Enzymes
Presented By
Dr. Salwa Abo El-khair
Catalytic Proteins 5: Enzymes
3
Enzyme Regulation
 Regulation of enzyme activity is important to
coordinate the different metabolic processes.
 It is also important for homeostasis i.e. to
maintain the internal environment of the
organism constant.
Regulation of enzyme activity:
It can be achieved by two general mechanisms:
A) Control of enzyme quantity
 Altering the rate of enzyme synthesis and
degradation.
 Induction.
 Repression.
 Concentration of substrates, coenzymes and metal
ion activators.
B) Altering the catalytic efficiency of the
enzyme:
 Allosteric regulation.
 Feedback inhibition.
 Proenzyme (zymogen).
 Covalent modification.
 Protein-protein interaction.
A) Control of enzyme quantity
1- Control of the rates of enzyme synthesis and
degradation.
 Enzymes are protein in nature, they are
synthesized from amino acids under gene control
and degraded again to amino acids.
 Enzyme quantity depends on the rate of enzyme
synthesis and the rate of its degradation.
 Increased enzyme quantity may be due to an
increase in the rate of synthesis, a decrease in
the rate of degradation or both.
 Decreased enzyme quantity may be due to a
decrease in the rate of synthesis, an increase in
the rate of degradation or both.
For example, the quantity of Liver Arginase
enzyme increases after protein rich meal due
to an increase in the rate of its synthesis; also
it increases in starved animals due to a
decrease in the rate of its degradation.
2- Induction
Induction means an increase in the rate of enzyme
synthesis by substances called inducers.
According to the response to inducers,
enzymes are classified into:
 Constitutive enzymes, the concentration of
these enzymes does not depend on inducers.
 Inducible enzymes, the concentration of these
enzymes depends on the presence of inducers.
 For example, induction of lactase enzyme in
bacteria grown on glucose media.
3- Repression
 Repression means a decrease in the rate of
enzyme synthesis by substances called
repressors.
 Repressors are low molecular weight
substances that decrease the rate of enzyme
synthesis at the level of gene expression.
 Repressors are usually end products of
biosynthetic reaction, so repression is sometimes
called feedback regulation.
 For example, dietary cholesterol decreases the
rate of synthesis of HMG CoA reductase (βhydroxy β-methyl glutaryl CoA reductase), which
is a key enzyme in cholesterol biosynthesis.
4- Derepression
Following removal of the repressor or its
exhaustion, enzyme synthesis retains its normal
rate.
5- Concentration of substrates, coenzymes and
metal ion activator:
 The susceptibility of enzyme to degradation
depends on its conformation.
 Presence of substrate, coenzyme or metal
ion activator causes changes in the enzyme
conformation
degradation.
decreasing its rate of
Got any Questions?!
AP Biology
2007-2008
Activity
 Enumerate ways to control quantity of
enzyme and discuss one of them
 Discuss feed back regulation (def, mechanism
,example)
B) Control of catalytic efficiency of enzymes
1- Allosteric Regulation
 Allosteric enzyme is formed of more than
one protein subunit.
 It has two sites; a catalytic site for substrate
binding and another site (allosteric site), that is
the regulatory site, to which an effector binds.
Allosteric regulation
Allosteric means “other site”
Active site
E
Allosteric
site
 Allosteric means another site
 If binding of the effector to the enzyme
increases it activity, it is called positive effector
or allosteric activator e.g. ADP is allosteric
activator for phosphofructokinase enzyme.
 If binding of the effector to the enzyme causes
a decrease in its activity, it is called negative
effector or allosteric inhibitor e.g.
 ATP and citrate are allosteric inhibitors for
phosphofructokinase enzyme.
 Glucose-6-phosphate is allosteric inhibitor for
hexokinase enzyme.
The allosteric site the enzyme “onoff” switch
Active
site
Substrate
fits into
the active
site
E
Allosteric
site empty
The inhibitor
molecule is
absent
Conformational
change
Substrate
cannot fit
into the
active site
E
Inhibitor
molecule
is present
Inhibitor fits
into allosteric
site
Mechanism of allosteric regulation
Binding of the allosteric effector to the
regulatory site causes conformational changes
in the catalytic site, which becomes more fit for
substrate binding in positive effector (allosteric
activator), and becomes unfit for substrate
binding in negative effector (allosteric inhibitor)
Allosteric regulation
 Conformational changes by regulatory
molecules

