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1. THEME:
STRUCTURE AND PROPERTIES OF ENZYMES. THE
MECHANISM OF ENZYMES ACTION.
CLASSIFICATION OF ENZYMES. ISOENZYMES.

2. Definition
 Enzymes are protein catalysts for
biochemical reactions in living cells
 Enzymes are neither consumed nor altered
during their participation in enzymatic
reaction
 RNA molecules or ribozymes can have
enzymatic activities also.
 They are highly efficient and extremely
selective catalysts

3. Naming
The name enzyme (from Greek word "in yeast")
was not used until 1877,
but much earlier it was suspected that
biological catalysts
are involved in the fermentation of sugar
to form alcohol
(hence the earlier name "ferments").

4. Naming and Classification of
Enzymes
 Many enzymes have been named by adding the
suffix -ase to the name of the substrate, i.e., the
molecule on which the enzyme exerts catalytic
action.
 For example, urease catalyzes hydrolysis of
urea to ammonia and CO2, arginase catalyzes
the hydrolysis of arginine to ornithine and
urea, and phosphatase the hydrolysis of
phosphate esters.

5. Classification of enzymes
 Oxido-reductases (oxidation-reduction
reaction).
 Transferases (transfer of functional groups).
 Hydrolases (hydrolysis reaction).
 Lyases (addition to double bonds).
 Isomerases (izomerization reactions).
 Ligases (formation of bonds with ATP
cleavage).

6. The structure of enzymes
 Protein part + Non- protein part
 Apoenzyme + Cofactor = Holoenzyme
 Function of apoenzyme:
 It is responsible for the reaction
 Function of cofactor:
 It is responsible for the bonds formation between
enzyme and substrate
 Transfer of functional groups
 Takes plase in the formation of tertiary structure of
protein part

7. Cofactor
 1. Prosthetic group (when cofactor is very
tightly bound to the apoenzyme and has small
size )
 2. Metal ion
 3. Cofactors that are small organic molecules
are called coenzymes.
 4. Coenzymes (organic molecule derived from
the B vitamins which participate directly in
enzymatic reactions, but bind in a transient,
dissociable manner either to the enzyme or to
a substrate)

8. Prosthetic group
 Heme group of cytochromes
 Biothin group of acetyl-CoA carboxylase
 Pyridoxal phosphate
 Flavin mononucleotide( FMN)
 Flavin adenine dinucleotide(FAD)
 Thiamin pyrophosphate (TPP)

9. Metal ions
 Fe - cytochrome oxidase, catalase
 Cu - cytochrome oxidase, catalase
 Zn - alcohol dehydrogenase
 Mg - hexokinase, glucose-6-phosphatase
 K, Mg - pyruvate kinase
 Na, K – ATP-ase

10. Coenzyme
 B1
 TPP- Thiamine Pyro Phosphate
 B2
 FAD- Flavin Adenine Dinucleotide
 FMN- Flavin Mono Nucleotide
 Pantothenic acid
 Coenzyme A (CoA)
 B5
 NAD – Nicotinamide Adenine Dinucleotide
 NADP- Nicotinamide Adenine Dinucleotide
Phosphate

11.  It have a cleft or pocket form
 Takes up a relatively small part of the total
volume of an enzyme
 Substrates are bound to enzymes by multiple
weak attractions
 The specificity of enzyme depends on the
arrangement of atoms in an active site
Active Site

12.  There are two models present to explain how an enzyme binds
with substrate:
 - the lock-and –key model
 - the induced-fit model.
Enzyme substrate binding
Lock-and-Key Model of Enzyme-
Substrate Binding.
The active site of the unbound enzyme is
complementary to the substrates hape .

13. Induced-Fit Model
The enzyme changes shape during substrate binding. The active site
forms a shape complementary to the substrate only after the substrate
has been bound. When a substrate approaches and binds to an
enzyme they induce a conformational changes, complementary to
placing a hand (substrate) into a glove (enzyme).

14.  The basic enzymatic reaction can be represented as
follows
 E+S ES EP E+P
 The mechanism of enzymes action can be explained
by two reasons:
 Thermodynamic changes
 Processes at the active site
Mechanism of Action of Enzymes

15.  All enzymes accelerate reaction rates by
providing transition states with a lowered
∆G for formation of the transition states.
The lower activation
energy means that more
molecules have the
required energy to
reach the transition
state.
1) Thermodynamic changes

16. The Free Energy of
Activation
 Before a chemical reaction can take place, the
reactants must become activated.
 This needs a certain amount of energy which is
termed the energy of activation.
 It is defined as the minimum amount of energy
which is required of a molecule to take part in
a reaction.

17. The Free Energy of
Activation
 For example,decomposition of hydrogen
peroxide without a catalyst has an energy
activation about 18 000. When the enzyme
catalase is added, it is less than 2000.

18. The Free Energy of
Activation
 The rate of the reaction is proportional to
the energy of activation:
 Greater the energy of activation
 Slower will be the reaction
 While if the energy of activation is less,
 The reaction will be faster

19. Thermodynamic changes

20. Energy of Activation

21. Chemical Kinetics

22. The Michaelis-Menten
Equation
 In 1913 a general theory of enzyme action and kinetics
was developed by Leonor Michaelis and Maud Menten.

1. Point А.
2. Point В.
3. Point С.

23. Effect of pH on Enzymatic
Activity
 Most enzymes have a characteristic pH at
which their activity is maximal (pH-
optimum);
 above or below this pH the activity
declines. Although the pH-activity profiles
of many enzymes are bell-shaped, they may
be very considerably in form.

