Factors Affecting Enzyme Activity PDF

2. Factors affecting enzyme activity  Prof / Ayman El Baz Professor of Medical Biochemistry & Molecular Biology Dr/ Sara El Derbaly Assistant Professor of Medical Biochemistry& Molecular Biology  Contents Factors affecti...

2. Factors affecting enzyme activity  Prof / Ayman El Baz Professor of Medical Biochemistry & Molecular Biology Dr/ Sara El Derbaly Assistant Professor of Medical Biochemistry& Molecular Biology  Contents Factors affecting enzyme activity. Enzyme Kinetics. Course learning Outcomes At the end of the lecture, the students should be able to: List different factors affecting enzyme activities State effects of different factors on enzymatic activity Explain enzyme Kinetics Understand the effect of Km on enzyme action Factors that affect Rate of Enzyme Action 1. Temperature 2. pH 3. Time 4. Concentration of coenzymes 5. Concentration of ion activators 6. Enzyme concentration 7. Substrate concentration 8. Inhibitors 1- Effects of temperature - Rate of reaction increases gradually with the rise in temperature until reach a maximum at a certain temperature, called optimum temperature - Optimum temperature : Temperature at which maximum amount of substrate is converted to products - The optimum temperature is between 37- 40 °C in humans  Optimum temperature reaction rate What’s happening here?! 37° temperature Effect of temperature on reaction rate is due to: 1 Increase of temperature increase the initial energy of substrate and thus decrease the activation energy 2 Increase of collision of molecules. * After the optimum temperature, the rate of reaction decreases due to denaturation of the enzyme (60-65 °C). 2- Effect of PH - Each enzyme has an optimum PH at which its activity is maximal e.g.: Optimum PH of pepsin = 1.5 - 2 Optimum PH of pancreatic enzymes = 7.5 - 8 Optimum PH of salivary amylase = 6.8 Optimum PH for most enzymes is (7.35-7.45)  pH What’s happening here?! Salivary pepsin amylase trypsin reaction rate pepsin trypsin 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH  The stomach secrete HCl into the lumen, resulting in a pH 1 - 2. This strongly acidic environment is capable of irreversibly denaturing most proteins. Pepsin, its pH optimum is approximately 1.6 and it is active in the acidic environment of the stomach. As denatured dietary proteins pass into the intestinal lumen, the pH of the gastric juice is raised above 6 by secretion of bicarbonate from the pancreas. At this higher pH, chymotrypsin and other enzymes from the pancreas can act on the denatured proteins.  Change of PH above or below optimum PH decrease rate of enzyme action due to: 1. The enzyme activity depends on the ionization state of both enzyme and substrate active site which is affected by PH. 2. Marked change in PH will cause denaturation of enzyme. 3- Effect of time: In an enzymatically catalysed reaction, the rate of reaction is decreased by time. This may be due to: 1 The decrease in substrate concentration (exhaustion of substrates). 2 The accumulation of the end products (may cause negative feedback inhibition. 3 The change in PH away from optimum PH. 4- Concentration of coenzymes: In the conjugated enzymes that need coenzymes, the increase in the coenzyme concentration will increase the reaction rate. 5- Concentration of ion activators (metals): The increase in metal ion activator will increase the reaction rate Examples of enzymes that are activated by ions: 1 Chloride ion activates salivary amylase 2 Calcium ion activates thromobokinase enzyme 6- Effect of enzyme concentration The rate of enzymatically catalysed reaction is directly proportional to the concentration of enzyme provided that there are sufficient supply of substrate & constant conditions. 7- Effect of substrate concentration -The rate of reaction increases as the substrate concentration increases up to certain point at which the reaction rate is maximal (Vmax). - At Vmax, the enzyme is completely saturated with the substrate. - Any increase in substrate concentration doesn't affect the reaction rate.  Substrate concentration reaction rate What’s happening here?! substrate concentration  Enzyme kinetics The study of the rate at which an enzyme acts The Mechaelis-Menten equation Proposed that in an enzyme-catalysed reaction, the enzyme combines with its substrate to form an enzyme-substrate complex (EnzS), which then breaks down either to enzyme and substrate or to enzyme and product as follows: Mechaelis and Menten developed an equation that expresses the initial velocity of a reaction as (Vi) , the Vmax is the maximal velocity, [S] is the substrate concentration and a constant is defined as Michaelis constant (Km) equal to [ (K-1+K2)/ K1], The equation state that:  At substrate concentration at which S = Km, the equation will be: Km is the substrate concentration that produces half maximal velocity. Michaelis-Menten plot Describes the dependence of reaction velocity on substrate concentration Studied by plotting the velocity (V) on the Y axis and the substrate concentration [S] on the X axis ▪ It is hard to draw and hard to determine Vmax and Km precisely  Michaelis constant (Km) - It is the substrate concentration that produces half maximum velocity of enzyme  Enzymes with low Km: have high affinity to the substrate i.e. they act at maximal velocity at low substrate concentration e.g. Hexokinase acts on glucose at low concentration (fasting state) Enzymes with high Km: they have low affinity to the substrate i.e. they act at maximal velocity at high substrate concentration e.g. Glucokinase enzyme acts on glucose at high concentration (fed state)  Lineweaver-Burk plot (double-reciprocal) In practice, it is very difficult to assess Vmax accurately from direct plots of Vi versus [S] since the value of Vmax is almost underestimated. A better method for determining the values of Vmax and Km, is using the reciprocal of the Michaelis-Menten equation The curve is linear, easy to draw, more precise to determine Vmax and Km ▪ Vmax: the intercept on the Y axis. ▪ Km: the intercept on the X axis. 8- Enzymes inhibitor: Presence of enzyme inhibitor decreases or stops the enzyme activity. Enzyme inhibitors may be: 1- Competitive inhibitors. 2- Non-competitive inhibitors. Enzyme + Substrate E-S complex Product + E Theories of enzyme action: Lock & key, induced fit models. Summary Factors affecting rate of enzyme action: enz. Conc., Substrate & conc., temp., PH, coenz. Conc., ion activator conc., time, Wrap up inhibitors. Enzyme kinetics study the rate at which an enzyme acts: Michaelis-Menten plot, Lineweaver-Burk plot.  References Vasudevan’s Textbook of Biochemistry For Medical Students, 7th Edition. Chatterjea’s Textbook of Medical Biochemistry, 8th edition. BRS Biochemistry, Molecular Biology, and Genetics, 5th Edition.