inhibitors
 keeps enzyme in inactive form

activators
 keeps enzyme in active form
AP BiologyConformational
changes
Allosteric regulation
Conformational changes
Allosteric regulation
2- Feedback Inhibition
 In biosynthetic pathways, an end product may
directly inhibit an enzyme early in the pathway.
 This enzyme catalyzes the early functionally
irreversible step specific to a particular biosynthetic
pathway.
 Feedback inhibition may occur by simple
feedback loop.
Feedback Inhibition

final product is inhibitor of earlier step






ABCDEFG
1
2
3
4
5
6
X
enzyme enzyme enzyme enzyme enzyme enzyme
AP Biology
End product is an inhibitor of enzyme 1
 Feedback inhibition can occur by multiple
feedback inhibition loops as occurs in branched
biosynthetic pathways.
 Feedback regulation is different from
feedback inhibition.
Feedback regulation:
 It means that an end product in the reaction
decreases the rate of enzyme synthesis at the level
of gene expression.
 It decreases the enzyme quantity through the
action on the gene that encodes the enzyme.
 It does not affect the enzyme activity.
 It is a complicated process that takes hours to
days.
 For example, inhibition of HMG CoA reducatse
enzyme by dietary cholesterol.
Feedback inhibition
 It means that an end product directly inhibits
an enzyme early in biosynthetic pathways.
 It does not affect enzyme quantity.
 It decreases the enzyme activity.
 It is a direct and rapid process that occurs in
seconds to minutes.
 For example, CTP inhibits aspartate
transcarbamylase enzyme in pyrimidine synthesis.
Got any Questions?!
AP Biology
2007-2008
Activity
 1- During ___________the final product of a metabolic
pathway turn off the first step of metabolic pathway.
(A) Positive feed back
(B) Negative feed back
(C) Competitive feed back
(D) Both A and C
 2- An allosteric modulator influences enzyme activity by
(A) Competing for the catalytic site with the substrate
(B) Binding to a site on the enzyme molecule far from the
catalytic site
(C) Changing the nature of the product formed
(D) Covalently modifying enzyme
Activity
 Enumerate factors control catalytic activity of
enzyme and discuss one of them
 Discuss allosteric regulation (def, mechanism
,types )
 Compare between feedback regulation and
feed back inhibition
3- Proenzymes (Zymogens)
 Some enzymes are secreted in inactive forms
called proenzymes or zymogens.
 Examples for zymogens include:
1. Pepsinogen,
2. Trysinogen,
3. Chymotrypsinogen,
4. Prothrombin and Clotting factors.
 Zymogen is inactive because it contains an
additional polypeptide chain that masks (blocks)
the active site of the enzyme.
 Activation of zymogen occurs by removal of
the polypeptide chain that masks the active site.
Activation of chymotrypsinogen to chymotrypsin,
and of trypsinogen to trypsin
 Activation of zymogens can occur by one of
the following methods:
a) Activation by HCl
HCl
Pepsinogen
Pepsin
b) Activation by other enzymes
Enterokinase
Trypsinogen
Trypsin
Thrombokinase + Ca++
Prothrombin
Thrombin
c) Auto activation i.e. the enzyme activates
itself.
Pepsin
Pepsinogen
Pepsin
Biological importance of zymogens
 Some enzymes are secreted in zymogen from to
protect the tissues of origin from auto digestion.
 To insure rapid mobilization of enzyme activity
