Enzyme

1. ENZYME
Presented by:
Dr. Mohammed Al-huraiby

3. DEFINITION AND CHARACTERISTICS OF
ENZYMES
 Enzymes are biological catalysts which speed up
chemical reactions, but remain unchanged themselves.
 Catalysts: are substances that accelerate the rate of
chemical reactions.
 Enzymes are involved in conversion of substrate into
product
 Almost all enzymes are globular proteins consisting
either of a single polypeptide or of two or more
polypeptides held together (in quaternary structure) by
non-covalent bonds

4.  Substrate : is the substance upon which the enzyme acts

5. THE BASIC CHARACTERISTICS OF ENZYMES
INCLUDES
 (i) Almost all the enzymes are proteins and they follow
the physical and chemical reactions of proteins
 (ii) Enzymes are sensitive and labile to heat
 (iii) Enzymes are water soluble
 (iv) Enzymes could be precipitated by protein
precipitating agents such as ammonium sulfate and
trichloroacetic acid

6.  Most of the enzymes are produced by the cells of a
particular tissue and function within that cell. Such
enzymes are called as intracellular enzymes.
 Example : Enzymes of glycolysis, TCA cycle and fatty
acid synthesis
 certain enzymes, which are produced by the cells of a
particular tissue from where these are liberated for use
in the other tissues.
 Such enzymes are called as extracellular enzymes.
 Example :Various proteolytic enzymes of
gastrointestinal tract (Trypsin, chymotrypsin).

9. CHARACTERISTICS OF AN ENZYME ACTIVE SITE
 This active site contains specific amino acid residues,
(binding and catalytic residues) and possess three-
dimensional structure.
 The amino acid residues at the active site of an Enzyme
have two functions:
 1-The binding amino acid residues recognize and bind
the correct substrate to form enzyme-substrate (ES)
complex
 It is the part of the enzyme that binds with substrate. It
determines substrate specificity.

10.  2- The catalytic residues create a chemical environment
that enhances the rate of reaction and ES complex is
converted to an enzyme (E),and a product(P).

11. LOCK-AND-KEY MODEL
 In the lock-and-key model of enzyme action:
- the active site has a rigid shape
- only substrates with the matching shape can fit
- the substrate is a key that fits the lock of the active
site
 This is an older model, however, and does not work
for all enzymes

12. INDUCED FIT MODEL
 In the induced-fit model of enzyme action:
- the active site is flexible, not rigid
- the shapes of the enzyme, active site, and substrate
adjust to maximize the fit, which improves catalysis
- there is a greater range of substrate specificity
 This model is more consistent with a wider range of
enzymes

13. STRUCTURE OF ENZYME
 Enzymes are either simple or conjugated proteins
 If the enzyme is a conjugated protein, it Is called: holoenzyme
 Holoenzyme: Is made up of protein groups and non-protein
component.
 The protein component of this holoenzymes is called:
apoenzyme
 The non-protein component of the holoenzyme is called
Cofactor

14. ZYMOGEN OR PROENZYME
 A number of proteolytic enzymes found in the blood or in
the digestive tract are present in an inactive (precursor)
form, called zymogen or proenzymes
 For example, chymotrypsin is secreted by the pancreas
as chymotrypsinogen. It is activated in the digestive tract
by the proteolytic enzyme trypsin

15.  Precursor proteins or inactive enzyme names
have the prefix “pro” like prothrombin.
 suffix “ogen” like chymotrypsinogen,
trypsinogen.
 cofactor : enzymes with non protein component
cofactor: may be either loosely or tightly bound
to the protein portion of the enzyme.

16. These cofactors may be:
 – Organic compounds, called coenzymes
 – Inorganic ions, called activators
 Enzymes without its cofactor is referred to as an
apoenzyme
 the complete catalytically active enzyme is called
holoenzyme
 Apoenzyme + cofactor = holoenzyme

17.  Many vitamins function as coenzymes. Coenzymes
derived from vitamins

18. ROLE OF COENZYMES
 The functional role of coenzymes is to act as
transporters of chemical groups from one reactant to
another.

19. WHAT ARE FACTORS THAT AFFECT ENZYMES ACTIVITY?
 Substrate concentration
 Enzyme concentration
 pH
 Temperature
 Effect of product
 Inhibitor or activator

20. Substrate Concentration and Reaction Rate
• The rate of reaction increases as substrate
concentration increases (at constant enzyme
concentration)
• Maximum activity occurs when the enzyme is
saturated (when all enzymes are binding substrate)

23.  Michaelis constant (Km):
 1 ) when substrate concentration(S) is equal to Km. thus
Km can be defined as:
 substrate concentration that produces half maximum
velocity.

