Transcript Enzyme -3. Factors affecting enzyme activity Lecture NO: 1st MBBS

Enzyme 4 - the Inhibition
Lecture NO: 1st MBBS
Dr Muhammad Ramzan
Enzyme inhibition – the definition
• It is an interaction B/W a molecule and enzyme that blocks
the catalytic action of the enzyme in a normal way
• Enzyme inhibition interferes with catalysis and slows down or
stops the enzymatic reaction
Enzyme Inhibition - the background
• An enzyme inhibitor is a substance that decreases the rate of
catalyzed reaction of an enzyme .
• Enzyme Inhibition is the major mechanism for the physiological
regulation of enzymes
• This process is either Reversible or irreversible depending
upon the type of inhibitor
• Enzyme inhibition has scientific uses and a large no: of drugs
act by inhibiting enzymatic metabolic pathways
Types of enzyme inhibition - 2
• There are 2 types of enzyme inhibition, depending upon the
type/nature of inhibitor :
• Reversible inhibition
• Irreversible Inhibition
Enzyme inhibitors - types
Reversible inhibition - the mechanism
Inhibitors forms non covalent bonds with E
• Inhibitor, binds reversibly to an enzyme, establishing non
covalent interactions with the Enzyme,
• Once inhibitor binds with the enzyme, active site is blocked
and there is no further catalysis
• The inhibitor can be removed from E by:
• ↑ing the substrate or diluting the inhibitor
Comparison B/w reversible and irreversible inhibition
Types of reversible inhibition - 3
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There are 3 types of reversible inhibition
Competitive
Non competitive and
Feed back inhibition
Competitive inhibition (Blockers)
the imposter and substrate
• Competitive inhibitor binds to the enzyme’s active site and
competes with the normal substrate molecule.
• These imposters often have structures that are similar to
those of the normal substrate
• Competitive inhibition can be reversed by ↑ing the Substrate
or decreasing the conc. Of the Inhibitor
• In competitive inhibition, at any given moment, inhibitor is bound to
enzyme or the product or neither. (Shape of E/active site is unchanged)
• It cannot bind to both at the same time
Enzyme Inhibition- makes the E less active
Competitive inhibition – the example
the Statins
• Statins are the lipid lowering agents that compete with the
HMG COA – A substrate for the HMG CoA Reductase( HMGR)
• HMGR is a rate limiting Enzyme for the CH synthesis
• Statins thus prevent the De novo synthesis of CH
• HMG-Co A is a precursor for CH synthesis
Competitive inhibition of Enzymes
Pathway for Denovo CH synthesis
Example of Competitive inhibition –
the Statins
Competitive inhibition – Example 2
ACE Inhibitors –Lipitor/Captopril
• ACE inhibitors are the antihypertensive drugs that prevent
the conversion of Angiotensin 1 to Angiotensin 11
• These drugs compete with Angio11 for the Enzyme ACE in the
pulmonary circulation
• ACE – I is the Angiotensin converting Enzyme Inhibitors
• ACE inhibitors are Captopril (Capoten) and Atrovastatin (Lipotar)
Completive inhibition - ACE inhibitors (Captopril)
Non competitive inhibition (Allosteric)
inhibitors bind to Allosteric sites
• Noncompetitive inhibitor binds reversibly with the enzyme at
a site other than the active site :
1
• Changes the 3D shape of the enzyme and the active site, 2
• So that normal substrate no longer fits correctly to :
• give a catalytic reaction
3
• It makes the enzyme less active
4
Non competitive inhibitors (Allosteric)
Differences from competitive Inhibitors
• Structure of Inhibitor is different from Substrate
1
• Inhibitor Does not compete for the active site, binds to the
Allosteric site - A site other than active site
2
• It rather changes the shape of the Enzyme and active site
• Inhibitor cannot be easily dislodged from its Allosteric site
3
• Cannot be overcome by ↑ the substrate Conc.
4
Reversible or Feed back inhibition
inhibition by end product
• It is a negative feed back to regulate the production of a given
molecule like the synthesis of an amino acid
• The end product of an Enzymatic pathway, then binds with
the Allosteric site of the 1st enzyme and inhibits its activity
• The – ve feed back inhibition prevents the excessive
production of the end products
End product or Negative feed back inhibition
prevents ↑ production of the end product
Negative feed back inhibition
Threonine to Isoleusine
Irreversible inhibition – Poisoning of E
Inhibitor forms covalent bond with E
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Irreversible inhibitor modifies the E chemically and :
makes covalent bonds with the specific functional groups:
of the enzyme (Aminoacyl residues)
These groups are essential for S binding ,maintenance of
enzyme conformation and catalysis
• The covalent bonds are stable and inactivate the enzyme
permanently - poisoning of enzyme
• These include Cyanides, heavy metals and antibiotics
Irreversible Inhibition – Changes shape of E and
active site – S cannot bind to E
Irreversible inhibition – the Cyanides
poisoning of Cytochrome Oxidase
• Cyanide ion, CN¯, is a rapidly-acting, highly toxic inhibitor,
which interferes with the iron -containing enzyme:
• The Cytochrome Oxidase
• Cell respiration stops, and death occurs with in minutes
Irreversible inhibitors - Heavy metal poisoning and
antibiotics
• Heavy metals like Mercury and Lead ions which bind with the
SH group of enzymes causing the :
• Protein denaturation and permanent neurological damage
• Antibiotics are enzyme inhibitors that act on the life
processes that are essential to certain strains of bacteria
• These include the sulfa drugs and penicillin
• Penicillin prevents synthesis of bacterial cell wall and growth
Irreversible inhibition - Penicilline
prevents cell wall synthesis
Diagnostic importance – enzymes localization in
specific tissues
• Certain enzymes are found exclusively in specific tissues .
• Their presence in the blood indicates damage to the tissues
• ↑ in the serum level of the enzymes is propornal to the damage
of the affected organ
• Measurement of the level of E is the diagnostic tool for a no: of
diseases especially of :
• Heart; Liver, Pancrease, Prostate and bone
Enzymes – Diagnostic importance in MI
• Elevated levels of the following Enzymes is important in
Diagnosis of Myocardial Infarction (MI)
• Creatine Kinase (CK)
- CK MB
• Lactic Dehydrogenase - (LDH2)
• Aspartate Transaminase – AST and
• Cardiac Troponin I and Cardiac Troponin T tests
Enzymes in the diagnosis of liver diseases
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Elevated levels of enzymes in Liver diseases are :
Alanine Transaminase - ALT
Lactic Dehydrogenase - LDH and
AST and alkaline Phosphatase
Enzymes in the diagnosis of bones and prostate
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Elevated levels of enzymes in bone diseases are:
Alkaline Phosphatase
Enzymes in Prostate cancer with
Acid Phosphatase