Enzymes are protein catalysts that accelerate biochemical reactions. They are produced by living organisms and are responsible for essential reactions in microbes, plants, animals, and humans. Enzymes work efficiently and specifically, catalyzing reactions that otherwise would proceed slowly or not at all. They are regulated by factors like temperature, pH, inhibitors, and activators. Enzyme activity can be inhibited through competitive inhibition, where an inhibitor binds to the active site, or non-competitive inhibition, where an inhibitor binds elsewhere and alters the enzyme's shape.

4. *Enzymes are proteins with highly specialized catalytic functions, produced by
all living organisms.
*Enzymes are responsible for many essential biochemical.
reactions in microorganisms, plants, animals, and human beings.
*Enzymes are natural protein molecules that act as highly efficient catalysts in
biochemical reactions, that is, they help a chemical reaction take place
quickly and efficiently.
*Enzymes not only work efficiently and rapidly, they are also biodegradable.
*Enzymes are highly efficient in increasing the reaction rate of
biochemical processes that otherwise proceed very slowly, or in some
cases, not at all.
*All enzymes are protein in nature enzymes are protein in nature, so have the
same properties of proteins as denaturation, precipitation,
electrophoresis ………….etc.
Enzymes:

5. Chemical nature of enzymes:
All enzymes are protein in nature and it may be:
Simple protein enzyme:
consisting wholly of protein e.g. pepsin.
Conjugated protein enzyme:
Holoenzyme: it is an enzyme composed from protein part and
non-protein part.
This non-protein part may be loosely attached to protein part
(coenzyme) or firmly attached to protein part (prosthetic
group)

6. Inorganic catalyst
Enzyme
*Thermostable
*Inorganic, non biologically
substances
*Non protein in nature *non-
specific
*Low catalytic efficiency
*Thermolabile
*Organic, biologically
substances
*Protein in nature specific
*High catalytic efficiency

7. Differentiate between holoenzymes, apoenzymes, co-factor,
metal activated enzyme, prosthetic group, coenzyme,
metalloenzyme and isoenzyme.
A. Holoenzyme: it is an enzyme composed from proteinic part
and nonproteinic part.
B. Apoenzyme: it is the proteinic part of the holoenzyme.
C. Co-factor: it is the non-proteinic part of the holoenzyme.
D. Metal activated enzyme: holoenzymes which have a loosely
bound metals on its prosthetic inorganic group.
E. Co-enzyme: specific thermo stable low mol.wt non-protein
organic substance bound tightly in usual.
F.Metalloenzyme: enzyme which has tightly bound metals as its
prosthetic group.
G. Isoenzymes: enzymes which have different structures and
same function.
Cardiolipin is the prosthetic group of the enzyme cytochrome
oxidase.
Cu++, Fe++ ions are the metalloenzyme of cytochrome oxidase.

8. Some vitamins act as co-enzyme:
a.B1 thiamine : form TPP [ thiamine pyrophosphate ] co-enzyme for pyruvate
degyrogenase .
b. B2 : FAD [flavin adenine dinucleotide] , FMN [flavin mononucleotide].
c. B3 :NAD [ nicotinamide adenine dinucleotide ] , NADP nicotinamide adenine
dinucleotide phosphate ]
d. B5 pantothenic :co-enzyme A [transfer of acyl group]
e. B6 pyridoxine :form co-enzyme [pyridoxal phosphate].
f.B12 cobalamin : cabamide
g. Biotin :coenzyme in carboxylation reaction.

10. Localization of the enzyme
Intracellular enzymes :they act inside the cells that make
them.
Extracellular enzymes : they act outside the cells that
secret them such as:
digestive enzymes in the
GIT
coagulation enzymes in plasma.

11. Nomenclature of enzymes:
1-Some enzymes still retain their old names as digestion
enzymes still use –in pepsin, trypsin.
2-End in –ase:
Identifies a reacting substance
sucrase – reacts sucrose
lipase - reacts lipid
3-Describes function of enzyme:
oxidase – catalyzes oxidation
hydrolase – catalyzes hydrolysis
4-Enzyme named by the name of both the substrate acted
upon and the type of reaction catalyzed ex.
Succinic Dehydrogenase that remove hydrogen
from succinic acid.

