Formation of products: after transition
state – spontaneously.

12. Catalysts: principle

13. Enzyme-catalyzed reaction
Sequential mechanism

14. Enzyme-catalyzed reaction
• Sequential mechanism in which substrate
A and B are bound with enzyme and
products C and D are released.
1. Enzymes are able to bind the specific
substrate at the active site (1-2-3 steps)
• In this process the substrates A and B are
oriented in relation to each other to the
optimum orientation for the formation of
transition state (4th step)

15. Enzyme-catalyzed reactions
• Binding of substrates results in removal of
their hydration shells.
• Reaction inside of the enzyme (5th step)
• Release of products C and D (6th step)

16. Principles of enzymes catalysis

17. “Ping-pong” mechanism

18. “Ping-pong” mechanism
• The 1st substrate is bound and
immediately cleaved formed covalent
intermediate product-enzyme complex.
• A part of this substrate remains bound to
the enzyme and is then transferred to the
2nd substrate B and product P2 is
released.

19. Coenzymes
• In many enzyme-catalyzed reactions,
electrons or groups of atoms are transferred
from one substrate to another.
• Helper molecule of this type are called
coenzymes
A HELPER
MOLECULE
electrones, H+
B

20. Coenzymes
• Coenzymes cooperate with many
enzymes.
• They are not catalytically active
themselves.
• Classifications of coenzymes based on
the type of interaction with the enzyme

21. Classification of coenzymes (based
on the type of interaction)
• Soluble coenzymes (usually vitamins in nature). –are
bound like substrates, undergo change and then
released again.
• Prosthetic groups are tightly bound to enzyme and
remain associated with it during the reaction (eg. Heme-
containing enzymes)

22. Classification of coenzymes
• Soluble coenzymes can be removed from
enzymes by dialysis.
• Prosthetic group can not be removed from
enzyme by dialysis.

24. Oxidoreductases
(require coenzymes)

25. Coenzymes of oxidoreductases