azin nowrouzi, phd department of biochemistry tehran
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Azin Nowrouzi, PhD Department of Biochemistry
Tehran University of Medical Sciences
Topics
Activity of enzyme and its expression
Unit
Katal
Specific activity
Factors that affect activity
Enzyme Kinetics
definition MM equation
Lineweaver-Burk equation
Enzyme inhibition
Reversible
Competitive
Mixed or non-competitive
Uncompetitive
Irreversible
Allosteric enzymes
Multi-substrate reactions
Ping-pong mechanism
Sequential
Random
Ordered
Enzyme activity
1. Enzyme Unit:
A. One unit = 1 micromol/min= 1µmol/min • the amount of enzyme that converts 1 µmol substrate to product in 1 minute.
B. One unit = 1 nanomol/min= 1nmol/min
2. 1 katal = 1 mol/s
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A measure of the ability of a given enzyme to convert its substrate(s) into its product(s).
To find the activity of an enzyme: We measure either the amount of substrate that's disappearing, or the amount of product that's appearing over a specified period of time. Activity is expressed as Units or Katal.
Enzyme activity is a measure of the quantity of active enzyme present.
Activity is the same as: • speed • velocity • Rate
of the reaction
Enzyme specific activity
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Example for enzyme specific activity:
In a sample of blood the protein concentration is 4 g/dl, and the enzyme activity is 60 U/ml. what is the specific activity of the enzyme? a. 1.5 IU/ml b. 15 IU/ml c. 150 IU/ml d. 1500 IU/ml
Specific activity is the number of enzyme units per ml divided by the concentration
of protein in mg/ml. Specific activity values are therefore quoted as units/mg.
Factors that influence activity
Important factors in reaction rates:
1. Temperature
2. pH
3. Enzyme concentration [E]
4. Substrate concentration [S]
5. Inhibitors
6. Regulation (Effectors)
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A typical graph of rate against temperature
• The temperature at which the rate is fastest is called the optimum temperature for that enzyme.
1
2
3
① Little activity at low temperature (low number of collisions)
② Rate increases with temperature (more successful collisions) rate doubles or
triples for every 10°C increase in temperature
③ Activity lost with denaturation at high temperatures
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Optimum temperature
Enzymes are most active at optimum temperatures (usually 37 C in humans) Enzymes isolated from thermophilic organisms display maxima around 100°C Enzymes isolated from psychrophilic (cryophilic) organisms display maxima
around 10°C
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Optimum pH
• Effect of pH on ionization of active site.
• Effect of pH on enzyme denaturation.
• Each enzyme has an optimal pH (most of the times ≈ 6 - 8 ) – Exceptions :
digestive enzymes in the stomach( pH 2)
digestive enzymes intestine (pH 8)
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Effect of pH on ionization of active site
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Too much Basic condition
Too much Acidic condition
Correct pH in the microenvironment
Enzyme concentration
• The Rate (v) of reaction Increases proportional to the enzyme concentration [E] ([S] is high).
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Enzyme saturation with substrate
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Enzyme kinetics
• Kinetics:
The study of reaction rates (velocities)
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Dependence of velocity on [substrate] concentration.
many enzymes follow Michaelis-Menten equation:
2 1 3 4 5 6 7 8 0
0 2 4 6 8
Substrate (mmole) [S]
Pro
du
ct (
v)
80
60
40
20
0
S + E
(in a fixed
period of
time)
Enzyme Velocity Curve
P
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Michaelis-Menten equation
S E k1
k-1 E S
k2 P
0
1
2
3
4
5
0 10 20 30 40 50
v,
µm
ol/
min
[S], mM
0.5Vmax
Km
Vmax ,سرعت ماکزیمم
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منتن-منحنی سهمی میکائیلیس
Order of reaction
1. When [S] << Km
vo = (Vmax / Km )[S]
2. When [S] = Km
vo = Vmax /2
3. When [S] >> Km
vo = Vmax
zero order
First order
Mixed order
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MM equation derivation (steady state)
Substrate concentration
• When enzyme concentration is constant, increasing [S] increases the rate of reaction, BUT
• Maximum activity reaches when all E combines with S (when all the enzyme is in the ES, ,form).
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Example
• In an enzyme catalyzed reaction [S] is 1mmol and initial velocity is equal to 2/3Vmax. Calculate Km?
• In an enzymatic reaction if substrate concentration is equal to Km, what would be the relationship between Vi and Vmax?
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Lineweaver-Burk plot
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1
vKm
Vmax
1
[S]1
Vmaxv
Vmax [S]
Km [S]
Direct plot Double reciprocal
Enzymes are subject to reversible inhibition
Competitive inhibition
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• Competitive Inhibition:
o A competitive inhibitor binds to the active site of the enzyme.
o A competitive inhibitor does not change the maximum rate for the reaction.
o A competitive inhibitor lowers the enzymes affinity for its substrate.