25. ENZYME CONCENTRATION
 In order to study the effect of increasing the Enzyme
concentration upon the reaction rate, the
substrate must be present in an excess amount.
 Increased Enzyme concentration increased the rate
of activity.

26. ENVIRONMENTAL FACTORS
 pH also affects the rate of enzyme-substrate
complexes
 Most enzymes have an optimum pH of around 7 (neutral)
 However, some prefer acidic or basic conditions

27. ENVIRONMENTAL FACTORS
 Optimum temperature The temp at which enzymatic
reaction occur fastest.

28. EFFECT OF ACTIVATORS AND CO-ENZYMES
 The activity of many enzymes is dependent on the
activators (metallic ions) like Mg2+, Mn2+, Zn2+,
Ca2+,Co2+, Cu2+, etc. and coenzymes for their
optimum activity. In absence of these activators and
coenzymes, enzymes become functionally inactive.

29. EFFECT OF INHIBITORS
 The substances which stop the enzymatic reaction are
called inhibitors. Presence of these substances in
reaction medium decreases the rate of enzyme
reaction.

34.  Dicumarol
 It is an anticoagulant drug structurally similar to
vitamin K. It inhibits the vitamin K activity and
inhibits the formation of prothrombin.

41. CLASSIFICATION OF E NZYMES
 Enzymes are classified into six groups according
to the IUB (International Union of Biochemistry).
According to this classification, each enzyme is
characterized by acode number called enzyme
code number or EC ’ number,
 1. EC-1 : Oxidoreductases
 2. EC-2 : Transferases
 3. EC-3 : Hydrolases
 4. EC-4 : Lyases
 5. EC-5 : Isomerases
 6. EC-6 : Ligases.

42.  EC-1 Oxidoreductases
 Those enzymes that catalyze oxidation-reduction
reactions, are included in this class which can be
illustrated schematically as
 follows:
 Enzymes in this category include :
 • Dehydrogenases
 • Reductases
 • Oxidases
 • Peroxidases.

43.  Specific Example
 EC-2 Transferases
Those enzymes that catalyze the transfer of a group
suchas, amino, carboxyl, methyl or phosphoryl, etc.
from one molecule to another are called transferases.

44.  Some common enzymes in this category include :
 • Amino transferase or transaminase
 • Kinase
 • Transcarboxylase.
 Specific Example

45.  EC-3 Hydrolases
 Enzymes of this class catalyze the cleavage of C-
O,C-N , C-C and some other bonds with the
addition of water
 Some common enzymes in this category are:
 • Acid phosphatase
 • All digestive enzymes like α -amylase, pepsin,
 trypsin, chymotrypsin, etc.

46.  EC-4 Lyases
 These are enzymes cleaving C—C,C—O,C—
N,and other bonds by elimination,leaving
 Double bonds or rings,or conversely
adding groups to double bonds (without the
addition of water).
 Examples: Fumaras and enolase

47.  EC-5 Isomerases
 Isomerases catalyze intramolecular structural
rearrangement in a molecule. They are called
epimerases,isomerases or mutases, depending on
the type of isomerism involved.

48.  EC-6 Ligases (Synthetases)
 Synthesize substance by joining two substrates
using energy.

49. ENZYME SPECIFICITY
 Specificity refers to the ability of an enzyme to
discriminate between two competing substrates.
 Enzymes are highly specific both in the reaction
catalyzed and in their choice of substrates
 Types of Specificity
 1. Substrate specificity
 2. Reaction specificity
 3. Stereo specificity

50.  Substrate Specificity
 Pepsin hydrolyzes residues of only aromatic amino
acids while trypsin hydrolyzes residues of
the basic amino acids only
 Absolute specificity :Glucokinase acts on
glucose only
 In group specificity, an enzyme acts on more than
one substrate containing a particular group,
:Hexokinase catalyzes hexoses.

51.  Bond specificity :Refers to the action of
proteolytic enzymes, glycosidase and lipases which
act on peptide bonds of proteins, glycosidic
bonds of carbohydrates and ester bonds of lipids
respectively
 Stereo specificity :The group of enzyme catalyzes
either L or D isomer
 Reaction specificity: One enzyme catalyzes only
one type of reaction

52. ISOENZYMES
 Isoenzymes or isozymes are multiple forms
(isomers)of the same enzyme that catalyze the
same biochemical reaction. But differ structurally,
physically, electrophoretically and chemically.

53.  Not all enzymes have isoenzymes. In fact, it was
found that only those enzymes, which are active in
polymeric form demonstrate isoenzyme. For
example:
 1. Lactate dehydrogenase (LDH)
 2. Creatine kinase (CK) (formerly called creatine
phosphokinase (CPK)