12. The substrate
The substrate of an enzyme are the reactants that are
activated by the enzyme
Enzymes are specific to their substrats
The specificity is determined by the active site

13. Enzyme Action:
An enzyme binds a substrate in a region called the active site
Only certain substrates can fit the active site forming enzyme
substrate complex.
The active site contain specific groups of Amino acid help
substrate bind.
Enzyme-substrate complex decomposes, giving rise to free
enzyme and products of the reaction.

14. ES
+ P
+
S
E E

15. Lock and Key Model
The active site of enzyme by itself is pre-shaped to
fit the substrate ( it is complementary in shape to
that of substrate.
+
+
E + S
ES complex E + P
S
P
P
S

16. Induced Fit Model
The active site of the enzyme is flexible, not rigid
When the active site identifies the substrate it brings
a change in the active site shape so it can
accommodate the substrate.
It has a shape complementary to that of the
substrate only after the substrate is bound to the
enzyme.
This model similar to the rubber gloves.

17. Induced Fit Model
E + S ES complex E + P
S
P
P
S
S

19. Factors Affecting Enzyme Action
The activity of the enzyme is evaluated by measuring
the rate or the velocity of the reaction.
velocity of the reaction is measured by how many
moles of the substrate are converted into products
per unit of time (minute).

20. 1-Temperature
*Little activity at low
temperature.
*Rate increases with temperature.
*Each enzyme has an optimum
temperatures at which the
enzyme acts maximally.
*Most active at optimum
temperatures (usually 37°C
in humans and 65°C for
plant enzymes).
*Activity lost with denaturation
at high temperatures.

21. 2-Enzyme Concentration
*Increasing enzyme concentration increases the rate of
reaction.
*Further increase in the enzyme can not increase the
velocity of reaction because the amount of substrate
may not be sufficient to permit of maximum velocity.

22. Maximum activity
velocity
Reaction
Rate
enzyme concentration

23. 3-Substrate Concentration
*Increasing substrate
concentration increases the
rate of reaction (enzyme
concentration is constant).
*Maximum activity reached
when all of enzyme
combines with substrate.

24. Michaelis constant (Km)
*Km is equal to the substrate
concentration [S] at which the
reaction is half of its maximum
(1/2Vmax).
*It expresses the affinity of the
enzyme to its substrate.
*Low Km means high affinity of the
enzyme to the substrate
*High Km means low affinity of the
enzyme to the substrate

25. 4-pH
*Each enzyme has an
optimum pH at which the
enzyme acts maximally.
*Maximum activity at
optimum pH
*Most lose activity in low or
high pH

26. 5-Hormones
e.g. insulin hormone stimulates glucokinase
enzyme while glucocorticoids inhibit it.
Steroid hormones are known to increase the rate
of synthesis of many enzyme.

27. 6-Time
As time is passed the rate of the enzyme catalyzed
reaction diminishes due to:
*Decline of substrate concentration.
*The accumulated product may cause feedback
inhibition of the enzyme.

28. 7-Product concentration:
As you increase the product
concentration you decrease
the rate of the reaction.
The excess amount of product
accumulates and occupies
the active site of the
enzyme.

29. 8-Radiation and light
Light inhibit most enzyme activity although some
enzymes e.g. amylase is activated by red or
green light.
Ultraviolet rays and ionized radiations cause
denaturation of most enzymes.

30. 9-Enzyme activators
Certain inorganic ions e.g.:
CL¯activate salivary and pancreatic amylase.
Ca++ activate thrombokinase.
Bile salts activate lipase enzyme.
Some enzymes are secreted in inactive form and are
activated by:
pH: pepsinogen is activated by HCL giving pepsin.
Autoactivation: pepsinogen pepsin
Kinase: which activate zymogen or proenzymes e.g.:
Trypsinogen enterokinase trypsin

31. 10-Enzyme Inhibitors
Definition: substances which inhibit (stop) the enzyme activity.
It causes a loss of catalytic activity
Change the protein structure of an enzyme.
It may be :
Non-specific or specific.