Competitive inhibition of succinate dehydrogenase
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Example: Competitive inhibition
• The antibiotic sulfanilamide is similar in structure to para-aminobenzoic acid (PABA), an intermediate in the biosynthetic pathway for folic acid.
• Sulfanilamide can competitively inhibit the enzyme that has PABA as its normal substrate by competitively occupying the active site of the enzyme.
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Example: Enzyme inhibitors in medicine
•Many current pharmaceuticals are enzyme inhibitors
(e.g. HIV protease inhibitors for treatment of AIDS)
•An example: Ethanol is used as a competitive
inhibitor to treat methanol poisoning.
Methanol formaldehyde (very toxic)
Ethanol competes for the same enzyme.
Administration of ethanol occupies the enzyme
thereby delaying methanol metabolism long enough
for clearance through the kidneys.
Alcohol dehydrogenase
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Uncompetitive inhibition
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• Un-Competitive or Anti-competitive Inhibition:
o binds to a site other than the active site.
o slows the maximum attainable rate of the reaction.
o lowers the enzymes affinity for its substrate.
Mixed inhibition
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o A non-competitive inhibitor slightly decreases an enzymes affinity for its substrate by altering the shape of the active site.
Pure Non-competitive inhibition
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• Non-Competitive Inhibition:
o A non-competitive inhibitor binds to a site other than the active site.
o A non-competitive inhibitor slows the maximum attainable rate of the reaction.
Reversible Inhibition = Temporary decrease of enzyme function
• Three types based on “how increasing [S] affects degree of inhibition”:
1. Competitive – degree of inhibition decreases
2. Mixed or Non-competitive – degree of inhibition is unaffected
3. Anti- or Uncompetitive – degree of inhibition increases
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Types of enzyme inhibitors
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Uncompetitive
Irreversible Inhibition = Enzyme stops working permanently
1. Destruction of enzyme (heat or extremes of pH) 2. Irreversible Inhibitor = forms covalent bonds to E (inactive E) Examples:
– Diisopropylfluorophosphate • inhibits acetylcholine esterase • binds irreversibly to –OH of serine residue
– Cyanide and sulfide • Inhibit cytochrome oxidase • bind to the iron atom
– Fluorouracil • inhibits thymidine synthase (suicide inhibition - metabolic product
is toxic )
– Aspirin • Inhibits prostaglandin synthase • acylates an amino group of the cyclooxygenase
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(1) Irreversible Inhibition: inhibitor binds tightly, often covalently, to the enzyme,
permanently inactivating it.
DIPF = DIFP = diisopropylfluorophosphate
kinetics
• Dependence of velocity on [substrate] is described for many enzymes by the Michaelis-Menten equation:
• kinetic parameters: – Km (the Michaelis constant) – Kcat (the turnover number, which relates Vmax, the maximum velocity, to [Et],
the total active site concentration) – kcat/Km (the catalytic efficiency of the enzyme) – can't be greater than limit imposed by diffusion control, ~108-109 M–1sec–1
• Kinetic parameters can be determined graphically by measuring velocity of enzyme-catalyzed reaction a(V vs. [substrate]).
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Why study kinetics
• Study of enzyme kinetics is useful for measuring 1. concentration of an enzyme in a mixture (by its catalytic activity), 2. its purity (specific activity), 3. its catalytic efficiency and/or specificity for different substrates 4. comparison of different forms of the same enzyme in different tissues or
organisms, 5. effects of inhibitors (which can give information about catalytic mechanism,
structure of active site, potential therapeutic agents...)
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Allosteric enzymes
• Why the sigmoid shape?
• Allosteric enzymes are multi-subunit enzymes, each with an active site.
• They show a cooperative response to substrates
Sigmoidal curve
hyperbolic curve michaelis-menten
kinetics
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Allosteric enzymes
• They have oligomeric organization – They have more than one site, where effector binding at one site
induces a conformational change in the enzyme, altering its affinity for a substrate.
• Their kinetics do not obey the Mikaelis-Menten equation • Their v versus [S] plot yields sigmoid or S-shaped curves rather than
rectangular hyperbolic – sensitive to a small change in [S].
• Substrate binding is cooperative – Binding of one substrate alters the enzyme’s conformation and
enhances the binding of subsequent substrates.
• Inhibition of a regulatory enzyme does not conform to any normal
inhibition pattern. – Allosteric inhibition – Allosteric activation
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CO2 Also Promotes the Dissociation of O2 from Hemoglobin
Oxygen binding curves of blood and of hemoglobin in the absence and presence of CO2 and BPG.