at the time of needs in response to physiological
demands.
4- Protein-protein interaction
 In enzymes that are formed from of many protein
subunits, the enzyme may be present in an inactive
from through interaction between its protein
subunits.
 The whole enzyme, formed of regulatory and
catalytic subunits, is inactive.
 Activation of the enzyme occurs by separation of
the catalytic subunits from the regulatory subunits.
• Protein kinase A enzyme is an example for
regulation of enzyme activity by protein interaction.
• It is formed of 4 subunits, 2 regulatory (2R) and 2
catalytic (2C) subunits.
• The whole enzyme (2R2C) is inactive.
• cAMP (cyclic adenosine monophosphate) activates
the enzyme by binding to the 2 regulatory (2R)
subunits releasing the 2 catalytic (2C) subunits.
5- Covalent modification
 It means modification of enzyme activity through
formation of covalent bonds e.g.
 Methylation (addition of methyl group).
 Hydroxylation (addition of hydroxyl group).
 Adenylation (addition of adenylic acid).
 Phosphorylation (addition of phosphate group).
Reversible covalent modification
What’s covalently modulated enzymes?
•Activity is modulated by covalent
modification of one or more of its amino
acid residues in the enzyme molecule.
• Common modifying groups include:
phosphoryl, adenylyl, methyl and
hydroxyl.
• These groups are generally linked to
and removed from the regulatory
enzyme by separate enzymes.
 Phosphorylation is the most covalent
modification used to regulate enzyme activity.
 Phosphorylation of enzyme occurs by addition of
phosphate group to the enzyme at the hydroxyl
group of serine, threonine or tyrosine.
 This occurs by protein kinase enzyme.
Protein kinases catalyze the
phosphorylation of proteins
 Dephosphorylation of the enzyme occurs by
removal of phosphate group from the hydroxyl
group of serine, threonine or tyrosine.
 This occurs by phosphatase enzyme.
Protein phosphatases remove phosphate
groups from phosphorylated proteins
ATP Protein Kinase ADP
OH
+
+
Pi Protein Phosphatase
•Phosphorylation and
dephosphorylation are
not the reverse of one
another.
O • The rate of cycling
OP O between the
O-
phosphorylated and
the dephosphorylated
states depends on the
relative activities of
kinases and
phosphatases.
The phosphorylated from is the active form in
some enzymes, while the dephosphorylated form
is the active form in other enzymes.
(active form)
(inactive form)
(inactive form)
(active form)
Enzymes activated by phosphorylation:
 These are usually enzymes of degradative
(breakdown) reactions e.g.
1. Glycogen phosphorylase that breaks down
glycogen into glucose.
2. Citrate lyase, which breaks down citrate.
3. Lipase that hydrolyzes triglyceride into glycerol
and 3 fatty acids.
Enzymes inactivated by phosphorylation:
 These are enzymes of biosynthetic reactions
1. Glycogen Synthetase, which catalyzes
biosynthesis of glycogen.
2. Acetyl CoA carboxylase, an enzyme in fatty
acid biosynthesis.
3. HMG CoA reductase, an enzyme in cholesterol
biosynthesis.
Got any Questions?!
AP Biology
2007-2008
Activity
 1- The inactive precursor of an active enzyme is called
A) Zymogen
B) Ribozyme
C) Isozyme
D) Apoenzyme
2- Activation or inactivation of certain key regulatory
enzymes is accomplished by covalent modification of
the amino acid:

(A) Alanine

(B) Lysine

(C) Phenylalanine

(D) Serine
Activity
 Discuss protein- protein interaction ( mechanism
,one example )
 Discuss proenzyme and its biomedical importance
 Enumerate methods of covalent modification of
enzyme activity
 Discuss covalent modification by phosphorylation
(types, enzymes involved, examples)
Isoenzymes
 Isoenzymes (isozymes) are multiple forms of the
enzyme that have the same catalytic activity.
 Although they have the same catalytic activity,
they are physically distinct and differ in
electrophoretic mobility and liability to inhibitors.
 Iso means the same and isoenzyme means the
same enzyme.
Example of isoenzymes
Many enzymes are present in isoenzyme form:
1. Lactate dehydrogenase
2. Creatine kinase
3. Acid phosphatase
4. Alkaline phosphatase
Lactate dehydrogenase (LDH)
It is an enzyme that catalyzes the removal of 2
hydrogen atoms from lactic acid forming pyruvic
acid.
Lactate dehydrogenase
Lactic acid
Pyruvic acid
NAD
NADH+H
Its level in plasma increases in:
1. Myocardial infarction (heart diseases).
2. Viral hepatitis (liver disease).
3. Leukaemia (blood disease).
 LDH enzyme is a tetramer formed of 4 protein
subunits; each subunit is called protomer.
 The protomers of LDH are of 2 types, H (after
heart) and M (after muscle).
 LDH isoenzymes are clinically important to
differentiate between heart, liver and blood
diseases.
LDH has 5 isoenzymes:
 LDH1 is formed of HHHH. It increases in myocardial infarction.
 LDH2 is formed of HHHM. It increases in myocardial infarction.
 LDH3 is formed of HHMM. It increases in leukaemia.
 LDH4 is formed of HMMM. It increases in viral hepatitis.
 LDH5 is formed of MMMM. It increases in viral hepatitis.
H
Heart type
H H
HH
H H
HM
M
Muscle type
M M
MM
H H
MM
H M
MM
Creatine kinase (CK)
It is an enzyme that catalyzes phosphorylation of
creatine.
Creatine Kinase
Creatine
ATP
Creatine phosphate
ADP
 Its level in plasma increases in
1. Brain tumors.
2. Myocardial infarction (heart disease).
3. Skeletal muscle diseases.
 CK isoenzymes are clinically important to
differentiate between brain, heart and
skeletal muscle diseases.
 CK enzyme is a dimmer formed of 2 protein
subunits (protomers), B (after brain) and M (after
muscle).
 CK has 3 isoenzymes:
 CK BB which increases in brain tumors.
 CK MB which increases in heart diseases.
 CK MM which increases in skeletal muscle diseases
Source of isoenzymes
 Isoenzymes may be produced by the same gene
but the subunits undergo different posttranslation modifications in different organs.
 Isoenzymes may be produced by more than one
gene; each gene produces one subunit.
Medical importance of isoenzymes
 Isoenzymes are not only important for diagnosis
but also indicate the diseased organ.
 Lactate dehydrogenase enzyme (LDH) increases
in myocardial infarction (heart disease), viral
hepatitis (liver disease) and leukaemia (blood
disease).
 LDH isoenzymes indicate the diseased organ:
 LDH1 and LDH2 isoenzymes increase only in
myocardial infarction,
 LDH3 increases in leukaemia
 LDH4 and LDH5 increase in viral hepatitis.
Antienzymes
 These are substances secreted by living cells or
organisms that inhibit enzyme activity e.g.:
 Ascaris worms living in the intestine secrete
antienzymes (anti-trypsin and anti-pepsin) so;
they are not digested by proteolytic enzymes
present in the digestive juices.
 Mucin lining the stomach contains antienzyme
(anti-pepsin) that prevents digestion of stomach
wall by pepsin.
 Blood plasma contains natural antienzyme
(anti-thrombin) that inactivates thrombin after
blood coagulation to prevent its intra-vascular
spreading.
Ribozymes
Ribozymes are enzymes but they are not protein in
nature, they are nucleic acid in nature formed of
RNA.
 Ribozymes catalyze cleavage of RNA by
hydrolysis of phosphate diester bonds e.g.
cleavage of pre-mRNA to form mRNA.
Got any Questions?!
AP Biology
2007-2008

Activity
1- The enzyme Creatine kinase levels are increased in the
blood of patients with
A) Prostate cancer .
B) Hepatitis
C) Heart attack
D) Osteoporosis
2- The isoenzymes of LDH
(A) Differ only in a single amino acid
(B) Differ in catalytic activity
(C) Exist in 5 forms depending on M and H monomer
contents
(D) Occur as monomers
Activity
 Discuss antienzymes (def , examples , biomedical
importance).
 Discuss iso-enzymes (def, 2 examples & clinical
importance).
 Creatine kinase & Lactate dehydrogenase can be
used for diagnosis and follow up of some diseases
(explain )
Thank you