Enzyme 4 - the Inhibition Lecture NO: 1st MBBS Dr Muhammad Ramzan

Enzyme inhibition (EI) – 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 inhibitor is a substance that slows down /stops the enzyme catalyzed reaction www.medicaldictionary.com

Enzyme Inhibition - the background Enzyme inhibition is the major mechanism for the physiological regulation of enzymes Some body processes are controlled by enzyme inhibition like blood coagulation and clot dissolution/Fibrinolysis This process has led to the development of drugs that act by inhibiting enzyme metabolic pathway www.sciencedirect.com

Types of enzyme inhibition - 2 There are 2 types of enzyme inhibition, depending upon the type/nature of inhibitor : Reversible inhibition Irreversible Inhibition www.chemistry.elmhurst.edu

Enzyme inhibitors - types

Reversible inhibition 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

Irreversible inhibition Inhibitor, binds irreversibly to an enzyme through Covalent bonds These bonds are permanent and cannot be removed There is loss of folding, 3D tertiary structure and function Are also called as suicide substrates Inhibitors can also bind at the allosteric site

Difference B/W types of inhibitions Serial NO: Parameter Reversible Irreversible 1 Type of bonds Temporary – non covalent Permanent- Covalent bond 2 Chemical change No change in active site/Enzyme Permanent change in active site / E loss of folding 3D 3 Loss of function Reversible by removal of inhibitor or addition of substrate

Types of reversible inhibition - 3 There are 4 types of reversible inhibition Competitive Non competitive Uncompetitive 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 The inhibitor thus decreases the enzyme activity Competitive inhibition can be reversed by ↑ing the Substrate or decreasing the conc. Of the Inhibitor

Competitive inhibition cont. In competitive inhibition, inhibitor is bound to: Active site or the product or neither. It cannot bind to both at the same time Shape of the enzyme or active site is unchanged Statins and ACE inhibitors are the commonest inhibitors

Competitive inhibition

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( HMGCR) HMGCR is a rate limiting Enzyme for the CH synthesis HMGCR : 3 - hydroxy-3- methyl- Glutaryl CoA Reductase Statins thus prevent the De novo synthesis of CH HMG-Co A is a precursor for 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 Angio1 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 : Changes the 3D structure of the active site, So that substrate no longer fits correctly to : give a catalytic reaction It makes the enzyme less active

Non competitive inhibition (Allosteric)

Competitive VS. non competitive inhibition Serial number Parameter Competitive inhibition Non competitive inhibition 1 Structure of inhibitor similar to substrate Different from substrate 2 Site of binding Active site Allosteric site 3 Changes in active site /enzymes No change Changed 4 Dilution Can be diluted Cannot be diluted

Uncompetitive inhibition inhibitor binds to ESC The inhibitor binds to the Enzyme substrate complex - ESC This decreases the maximum velocity of the enzyme due to delayed removal of the activated complex Substrate cannot bind to the active site during this period and activity of the enzyme is slowed down Mixed inhibition is a term in which : inhibitor can bind to active site in competitive and to ESC in uncompetitive inhibition

End product or Negative Feed back inhibition inhibition by end product It is a negative feed back to regulate the production of a given mole like AA. Via enzymatic metabolic pathway End product of this pathway; then binds with the allosteric site of the 1st enzyme of the pathway to inhibit its activity This process prevents the excessive production of end products like AA. Isoleucine from AA. Threonine Threonine is an initial substrate and Isoleucine is the end product

End product or negative feed back inhibition

Negative feed back inhibition Threonine to Isoleusine

Irreversible inhibition – Poisoning of E Inhibitor forms covalent bond with E 1. Irreversible inhibitor modifies the E chemically and makes covalent bonds with Aminoacyl groups at active site These groups are essential for S binding ,catalysis at the active site and enzyme conformation E loses its folding, 3D tertiary structure and function - Denaturation These include Cyanides, heavy metals and antibiotics

Cyanide - An irreversible Inhibitor Cyanide is a poison which prevents ATP production via aerobic respiration, leading to eventual death It binds to an allosteric site on Cytochrome Oxidase A carrier molecule that forms part of the ECT It changes shape of the active site and enzyme can no longer pass electrons to the final acceptor (oxygen) Consequently, the ETC cannot continue to function and ATP is not produced via aerobic respiration

Cyanide poisoning

Irreversible inhibitors - Heavy metal poisoning and antibiotics Heavy metals like Mercury and Lead ions produce protein Denaturation and neurological damage by: binding with SH group of the enzymes 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

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 Enzymes 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) Cardiac Troponin I and Cardiac Troponin T tests

Enzymes in the diagnosis of liver diseases Elevated levels of enzymes in Liver diseases are : Alanine amino Transferase - ALT Lactic Dehydrogenase - LDH and AST and alkaline Phosphatase

Enzymes in the diagnosis of bones and prostate Elevated levels of enzymes in bone diseases are: Alkaline Phosphatase Enzymes in Prostate cancer with Acid Phosphatase