32. Non-specific inhibitors:
These are inhibitors which exert their effect on all
enzymes or on wide variety of enzymes; e.g.
Agents which precipitate or denaturate proteins.
specific inhibitors:
These are inhibitors which exert their effect on one
enzyme or on a small group of related enzymes.
May be competitive or noncompetitive

34. Competitive Inhibition
A competitive inhibitor:
*The molecule resembling the substrate.
*Can bind to the active site of the enzyme and so it can form
enzyme inhibitor complex EI.
*Decreases the affinity of enzymes for substrate.
*Excessive concentrations of substrate will break the EI
complex and then S can bind to the enzyme.
*Reversible.
*It depends on Substrate and Inhibitor.

35. Enzyme inhibitor
complex
Reversible
reaction
E + I EI

36. Examples of competitive inhibitors:
Allopurinol competes with hypoxanthine for xanthine oxidase
inhibiting the formation of uric acid, so it is used in treatment
of hyperuricemia (gout).
Dicumarol or Warfarine compete with vitamin K, for epoxide
reductase, so they are used to reduce prothrombin synthesis.
Statins (e.g. atorvastatin) competes with HMGCoA for its
reductase,so, it inhibits cholesterol synthesis.
Methotrexate competes with dihydrofolic acid for dihydrofolate
reductase, so, it inhibits DNA synthesis and used in treatment of
cancers.

37. The effect of enzyme inhibition
Succinate Fumarate + 2H++ 2e-
Succinate dehydrogenase
CH2COOH
CH2COOH CHCOOH
CHCOOH
COOH
COOH
CH2
Malonate

38. Competitive Inhibition
Succinate Glutarate Malonate Oxalate
Succinate Dehydrogenase
Substrate Competitive Inhibitor
Product
C-OO-
C-H
C-H
C-OO-
C-OO-
H-C-H
H-C-H
C-OO-
C-OO-
H-C-H
H-C-H
H-C-H
C-OO-
C-OO-
C-OO-
C-OO-
H-C-H
C-OO-

39. Non-competitive inhibitors
Does not have a structure like substrate
Binds to the enzyme but not active site
Changes the shape of enzyme and active site
Substrate cannot fit altered active site
No reaction occurs
Effect is not reversed by adding substrate

40. Enzyme Inhibition (Mechanism)
S I
I
I
I
S
Competitive Non-competitive
E
Different site
Compete for
active site
Inhibitor
Substrate
[I] binds to free [E] only,
and competes with [S];
increasing [S] overcomes
Inhibition by [I].
[I] binds to free [E] or [ES]
complex; Increasing [S] can
not overcome [I] inhibition.
E + S→ES→E + P
+
I
↓
EI
←
↑
E + S→ES→E + P
+ +
I I
↓ ↓
EI+S→EIS
←
↑ ↑
E
I
S

41. Allosteric inhibitors
Allosteric inhibitors are low-molecular weight substances, they regulate the
enzyme activity.
The inhibitor is not similar in structure to the substrate and it is bound to
apoenzyme at sites far from the active site this site is called allosteric
site.
It is not reversable by increasing the concentration of the substrate.
This interaction causes conformational changes in the catalytic site that
makes it unfavorable for binding to Substrate.
When Allosteric inhibitor is consumed the activity of the enzyme is
regained ,so Allosteric inhibition is reversable.
e.g. glucose-6-phosphate is allosteric inhibitor for hexokinase enzyme.
Some time conformational change of apoenzyme produce by binding to
the allosteric effector, makw the active center more fit to bind with the
substrate, thus the enzyme activity is increased. In this case the effector
is called allosteric activator e.g. AMP and ADP are allosteric activator
to phosphofructokinase.

42. Feedback Inhibition
End- product of a metabolic
pathway inhibits the initial
enzyme in the pathway, this
called feed-back inhibition.

43. Inhibitorsblocking activators
and coenzymes
*Agents that block the coenzyme will stop enzyme
action e.g. phenylhydrazine will block the
aldehyde group of pyridoxal phosphate, which is
the coenzyme of transaminase.
*Agents that block the prosthetic group will stop
enzyme action e.g.cyanide and carbon monoxide
block the iron of heme of cytochrome oxidase
enzyme.

44. 11-Antienzymesand antibodies
Antienzymes: ascaris living in the lumen of
intestine secrete antipepsin and antitrypsin to
prevent digestion of the worm by these
enzymes.
Antibodies: if enzyme is injected, the immune
system of the body produces antibodies which
will inactivate these enzymes.