Multi-substrate reactions
• Ping Pong mechanism for two substrates
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Multi-substrate reactions
• Sequential mechanism
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Regulation of enzyme activity
Enzyme concentration
Gene trascription
Gene induction Gene
suppression
Control of translation
(mRNA stability)
Protein half life
Enzyme activity
Compartmentalization
Isoenzymes
Lactate dehydrogease
Creatine phosphate
Hexokinase Glucokinase
Allosteric regulation)
Product inhibition
Endproduct inhibition (feedback inhibition)
Substrate
(feedforward activation)
Covalent modification
Regulatory subunit
proteolysis
Allosteric regulation
• Allosteric enzymes – Enzymes whose activity can be changed by molecules (effector molecules) other than substrate.
• Allosteic means “other site” or “other structure”
• The interaction of an inhibitor at an allosteric site changes the structure of the enzyme so that the active site is also changed.
• Negative allosterism
• Positive allosterism
• Binding of the effector molecule regulates enzyme activity by determining whether it will be active or not.
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Enz 2 Enz 1 Enz 3 Enz 4 B C D E A
Product inhibition and end product inhibition
-
Negative feedback inhibition
-
Product inhibition
Rate determining steps
Rate limiting steps
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Product inhibition
Hexokinase- first reaction in glycolysis, hexokinaseis inhibited by glucose-6-phosphate (G6P, the product)
ADP ATP Glucose Glucose-6-phosphate
Hexokinase
×
Product inhibition
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اولین مرحله در سنتس اوراسیل و
سیتوزین
Example
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End-product inhibition
Proenzymes (zymogen)
Proteolytic activation Activation of a zymogen.
• Some enzymes are secreted as inactive precursors, called zymogens. • Pancreatic proteases - trypsin, chymotrypsin, elastase,
carboxypeptidase are all synthesized as zymogens - trypsinogen, chymotrypsinogen, proelastase and procarboypeptidase
• Clotting factors Fibrinogen and pro-thrombin • Hormone peptides (Conversion of Pro-insulin to
Insulin) • an on/off switch more than regulation. • Fibrous proteins: Collagen is synthesized as
procollagen
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Real life examples of activation by proteolytic cleavage
Trypsin activation
45 Chymotrypsin activation
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Different types of covalent modification
Phosphorylation
Conformational Change dephosphorylastion
Phosphatase
P
Protein
OH
SerThr Tyr(His)
Inactive Active
Glycogen phosphorylase b Glycogen phosphorylase a
Juang RH (2004) BCbasics
Kinase
phosphorylation
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ADP ATP Glucose Glucose-6-phosphate
Hexokinase
Isoenzymes (isozymes)
• Multiple forms of same enzyme • Catalyse the same chemical reaction • Different chemical and physical properties:
– Electrophoretic mobility – Kinetic properties – Amino acid sequence – Amino acid composition
Hexokinase Glucokinase
Location in cell
All cell types (cytoplasm)
Liver (mitochondria)
Km 5 x 10-5M 0.05 mM
5 x 10-3M 5mM
Glucokinase
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Aminotransferases
Aspartate aminotransferase
(AST or SGOT)
Alanine aminotransferase
(ALT, or SGPT)
Myocardial infarction
Viral hepatitis
Lactate Dehydrogenase (LDH) myocardial infarction
Creatine Kinase (CK) Myocardial infarc., brain,
skeletal muscle disorder
Cholinesterase Liver, erythrocytes
Gamma-glutamyltransferase Liver damage
Acid phosphatase Carcinoma of prostate
Alkaline phosphatase (AP) Bone disease
Lipase Acute pancreatitis
Alpha-amylase Intestinal obstruction Som
e d
iag
no
stically
im
port
an
t e
nzym
es
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Isozymes of lactate dehydrogenase
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In LDH • Subunits M and H can combine to make five different tetramers.
H4
M4
H3M
HM3
(LDH, EC 1.1.1.27.)
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• CK has 3 forms dimer B and M chains: • CK1= BB • CK2= MB • CK3=MM • Heart is the only tissue rich in CK2, increases 4-8 hr
after chest pains- peaks at 24 hr. • LDH peaks 2-3 days after MI.
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Isoenzymes of Creatine kinase
Electrophoresis of LDH and CK isoenzymes in blood
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Isoenzyme Name Circulating Lifetime
Mitochondrial AST 1hr
LDH-5 (M4) 12 hrs
CK 18 hrs
Cytoplasmic AST 1 day
ALT 2 days
LDH-1 (H4) 5 days
Increase in important isoenzymes in blood after chest pain
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