Michaelis Menten Kinetics- Enzyme Inhibition

Transcription

1 Michaelis Menten Kinetics- Enzyme Inhibition Dr. M. Vijayalakshmi School of Chemical and Biotechnology SASTRA University Joint Initiative of IITs and IISc Funded by MHRD Page 1 of 9

2 Table of Contents 1 INTRODUCTION COMPETITIVE INHIBITION UNCOMPETITIVE INHIBITION NON-COMPETITIVE INHIBITION MIXED INHIBITION REFERENCES TEXT BOOKS... 9 Joint Initiative of IITs and IISc Funded by MHRD Page 2 of 9

3 1 Introduction Enzyme inhibition can happen when the inhibitors (structural analogs of substrate) binds to the active site of the enzyme and prevents the catalysis. Enzyme inhibition is specific and is different from the alteration of structure of enzyme and reduction of reaction rate by environmental factors such as ph, temperature, presence of hydrophobic compounds, detergents etc., which are non specific. For e.g. consider sudden addition of an acid or base to the reaction mix which changes the ph and thereby influences the rate of the reaction. These are often confused with inhibition as they also reduce the turnover rate of enzymes. In general, the binding of enzyme to the inhibitor is reversible but few of them bind covalently and become irreversible. The reversible and irreversible inhibitors have different kinetics. Michaelis-Menten kinetics explains the inhibition of enzyme in a single substrate complex, but the complexity increases with the number of substrates. The inhibitors also depend on their homology with the substrate apart from the nature of binding site and binding affinity. Some inhibitors even bind stronger than the natural substrate because of specific interactions and act as antagonists. Most therapeutic drug molecules act in this way. Yet there are different forms of inhibition based on the affinity of inhibitor to the enzyme and substrate. 1.1 Competitive inhibition Competitive inhibition occurs when the inhibitor is highly homologous to the substrate molecule and competes with substrate to bind to the free enzyme. So, either one of them can bind with an enzyme and not both together Fig 1. In this condition, there is a need for excess substrate to overcome the competition with inhibitor. Classical example for competitive inhibition is the molecule methotrexate which inhibits the action of dihydrofolate reductase to convert dihydrofolate to tetrahydrofolate. Joint Initiative of IITs and IISc Funded by MHRD Page 3 of 9

4 E + S E.S E + P + I E.I Fig 1. Schematic representation of competitive inhibition The rate of product formation in the reaction is given by, And from the Michaelis-Menten kinetics, Similarly, rate of formation of enzyme inhibitor complex will give, The total enzyme concentration in the system will be the sum of the concentration of three forms in which the enzymes exists: Joint Initiative of IITs and IISc Funded by MHRD Page 4 of 9

5 Hence in competitive inhibition, only the K m is influenced and not the maximum velocity (V max ). 1.2 Uncompetitive inhibition Uncompetitive inhibition occurs when the inhibitor does not bind to the free enzyme and instead binds to the already formed enzyme substrate complex and makes the complex inactive Fig 2. This phenomenon of inhibition is commonly observed in multimeric enzymes. E + S E.S E + P + I E.S.I The rate of product formation in the reaction is given by, And from the Michaelis-Menten kinetics, Joint Initiative of IITs and IISc Funded by MHRD Page 5 of 9

6 Similarly, rate of formation of enzyme-substrate-inhibitor complex will give, Fig 2. Schematic representation of uncompetitive inhibition The total enzyme concentration in the system will be the sum of the concentration of three forms in which the enzymes exists: Joint Initiative of IITs and IISc Funded by MHRD Page 6 of 9

7 In uncompetitive inhibition, both K m as well as maximum velocity (V max ) is influenced. 1.3 Non-Competitive inhibition Non-Competitive inhibition is where; the inhibitor binds to the different site in the enzyme. So, in contrast to competitive inhibition, they can bind along with substrate to the enzyme and here both EI and ESI is inactive Fig 3. E + S E.S E + P k i + I K ii + I +S E.I E.S.I Fig 3. Schematic representation of non competitive inhibition The rate of product formation in the reaction is given by, And from the Michaelis-Menten kinetics, Joint Initiative of IITs and IISc Funded by MHRD Page 7 of 9

8 Similarly, rate of formation of enzyme-substrate-inhibitor complex will give, The total enzyme concentration in the system will be the sum of the concentration of four forms in which the enzymes exists: Non competitive inhibition influences the maximum velocity while the K m does not changes. V max is changed because high substrate concentration cannot prevent the binding of inhibitor. 1.4 Mixed inhibition Another mode of inhibition which is similar to that of non competitive inhibition but with an active ESI complex is termed the mixed inhibition. Such inhibition is common in metabolic feedback pathways. Enzymes showing this form of inhibition are generally allosteric in nature. Joint Initiative of IITs and IISc Funded by MHRD Page 8 of 9

9 2 References 2.1 Text Books 1. Bisswanger H, Enzyme Kinetics, Principles and Methods, WILEY-VCH, (2002). 2. J. D. Murray, Mathematical Biology, Springer-Verlag, (1989). 3. Berg JM, Tymoczko JL, Stryer L. Biochemistry, 5/e, W H Freeman, (2002). Joint Initiative of IITs and IISc Funded by MHRD Page 9 of 9