45. 12-Concentration of cofactors
The rate of enzyme reactin is directly proportional to the
concentration of the cofactors.
Definition: An additional non-protein molecule that is
needed by some enzymes to help the reaction
Tightly bound cofactors are called prosthetic groups
Cofactors that are bound and released easily are called
coenzymes Many vitamins are coenzymes.

46. Common Coenzymes

47. Coenzyme = thiamine pyrophosphate (TPP)
 used in decarboxylation and transketolation
contains pyrimidine and thiazole.
disease due to deficiency: beri-beri, Wernicke’s disease.
peripheral nerves, muscle cramps, numbness
Wernicke’s disease.
beri-beri

48. Coenzyme :flavin mononucleotide
(FMN), flavin adenine dinucleotide (FAD)
both act as prosthetic groups
-- use = redox reactions
-- its vitamin = riboflavin or B2

49. Nicotinamide-adenine dinucleotide (NAD),
nicotinamide-adenine dinucleotide phosphate
(NADP)
used in redox reactions with H transfer.
Its vitamin =niacin or B3 =nicotinamide
& nicotinic acid
Disease due to deficiency pellagra, skin lesions,
swollen tongue, nervous/mental disorders
pellagra

50. P
Coenzyme

51. Coenzyme = pyrodoxal phosphate
Used in decarboxylations, transaminations and
racemases.
Its vitamin = pyridoxine, or vitamin B6

52. Coenzyme = Coenzyme A (CoA)
use = activates carbonyl groups and in acyl transfer
(acetyl- CoA, synthesis
of fats and steroids)
its vitamin = pantothenic acid
disease due to deficiency GI problems, emotional
instability, burning sensation in extemities
acetyl
Acetyl CoA

53. Coenzyme = folate or tetrahydrofolate (the
reduced form)
Used in transfer of one carbon unit or formate
Its vitamin = folic acid
Disease due to deficiency megablastic anemia, birth
defects
megablastic anemia

54. Coenzyme = biotin
a prosthetic group
-- use = carboxylations
-- its vitamin = biotin

55. Coenzyme = cyanocobalamin
Used in methyl group transfer;
folate metabolism,
myelin synthesis
Its vitamin is cyanocobalamin or vitamin B12
disease due to deficiency pernicious anemia
pernicious anemia

56. Coenzyme = lipoic acid
(reduced SH or oxidized form -S-S-)prosthetic group
Used in redox reactions
Its vitamin = lipoic acid
(humans probably produce enough so it is not always
considered a vitamin)
oxidized reduced

57. Classification according to enzyme specifity:
One of the properties of enzymes that makes them so
important as diagnostic and research tools is the
specificity they exhibit relative to the reactions they
catalyze. A few enzymes exhibit absolute specificity; that
is, they will catalyze only one particular reaction. Other
enzymes will be specific for a particular type of chemical
bond or functional group.
Classification of Enzymes

58. In general, there are four distinct types of specificity:
Absolute specificity: the enzyme will catalyze only one reaction
e.g. ureas enzyme acts only on urea .
Relative specificity: the enzyme will catalyze agroup of closly
related sbstratei.e. which are similar in structure and posses
the same type of the bond e.g.lipase catalysis the process of
hydrolysis of ester linkage present in triglycerides containig
different type of fatty acids.
Dual specifity:
The enzyme acts on two different substrates e.g. xanthine
oxidase can oxidize hypoxanthine and xanthine to uric acid.
Stereochemical specificity: the enzyme will act on a particular
steric or optical isomer. E.g. L-amino acid oxidase acts on L-
amino acid only
Exception:there is only one exception which is racemase
enzyme catalyses reversable interconversion of D andL
isomers.

59. Classification of Enzymes according
to function:

67. Autolysis
More commonly known as self-digestion, refers to the
destruction of a cell through the action of its own
enzymes. It may also refer to the digestion of an
enzyme by another molecule of the same enzyme.
In the living body these enzymes are usually inactive
since their optimum pH is slightly acidic, while the
pH of the body fluids is slightly alkaline and
unsuitable for their activity.
After death, lactic acid accumulate in the tissues and
cathepsins become activated