24. Effect of pH on Enzymatic
Activity

25. Effect of Temperature on
Enzymatic Reactions
.The rate of enzyme catalysed reaction generally
increases with temperature range in which the
enzyme is stable. The rate of most enzymatic
reactions doubles for each 100 C rise in
temperature. This is true only up to about 500 C.
Above this temperature, we observe heat
inactivation of enzymes.
The optimum temperature of an enzyme is that
temperature at which the greatest amount of
substrate is changed in unit time.

26. Effect of Temperature on
Enzymatic Reactions

27. Allosteric enzymes have a second regulatory site
(allosteric site) distinct from the active site
Allosteric enzymes contain more than one polypeptide
chain (have quaternary structure).
Allosteric modulators bind noncovalently to allosteric
site and regulate enzyme activity via conformational
changes
Allosteric enzymes

28. 2 types of modulators (inhibitors or activators)
• Negative modulator (inhibitor)
–binds to the allosteric site and inhibits the action of the
enzyme
–usually it is the end product of a biosynthetic pathway
- end-product (feedback) inhibition
• Positive modulator (activator)
–binds to the allosteric site and stimulates activity
–usually it is the substrate of the reaction

29. • PFK-1 catalyzes an early step in glycolysis
• Phosphoenol pyruvate (PEP), an intermediate
near the end of the pathway is an allosteric
inhibitor of PFK-1
Example of allosteric enzyme - phosphofructokinase-1
(PFK-1)
PEP

30. Regulation of enzyme activity by
covalent modification
Covalent attachment of a molecule to an amino acid side chain of a
protein can modify activity of enzyme

31. Phosphorylation reaction

32. Dephosphorylation reaction
Usually phosphorylated enzymes are
active, but there are exceptions (glycogen
synthase)
Enzymes taking part in phospho-rylation are
called protein kinases
Enzymes taking part in dephosphorylation
are called phosphatases

33. Activation by proteolytic cleavage
• Many enzymes are synthesized as inactive precursors
(zymogens) that are activated by proteolytic cleavage
• Proteolytic activation only occurs once in the life of an enzyme
molecule
Examples of specific proteolysis
•Digestive enzymes
–Synthesized as zymogens in stomach and pancreas
•Blood clotting enzymes
–Cascade of proteolytic activations
•Protein hormones
–Proinsulin to insulin by removal of a peptide

35. • Multienzyme complexes - different enzymes that
catalyze sequential reactions in the same pathway are
bound together
• Multifunctional enzymes - different activities may
be found on a single, multifunctional polypeptide
chain
Multienzyme Complexes and
Multifunctional Enzymes

36. Metabolite channeling
• Metabolite channeling - “channeling” of reactants
between active sites
• Occurs when the product of one reaction is transferred
directly to the next active site without entering the bulk
solvent
• Can greatly increase rate of a reactions
• Channeling is possible in multienzyme complexes and
multifunctional enzymes

37. Enzyme Inhibition
1. Reversible inhibition
A. Competitive
B. Non-competitive
C. Uncompetitive
2. Irreversible inhibition

38. Competitive Inhibition

39. Usage competitive inhibition in
medicine
 The antibacterial effects of sulfanilamides
are also explained by their close
resemblance to para-amino-benzoic acid
which is a part of folic acid, an essential
normal constituent of bacterial cells. The
sulfanilamides inhibit the formation of folic
acid by bacterial cells and thus the bacterial
multiplication is prevented and they soon
die.

40. Non-competitive Inhibition
 In this case, there is no structural
resemblance between the inhibitor and the
substrate. The inhibitor does not combine
with the enzyme at its active site but
combines at some other site.
 E + S +I =ESI (INACTIVE COMPLEX)
E + S = ES
ES + I = ESI

41. Uncompetitive inhibition
 E + S +I =ESI (No active complex)

42. Irreversible Inhibition
 The inhibitor is covalently linked to the
enzyme.
 The example:
 Action of nerve gas poisons on
acetylcholinesterase,an enzyme that has an
important role in the transmission of nerve
impulse.

43. These are the enzymes from the same
organism which catalyse the same reaction
but are chemically and physically different
from each other.
Isoenzymes

44. Lactate dehydrogenase
 It occurs in 5 possible forms in the blood
serum:
 LDH1
 LDH2
 LDH3
 LDH4
 LDH5

45. Structure of LDH
 Each contains 4 polypeptide chains which
are of 2 types: A and B which are usually
called M (muscle) and H (heart).
 LDH1 –H H H H
 LDH2 – H H H M
 LDH3 – H H M M
 LDH4 – H M M M
 LDH5 – M M M M

46. Clinical importance of LDH
 Acute myocardial infarction
 LDH1 and LDH2
 Acute liver damage
 LDH4 and LDH5

47. Creatine kinase
 It has 3 isoenzymes:
 CK1
 CK2
 CK3
 Clinical importance:
 When patient have acute myocardial infarction
CK appears in the blood 4 to 8 hours after onset of
infarction and reaches a peak in activity after 24
hours.

48. Enzyme-Activity Units
 The most widely used unit of enzyme activity is
international unit defined as that amount which
causes transformation of 1.0 mkmol of substrate
per minute at 25°C under
 The specific activity is the number of enzyme units
per milligram of protein.

49. Enzyme-Activity Units
 The molar or molecular activity, is the
number of substrate molecules transformed
per minute by a single enzyme molecule
 The katal (abbreviated kat), defined as the
amount of enzyme that transforms 1 mol of
substrate per 1 sec.