The document provides an introduction to enzymes, their classification, and mechanisms of action, highlighting their role as biological catalysts that significantly speed up metabolic reactions. It discusses the chemical nature of enzymes, their specificity, the factors affecting their activity such as temperature, pH, and the presence of inhibitors. Enzymes are categorized into six classes based on the reactions they catalyze and can be influenced by various factors including substrate and cofactor concentration.

1. ‫الرحيم‬ ‫الرحمن‬ ‫الله‬ ‫بسم‬
‫أشرف‬ ‫على‬ ‫والسالم‬ ‫والصالة‬
‫األنبياء‬
‫محمد‬ ‫سيدنا‬ ‫المرسلين‬ ‫و‬
‫وسلم‬ ‫عليه‬ ‫الله‬ ‫صلى‬

2. Biochemistry
CHEM 250
Chapter 8
Lecture 8

4. Introduction
• Virtually all reactions in the body are mediated by enzymes, thus
enzymes direct all metabolic events.
• Enzymes are biological catalysts produced by living cells. They can
work outside these cells.
• Virtually all enzymes are proteins, however, a few examples of RNA
as show enzymatic activity; they are called ribozymes.
• Enzymes accelerate chemical reactions without changing the
equilibrium point.
• They function in minute amounts and remain unchanged chemically
during the reaction.
• They are highly specific in their action.
• They are very sensitive to slight changes in temperature and pH.

5. The turnover number (catalytic efficiency)
:
• Most enzyme-catalyzed reactions are highly efficient, proceeding
from 103
to 108
times faster than uncatalyzed reactions.
• Each enzyme molecule is capable of transforming 100 to 1000
substrate molecules into product each second.
• “The number of molecules of substrate converted to product per
enzyme molecule per second is called
the turnover number”

6. Chemical nature of enzymes
:
Virtually all enzymes are proteins; some are
 simple proteins e.g. pepsin while others are
 conjugated proteins.
• In the latter type the enzyme consists of
• a protein part, the apoenzyme and
• a non-protein part.
• Both of which form together the “holoenzyme”.
• 1-Coenzyme is the organic nonprotein part of the holoenzyme. It is
loosely attached to the apoenzyme; when it is firmly attached it is
called prosthetic group.
• Coenzymes function as group transfer agents. They temporarily
carry part of the reactants or products during the reaction.

7. • 2-Activator is an inorganic non-protein part of the holoenzyme
that is loosely attached to the apoenzyme. When it is firmly
attached it is called prosthetic group.
• Inorganic metal ions are required for the activity of more than
25% of enzymes.
Holoenzyme
Activator (inorganic)
Protein part (apoenzyme) Non protein part
Coenzyme (organic)

8. Enzyme terminology :
1-Hydrolyzing enzymes are usually named by adding the
suffix -ase to the name of the substrate
• e.g. amylase, lactase and lipase which hydrolyze starch,
lactose and triacyiglycerol respectively.
2-Other enzymes are named by adding their function to the
name of the substrate.
• Thus lactate dehydrogenase removes hydrogen from lactic
acid
and alanine transaminase transfers amino group from alanine.
3-Some enzymes retain their original name,
• which give no hint of the associated enzymatic reaction e.g.
pepsin and trypsin.

9. CLASSIFICATION OF ENZYMES
• According to the type of chemical reaction catalyzed, enzymes
are classified into 6 major classes:
1
.
Oxidoreductases: oxidation-reduction reactions
2
.
Transferases: transfer of functional groups
3
.
Hydrolases: hydrolysis reactions (cleavage and introduction of
water)
4. Lyases: group elimination to form double bond
5. Isomerases: isomerization (intramolecular rearrangements
6. Ligases (synthases): bond formation coupled with ATP
hydrolysis

10. MECHANISM OF ENZYME ACTION (HOW ENZYMES WORK)
.
• Enzymes can only catalyze thermodynamically
possible reactions.
• For such reactions to proceed a certain amount
of energy is required called “the energy of
activation”.
• Enzymes speed up chemical reactions by
lowering the energy required for
activation, thus more substrate molecules
are sufficiently activated for the reaction to
proceed.

11. Free energy of activation
Progress of reaction
Transition
state
T*
Free energy of
activation
(uncatalyzed)
Free
energy
A
Reactants
(initial state)
Free energy of
activation
(catalyzed)
ΔG
Product
(final state)
B
This peak of energy represents
the transition state in which a
high- energy intermediate is
formed during the conversion of
reactant to product. Because of
the large activation energy, the
rate of uncatalyzed chemical
reactions is often slow.
There is no
difference In the free
energy of the overall
reaction (energy of
reactants minus
energy of products)
between the
catalyzed and
uncatalyzed
reactions.

12. Substrate Enzyme
Products
+
E-S complex
Enzyme
+

13. • It was formally suggested that the catalytic
site is a rigid area on the enzyme surface and
that the substrate binds to the enzyme like
• “a key into its lock”.
• Now there is evidence that the catalytic site
is a flexible area.
• The presence of the substrate induces a
conformational change in the enzyme that
allows the binding of the substrate and
catalysis to occur. This means that we get
“induced fit” ‫ما‬ ‫ل‬D‫شك‬ ‫ى‬D‫عل‬ ‫ق‬D‫ينطب‬ of the substrate
on the surface of the enzyme.