68. Isoenzymes
These are different isomers of the same enzyme which differ by having a different
electrophoretic mobility and different tissue source.
Each of these is called isoenzyme and all of them have the same catalytic activity
e.g. lactate dehydrogenase (LDH) and creatin kinase (CK).
CPK is made of three slightly different substances:
CPK-1 (also called CPK-BB) is found mostly in the brain and lungs.
CPK-2 (also called CPK-MB) is found mostly in the heart.
CPK-3 (also called CPK-MM) is found mostly in skeletal muscle.
LDH exists in 5 isoenzymes , which differ slightly in subunits.
LDH-1 HHHHis found primarily in heart muscle and red blood cells.
LDH-2 HHHM is concentrated in white blood cells.
LDH-3 HHMM is highest in the lung.
LDH-4 HMMM is highest in the kidney, placenta, and pancreas.
LDH-5 MMMM is highest in the liver and skeletal muscle.

69. Regulation of Enzyme activity
Enzyme activity can be regulated by control of enzyme synthesis.
Enzyme synthesis can be controlled by induction or by repression and
depression.
1-Induction of Enzyme synthesis:
Increase of substrate concentration increases rate of synthesis of certain
enzyme.
This is an example of positive feedback regulation at the gene level.
It is commonly observed in metabolism of microorganism and bacteria.
A compound that is simillar in structur to substrate that is not utilized by
the enzyme but is able to induce the enzyme synthesis is
called”gratuitous inducer:.
One inducer may induce synthesize of a number of enzymes at the same
time , this phenomenon is called “coordinate induction”.
Enzym whose concentration in the cell dose not depend on an added
inducer is called “constitutive enzymes”.

70. 2-Enzyme Repression:
accumulation of a product of certain enzyme leads to decrease the
rate of synthesis of that enzyme,i.e. Enzyme Repression.
This is an example of negative feedback regulation at the gene level.
Certain bacteria is able to synthesis a particular amino acid .
The availability of the amino acid in large amount decrease rate of
synthesis of the enzyme synthesizing that amino acid by the
bacteria, in this case the added amino acid is called corepressor.
When coreprosser amino acid is removed from the medium,
depression occurs and the amino acid can be synthesized by the
bacteria again.
This phenomenon is not restricted to amino acid in bacteria , but
operates to all synthetic pathways in the body, too.
Cholesterol and its derivatives are strong repressors for the
expression of the key regulatory enzymes for cholesterol synthesis.

72. Clinical importance of
enzymes:
Enzyme activity determination in plama or serum is often
important in clinical practice.
Plasma enzymes are classified into:
1-Functional plasma enzymes
2-Non-functional plasma enzymes

73. 1-Functional plasma enzymes
They are enzymes which have a function in the
plasma because their substrates are present in the
plasma. e.g.:
Lipoprotein lipase.
Blood clotting enzymes.

74. 2-Non-functional plasma enzymes
These are enzymes which have no specific function
in blood and having no substrate to act upon.
They are present in low concentration in blood and
originally they are present in different organs
inside their cells.
If the cells of these organs is destroyed by any
disease, these enzymes are librated and appear in
higher concentration in blood.
So the presence of these Non-functional enzymes in
higher concentrations than normal in blood is of
great clinical importance.

75. Examples
Lipase: it is elevated in acute pancreatitis and pancreatic
carcinoma.
Trypsin increased in pancreatic disease.
Serum alkaline phosphatase: elevated in :liver disease, bone
diseases ,hyperthyrodism and malignancy.
Serum acid phosphataseis elevated in cancer prostate.
Lactic acid dehydrogenase (LDH) is elevated in case of
myocardial infarction, liver and muscle diseases.
Creatine phosphokinase is elevated in cardiac and skeletal
muscle diseases.
Transaminase:
Glutamic pyruvic transaminase GPT increased in liver diseases
e.g. infected hepatitis.
Glutamic oxaloacetic transaminase GOT increased in damage of
heart e.g. coronary thrombosis and liver diseases.

76. References
*Biochemistry LECTURES “ENZYMES” ,
Haitham H. Alnemari, COLLEGE OF MEDICIN
AND MEDICAL SCIENCES
*http://www.worthington-
biochem.com/introbiochem/Enzymes.pdf
*http://class.fst.ohio-
state.edu/fst605/605p/Enzymes.pdf.
*Review of medical biochemistry,enzymes, part 1,
prof. Mohamed Amin Bakry.faculty of medicin
zagazig university .2nd edition.