Enz inhi 5 lec
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Transcript of Enz inhi 5 lec
Enzyme InhibitionDr.Geeta Jaiswal
InhibitorsEnzymes are proteins. They
can be inactivated by agents which denature them.
Chemical substances that inactivate enzymes are called Inhibitors
Inhibitors are organic substances or inorganic ions
Inhibitors decrease enzyme activity without altering the ionic strength or pH of the medium
Inhibition may be reversible , where the inhibitor does not react covalently with the enzyme.
Some agents react covalently with the functional groups of enzymes resulting in non-competitive irreversible inhibition.
Inhibition
Reversible Irreversible
Competitive Non-Competitive Uncompetitive
Classification of Inhibition
Competitive Inhibition
It is called substrate analogue inhibition
Inhibitors resembles substrate and competes for the active site of the enzyme.
Inhibitor binds with enzyme to form EI complex .
Due to structural similarity the substrate and inhibitor molecule both compete for the active site
This is a reversible type of inhibition, where both ES and EI complexes are formed.
The actual amount of ES and EI will depend upon:- (i) Affinity between enzyme
substrate/and inhibitor present. (ii)Actual concentration of
substrate and inhibitor present (iii) Time of pre-incubation with
substrate or inhibitor
Line weaver’s Burk plot representing Competitive Inhibition
Competitive Inhibition
Affinity Decreases
I Decreases as Km increases
Km
I Remains the same
Vmax
Efficiency Remains the same
Examples of Competitive Inhibitors
S.N Enzyme Substrate Inhibitor 1. LDH Lactate Oxamate
2. Aconitase Cis-Aconitase Trans-Aconitase
3. Succinate Dehydrogenase
Succinate Malonate
4. H.M.G Co.A reductase HMG Co.A HMG
5. Di Hydrofolate Reductase
7,8 dihydrofolate Amethopterin
6. Xanthine Oxidase Hypoxanthine Allopurinol used in Gout Treatment
Examples of Competitive Inhibitors used as Drugs Clinically
Allopurinol ---- Used in the treatment of Gout
Xanthine Oxidase Hypoxanthine Uric Acid Allopurinol
Sulphonamides:
Used as antibacterial agents.Similar in structure to PABA
For Folate synthesis PABA is essential Sulphonamide Needed for Bacterial Growth
Methotrexate
Methotrexate is 4-amino N10 methyl folic acid.
Used in cancer therapyMethotrexate resembles folic acid it
competitively inhibits “Folate Reductase”Prevents the formation of FH4
DNA Synthesis is inhibited
MAO Inhibitors MAO inhibitors are Ephedrine and
Amphetamine
Enzyme Mono Amine Oxidase oxidizes Epinephrine and Nor-epinephrine
MAO inhibitors competitively inhibit MAO ,prolong action of presser amines.
PhysotigminePhysotigmine is Acetylcholine esterase
inhibitorAcetylcholine Acetate + Choline
This drug prevents destruction of Acetylcholine,
Continued presence of Acetylcholine in post synaptic regions prolong neural impulse.
Dicoumarol
• Dicoumarol is used as an anticoagulant
• It competitively inhibits Vitamin K
Clinically useful Competitive Inhibitors
Drug Enzyme Inhibited Clinical use
Penicillin Transpeptidase Bacteria
Sulphonamide Pteroid synthetase Bacteria
6-Mercapto Purine Adenylo succinate Synthetase
Cancer
Neostigmine Acetyl Ch.esterase MyastheniaAlpha methyl DOPA DOPA Decarboxylase Hypertension
Lovastatin HMG Co A-reductase
Cholesterol Lowering
Non-Competitive InhibitionNo competition between the Inhibitor and
substrate.These inhibitors do not resemble the
substrate and bind to a site away from the active site.
Enzyme inhibitor has normal affinity for the substrate but produce products at a decreased rate.
Non-Competitive Inhibition
In Non-Competitive Inhibition (i) Affinity Remains the same
(ii) Efficiency decreases
(iii) 1/ Km remains the same as substrate concentration has no effect on the inhibitor
(iv) 1/ Vmax Increases as V has a decrease
Lineweaver Burk’s Plot for Non-Competitive Inhibition
1/[s]1/ Km
1 / v
1 / Vmax
1 / Vmax
+ Inhibitor
No Inhibitor
Non-competitive inhibition can be reversed if the inhibitor can be removed without affecting the enzyme activity.
Eg: Enzymes with –SH groups bind to heavy metals like Hg , Pb, resulting in non-competitive inhibition.
It can be reversed not by high levels of substrate but by increasing –SH in the medium.
Uncompetitive InhibitionIn this case the Inhibitor does
not bind to either enzyme or substrate, but combines with the ES complex to form ESI complex.
E + S ES + I ESI (no product forms)
Degree of inhibition increases with increase in substrate concentration.
There is a change in Vmax .It is lowered
This type of inhibition is very rare.
Uncompetitive Inhibition+
1/V
1/V
1/V
1/ [s]
1/V
1/Km 1/Km 1/Km
More Inhibitor
+ Inhibitor
Uncompetitive Inhibition
Suicide InhibitionFrom all the discussions it can
be assumed that the inhibitor binds to the enzyme reversibly.
The inhibitor just sits there, either in the active site or in a second binding site, but it is free to go back to solution.
There are another type of inhibitors that undergo irreversible chemistry with certain key amino acid residues (or prosthetic groups) in the active site, therefore killing the enzyme's activity, and preventing the inhibitor from being released out of the active site.
Many of these molecules are very effective drugs, because they are targeted specifically for a certain enzyme and kill the enzymes for good.
This inhibitors kill the enzyme for
good, but since they also 'die' in the process, they are called suicide or mechanism-based inhibitors.
Suicide inhibition It is a type of irreversible inhibition The inhibitor makes use of an enzyme’s
own reaction mechanism to inactivate itIn suicide inhibition, the structural
analogue is converted to a more effective inhibitor with the help of the enzyme to be inhibited.
This new product binds to the enzyme and inhibits further reaction.
Eg : Of Suicide inhibition1.Allopurinol a competitive inhibitor
for enzyme xanthine oxidase
When it comes in contact with the enzyme it is oxidized by xanthine oxidase to alloxanthine which is a stronger irreversible inhibitor of the enzyme
2. Anti-inflammatory Action of Aspirin
Membrane bound phospholipids are broken down first to Arichidonic acid (by phospholipases)
CyclooxygenaseArichidonic Acid Prostaglandins
Aspirin acetylates a serine residue in the active center of cyclo-oxygenase ,inhibiting prostaglandin synthesis and reducing inflammation
3. Difluromethyl ornithine against sleeping sickness TrypanosomiasisOrnithine Decarboxylase converts
Ornithine to putriscine a polyamineWhen this enzyme ODC
inTrypanosoma(parasite) is inhibited, multiplication of the parasite is arrested.
DFMO is initially inert. Binding with the enzyme it forms an irreversible covalent complex with co-enzyme PyPO4 and amino acid residues
S.No ENZYME Allosteric Inhibitor
Allosteric activator
1 HMG Co A -reductase Cholesterol
2 Phosphofructokinase ATP,Citrate AMP,F2,6,P
3 Pyruvate Carboxylase ADP Acetyl Co A4 Acetyl CoA Carboxylase AcylCoA Cirate
5 Citrate Synthase ATP
6 Carbamyl Phosphate Synthetase- I N-Acetyl Glut
7 Carbamyl PhosphateSynthetase-II UTP
8 Aspartate Transcarbamylase CTP ATP