14. • SPECIFICITY OF ENZYMES.
 Enzymes are highly specific in their action,
– 1-interacting with one or a few specific substrates and
– 2-catalyzing only one type of chemical reactions.
 The specificity of enzymes is due to
o the nature and
o arrangement of the chemical groups at the catalytic site.
This allows the enzyme to unite and activate only one
substrate or a small number of structurally related substrates

15. FACTORS AFFECTING THE RATE OF ENZYME-
CATALYZED REACTIONS
.
• The rate of enzyme-catalyzed reactions is directly
proportional to the concentration of the enzyme-
substrate complex. Thus, it is affected by the
• 1- concentration of the substrate, 2- enzyme,
cofactors, 3-inhibitors 4- Temperature, 5-pH, and
other factors also affect enzyme activity. In studying
the effect of any of these factors, only one factor has
to be varied at a time, all other factors being kept
constant.
- The rate of the reaction should
always be measured at the very
beginning of the reaction, the so-
called initial velocity (vo).

16. I- Concentration of the substrate
:
• At low substrate concentraton, only a few
enzyme molecules are in the form of E-S
complex and the velocity of the reaction
is low.
• 2-As the substrate concentration is
increased more enzyme molecules are
involved in the formation of E-S complex
and the velocity of the reaction increases.
• This is true up to the point when all enzyme
molecules are in the form of E-S complex at
which the maximal rate of the reaction Vmax
is reached.
3- Any further increase in the substrate
concentration will not increase the velocity
of the reaction, the enzyme concentration is
said to be the limiting factor.

17. Ill- Concentration of cofactors (C)
:
• If the enzyme requires a cofactor (coenzyme or activator) for its
activity the velocity of the reaction will be directly
proportional to the concentration of the cofactor.
• This is true up to the point when each enzyme molecule is
associated with the cofactor required.
• At this point any increase in cofactor concentration will
not increase the velocity of the reaction, the enzyme
concentration is the limiting factor.

18. • 1-At 0 °C, enzyme action virtually stops due to the
inhibition of movement and collision between the
substrate and enzyme molecules.
•
• 2-As the temperature rises the velocity of the
reaction increases due to increased kinetic energy
of the molecules and increased collision between
substrate and enzyme molecules, increasing the
formation of E-S complex.
• 3- the increase in the velocity of the reaction continue up to
a point, “the optimum temperature”, beyond which any 4-
further increase in temperature causes a decrease in the
reaction rate.
IV- Temperature
:

19. • For most enzymes, the
activity virtually stops at
about 70 °C , due to
denaturation‫البروتينى‬ ‫االفساد‬ of the
enzyme protein, disrupting
the organization of the
catalytic site.
• The optimum temperature for
most animal enzymes is about
37°C , while that of most plant
enzymes is about 50 °C. 70 °C
V
Optimum temperature
0

20. V- pH
• 1-Each enzyme has an optimum pH at which it
shows maximal activity.
• 2-Activity decreases as we go away from the
optimum pH,
• 3-it virtually stops about 2 units of pH above or
below this pH.
• Most enzymes have an optimum pH between
5 and 9.
• Slight changes in pH causes marked changes in
enzyme activity due to alteration of the charges on
the substrate and on the catalytic site of the enzyme.
- These alterations may affect chemical groups on
the enzyme responsible for substrate binding,
leading to changes in the affinity between the
enzyme and the substrate

21. • In addition, extremes of pH causes
denaturation and irreversible inhibition of
enzyme action.
pH
V
Optimum pH

22. • Any substance that can diminish the
velocity of an enzyme-catalyzed
reaction is called an inhibitor.
• The two most commonly encountered
types of inhibition are
• 1-competitive ‫تنافسي‬ ‫تثبيط‬and
• 2-noncompetitive.‫تنافسي‬ ‫ال‬ ‫تثبيط‬
ENZYME INHIBITORS
‫االنزيم‬ ‫مثبطات‬

23. A. Competitive inhibitors
:
‫تنافسي‬ ‫تثبيط‬
• This type of inhibition is due to
• 1- the structural similarity between the inhibitor (I)
and the substrate.
• This allows the inhibitor to unite reversibly with the
catalytic site of the enzyme, forming an enzyme-
inhibitor complex (El complex).
• Thus, Both the substrate and the inhibitor compete
with each other for the catalytic sites on the enzyme.
the formation of E-S complex is decreased,
inhibiting the activity of the enzyme. Competitive
inhibitors are also known as
• “substrate analogue ‫النظير‬ inhibitors”

24. • . 1-Thus, the degree of inhibition
depends on the ratio of the
concentrations of the inhibitor and the
substrate, but not on the absolute
concentration of each one of them.
• 2-At a fixed concentration of the
inhibitor, if more substrate is added the
degree of inhibition decreases, a fact
that differentiates competitive from
noncompetitive inhibitors.
•

25. B. Noncompetitive inhibitors
:
A noncompetitive inhibitor
(I) has no structural similarity to the substrate and
(2) does not bind to the catalytic site of the enzymes. It binds
to a different area on the enzyme surface
(3)and there is no competition between the substrate and the
inhibitor for binding.
• Both bind to the enzyme at the same time,
forming I-E-S complex.
• Binding of the inhibitor to enzyme decreases its
catalytic power, thus decreasing Vmax. However,
since the inhibitor does not interfere with
substrate binding.
• The degree of inhibition is not related to the
concentration of the (S)substrate but only to the
concentration of the(I) inhibitor.