Biochemistry Enzyms lecture aseel a.pptx

Enzyms

1. بسم الله الرحمن الرحيم والصلاة والسلام على أشرف الأنبياء و المرسلين سيدنا محمد صلى الله عليه وسلم

2. Biochemistry CHEM 250 Chapter 8 Lecture 8

3. Introduction Virtually all reactions in the body are mediated by enzymes, thus enzymes direct all metabolic events. Enzymes are biological catalysts produced by living cells. They can work outside these cells. Virtually all enzymes are proteins , however, a few examples of RNA as show enzymatic activity; they are called ribozymes . Enzymes accelerate chemical reactions without changing the equilibrium point. They function in minute amounts and remain unchanged chemically during the reaction. They are highly specific in their action. They are very sensitive to slight changes in temperature and pH.

4. The turnover number (catalytic efficiency): Most enzyme-catalyzed reactions are highly efficient, proceeding from 10 3 to 10 8 times faster than uncatalyzed reactions. Each enzyme molecule is capable of transforming 100 to 1000 substrate molecules into product each second. “The number of molecules of substrate converted to product per enzyme molecule per second is called the turnover number”

5. Chemical nature of enzymes: Virtually all enzymes are proteins; some are simple proteins e.g. pepsin while others are conjugated proteins . In the latter type the enzyme consists of a protein part , the apoenzyme and a non-protein part . Both of which form together the “ holoen zyme ”. 1 -Coenzyme is the organic nonprotein part of the holoenzyme . It is loosely attached to the apoenzyme ; when it is firmly attached it is called prosthetic group. Coenzymes function as group transfer agents. They temporarily carry part of the reactants or products during the reaction.

6. 2-Activator is an inorganic non-protein part of the holoenzyme that is loosely attached to the apoenzyme . When it is firmly attached it is called prosthetic g roup. Inorganic metal ions are required for the activity of more than 25% of enzymes. Holoenzyme Activator (inorganic) Protein part (apoenzyme) Non protein part Coenzyme (organic)

7. Enzyme terminology : 1-Hydrolyzing enzymes are usually named by adding the suffix - ase to the name of the substrate e.g. amylase, lactase and lipase which hydrolyze starch, lactose and triacyiglycerol respectively. 2-Other enzymes are named by adding their function to the name of the substrate. Thus lactate dehydrogenase removes hydrogen from lactic acid and alanine transaminase transfers amino group from alanine. 3-Some enzymes retain their original name , which give no hint of the associated enzymatic reaction e.g. pepsin and trypsin .

8. CLASSIFICATION OF ENZYMES According to the type of chemical reaction catalyzed, enzymes are classified into 6 major classes: 1. Oxidoreductases : oxidation-reduction reactions 2. Transferases : transfer of functional groups 3. Hydrolases : hydrolysis reactions (cleavage and introduction of water) 4. Lyases : group elimination to form double bond 5. Isomerases : isomerization ( intramolecular rearrangements 6. Ligases (synthases ): bond formation coupled with ATP hydrolysis

9. MECHANISM OF ENZYME ACTION (HOW ENZYMES WORK). Enzymes can only catalyze thermodynamically possible reactions. For such reactions to proceed a certain amount of energy is required called “the energy of activation”. Enzymes speed up chemical reactions by lowering the energy required for activation , thus more substrate molecules are sufficiently activated for the reaction to proceed.

10. Free energy of activation Progress of reaction Transition state T* Free energy of activation (uncatalyzed) Free energy A Reactants (initial state) Free energy of activation (catalyzed) Δ G Product (final state) B This peak of energy represents the transition state in which a high- energy intermediate is formed during the conversion of reactant to product. Because of the large activation energy, the rate of uncatalyzed chemical reactions is often slow. There is no difference In the free energy of the overall reaction (energy of reactants minus energy of products) between the catalyzed and uncatalyzed reactions.

11. Substrate Enzyme Products + E-S complex Enzyme +

12. It was formally suggested that the catalytic site is a rigid area on the enzyme surface and that the substrate binds to the enzyme like “a key into its lock”. Now there is evidence that the catalytic site is a flexible area . The presence of the substrate induces a conformational change in the enzyme that allows the binding of the substrate and catalysis to occur. This means that we get “induced fit” ينطبق على شكل ما of the substrate on the surface of the enzyme.

13. SPECIFICITY OF ENZYMES. Enzymes are highly specific in their action, 1-interacting with one or a few specific substrates and 2-catalyzing only one type of chemical reactions. The specificity of enzymes is due to the nature and arrangement of the chemical groups at the catalytic site. This allows the enzyme to unite and activate only one substrate or a small number of structurally related substrates

14. FACTORS AFFECTING THE RATE OF ENZYME-CATALYZED REACTIONS. The rate of enzyme-catalyzed reactions is directly proportional to the concentration of the enzyme-substrate complex. Thus, it is affected by the 1- concentration of the substrate, 2- enzyme, cofactors, 3-inhibitors 4- Temperature, 5-pH, and other factors also affect enzyme activity. In studying the effect of any of these factors, only one factor has to be varied at a time, all other factors being kept constant. - The rate of the reaction should always be measured at the very beginning of the reaction, the so-called initial velocity ( vo ).

15. I- Concentration of the substrate: At low substrate concentraton , only a few enzyme molecules are in the form of E-S complex and the velocity of the reaction is low. 2- As the substrate concentration is increased more enzyme molecules are involved in the formation of E-S complex and the velocity of the reaction increases . This is true up to the point when all enzyme molecules are in the form of E-S complex at which the maximal rate of the reaction Vmax is reached. 3- Any further increase in the substrate concentration will not increase the velocity of the reaction , the enzyme concentration is said to be the limiting factor .

16. Ill- Concentration of cofactors (C): If the enzyme requires a cofactor (coenzyme or activator) for its activity the velocity of the reaction will be directly proportional to the concentration of the cofactor . This is true up to the point when each enzyme molecule is associated with the cofactor required. At this point any increase in cofactor concentration will not increase the velocity of the reaction, the enzyme concentration is the limiting factor.

17. 1-At 0 °C , enzyme action virtually stops due to the inhibition of movement and collision between the substrate and enzyme molecules. 2-As the temperature rises the velocity of the reaction increases due to increased kinetic energy of the molecules and increased collision between substrate and enzyme molecules, increasing the formation of E-S complex. 3- the increase in the velocity of the reaction continue up to a point, “the optimum temperature” , beyond which any 4-further increase in temperature causes a decrease in the reaction rate . IV- Temperature:

18. For most enzymes, the activity virtually stops at about 70 °C , due to denaturation الافساد البروتينى of the enzyme protein, disrupting the organization of the catalytic site. The optimum temperature for most animal enzymes is about 37°C , while that of most plant enzymes is about 50 °C. 70 °C V Optimum temperature

19. V- pH 1-Each enzyme has an optimum pH at which it shows maximal activity. 2-Activity decreases as we go away from the optimum pH, 3-it virtually stops about 2 units of pH above or below this pH. Most enzymes have an optimum pH between 5 and 9 . Slight changes in pH causes marked changes in enzyme activity due to alteration of the charges on the substrate and on the catalytic site of the enzyme. - These alterations may affect chemical groups on the enzyme responsible for substrate binding, leading to changes in the affinity between the enzyme and the substrate

20. In addition, extremes of pH causes denaturation and irreversible inhibition of enzyme action. pH V Optimum pH

21. Any substance that can diminish the velocity of an enzyme-catalyzed reaction is called an inhibitor . The two most commonly encountered types of inhibition are 1-competitive تثبيط تنافسي and 2-noncompetitive. تثبيط لا تنافسي ENZYME INHIBITORS مثبطات الانزيم

22. A. Competitive inhibitors: تثبيط تنافسي This type of inhibition is due to 1- the structural similarity between the inhibitor (I) and the substrate. This allows the inhibitor to unite reversibly with the catalytic site of the enzyme, forming an enzyme-inhibitor complex (El complex ). Thus, Both the substrate and the inhibitor compete with each other for the catalytic sites on the enzyme. the formation of E-S complex is decreased, inhibiting the activity of the enzyme. Competitive inhibitors are also known as “substrate analogue النظير inhibitors”

23. . 1-Thus, the degree of inhibition depends on the ratio of the concentrations of the inhibitor and the substrate, but not on the absolute concentration of each one of them. 2-At a fixed concentration of the inhibitor, if more substrate is added the degree of inhibition decreases , a fact that differentiates competitive from noncompetitive inhibitors .

24. B. Noncompetitive inhibitors: A noncompetitive inhibitor (I) has no structural similarity to the substrate and (2) does not bind to the catalytic site of the enzymes. It binds to a different area on the enzyme surface (3)and there is no competition between the substrate and the inhibitor for bi nding. Both bind to the enzyme at the same time, forming I-E-S complex. Binding of the inhibitor to enzyme decreases its catalytic power, thus decreasing V max . However, since the inhibitor does not interfere with substrate binding . The degree of inhibition is not related to the concentration of the (S)substrate but only to the concentration of the(I) inhibitor.