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2. 2  enzymes = biological catalysts  Most Enzymes are proteins*  proteins ≡ Polypeptides Substrate attaches reversibly by noncovalent, or weak bonds: Ionic interactionssalt bridges between negative and positive group of ions Hydrogen bonds electrostatic attraction between dipoles van der Waals forcesweak interactions due to transitory polarities arising from electron movements Hydrophobic/hydrophilic interactionscauses molecules to associate together. *Recently RNA was found to catalyze some reactions. Enzymes and their classification substrate  The substance that is acted on by the enzyme is called the substrate active site  Enzymes react with substrates by means of the active site It has been estimated that the typical cell contains many thousands of different kinds of enzymes.

3. The lock-and-key model for enzyme activity. 1894-Emil Fisher rigidThe active site has a rigid shape. An enzyme only binds substrates that exactly fit the active site. Only substrates with the matching shape can fit. The substrate is the key that fits that lock. 3 The induced-fit model for enzyme activity. 1958-Daniel Koshland flexibleEnzyme structure is flexible, not rigid. Enzyme and substrate adjust the shape of the active site to bind the substrate. The range of substrate specificity increases. Shape changes improve catalysis during reaction.

4. Increase the rate of reaction by lowering the energy of activation: o Straining bonds in reactants to make it easier to achieve transition state o Positioning reactants together to facilitate bonding o Changing local environment o Direct participation through very temporary bonding 4  Enzymes are catalysts.

5. Enzymes may recognize and catalyze: A single substrate. A group of similar substrates. A particular type of bond. Enzyme Specificity and Efficiency 5

6. 6 Enzyme Catalyzed Reaction Mechanism

7. 7 Enzymes are divided into six major classes with subgroups and sub-subgroups to define their function more precisely.

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9. ase In most cases, enzyme names end in – ase Some are taken from the substrate Urea: remove -a, replace with -ase = urease Lactose: remove -ose, replace with -ase = lactase Amylase: substrate is amylose Others are named from the substrate and the reaction catalyzed Lactate dehydrogenase Pyruvate decarboxylase Some names are historical - no direct relationship to substrate or reaction type Pepsin Chymotrypsin Trypsin Papainfound in papaya Renninfound in milk Catalasefound in nearly all living organisms Naming Enzymes catalase catalyzes the decomposition of hydrogen peroxide to water and oxygen. 9

10. 10 o Different forms of an enzyme in which a quaternary structure is observed with slight variations in the primary structure. o Catalyze the same reaction in different tissues or different locations in cells. In some cases, the same reaction is catalyzed by different enzymes:Isoenzymes (Isozymes) ABABABABABAB E E’ E’’ E ≠ E’ ≠ E’’however, related multiple versions of the same enzyme Example: hexokinase and glucokinase G + PiG-6-PBrain and other tissues G + PiG-6-PLiver glucokinase hexokinase

11. Isozymes often have a number of subunits. Example: lactate dehydrogenase (LDH) = 4 subunits of 2 different types. o M and H represent different polypeptide subunits. o H is the main enzyme in the heart. o M is the main enzyme associated with muscle. o consists of five isoenzymes. 11

12. The deficiency or excess of particular enzymes can cause certain diseases or signal problems such as heart attacks and other organ damage. Isoenzymes concentration in blood serum can be used to identify the organ or tissue involved in damage or disease. A disease or medical condition can damage an organ and cells die, releasing their content into the blood. The blood levels of 3 enzymes are commonly assayed in MI (myocardial infraction) situations: creatine phosphokinase CPK aspartate transaminase AST lactate dehydrogenase LDH These are elevated following a heart attack and are used to determine the severity of the attack.

13. Enzymes 37°C in humans Are most active at an optimum temperature (usually 37°C in humans). Show little activity at low temperatures. Lose activity at high temperatures as denaturation occurs. Why is a high fever dangerous? Enzymes lose activity at high temperatures as denaturation occurs. Normal body T is about 37 °C (98.6 °F ) Fever classification Grade °C °F low grade38–39100.4–102.2 moderate39–40102.2–104.0 high-grade40–42104.0–107.6 hyperpyrexia>42>107.6 Factors Affecting Enzyme Activity & Inhibition 13

14. Enzymes Are most active at optimum pH. 7.4The body has an optimum pH of about 7.4. In certain organs, operate at lower and higher optimum pH values. proper chargesContain R groups of amino acids with proper charges at optimum pH. Lose activity in low or high pH as tertiary structure is disrupted. Acidosis is a serious medical condition caused by a drop in pH that, among other things, causes enzymes to denature.

15. Enzyme Concentration An increase in enzyme concentration Increases the rate of reaction (at constant substrate concentration). Binds more substrate with enzyme. An increase in substrate concentration Increases the rate of reaction (at constant enzyme concentration). Eventually saturates an enzyme with substrate to give maximum activity. Substrate Concentration 15

16. Reversible Involves noncovalent binding it can dissociate and allow the enzyme to regain activity. Irreversible Involves covalent bonding it can not dissociate and thus causing permanent inactivation. Inhibitors Are molecules that cause a loss of catalytic activity. Prevent substrates from fitting into the active sites. Medicines as well as poisons act as E inhibitors. E + SESE + P I + I I E I no P 16 competitive noncompetitive Inhibitors

17. In irreversible inhibition, a substance Bonds with R groups at the active site. Destroys enzyme activity. 17

18. Structures of selected penicillins in use today as antibiotics 18 Selective binding of penicillin to the active site of transpeptidase. Some bacteria are resistant to penicillin because they produce penicillinase, an enzyme that breaks down penicillin. Penicillin inhibits transpeptidase, an enzyme that catalyzes the formation of bacterial cell walls. It does not affect human cell membranes.

19. Structures of selected sulfa drugs in use today as antibiotics. 19 Sulfa drugs inhibit bacterial cell growth because it is structurally similar to PABA, para-aminobenzoic acid. Many bacteria need PABA in order to produce an important coenzyme, folic acid. Folic acid deficiency retards growth and eventually kills the bacteria.

20. Zymogens (proenzymes) Are inactive forms of enzymes. Are activated when one or more peptides are removed. Such as proinsulin is converted to insulin by removing a small peptide chain. Peptidases are needed to remove the peptide section that turns the inactive form into its active one. Removal of the fragment changes the tertiary structure and turns the zymogen into its enzyme. Regulation of Enzyme Activity 20

21. reaction sequence An allosteric enzyme is an enzyme in a reaction sequence that binds a regulator substance. A positive regulator when it enhances the binding of substrate and accelerates the rate of reaction. 21 Allosteric regulation In each biochemical pathway there are one or more enzymes whose catalytic activity can be modulated (increased or decreased) by the binding of a regulator. A negative regulator when it prevents the binding of the substrate to the active site and slows down the rate of reaction. Greek allostery translates to “other solid”, “other structure”. Allosteric effect occurs when the binding of a ligand results in a conformational change (other structure) in the enzyme. Such change is associated with increased or decreased enzyme activity.

22. E 1 is an allosteric enzyme: 1.An enzyme in a reaction sequence 2.Binds a regulator substance. A product can act as an inhibitor. An end product binds with the first enzyme (E 1 ) in a sequence when sufficient product is present. 22 Feedback inhibition = feedback control

23. Enzymes: Can function on their own Or need some help in order to function The needed help is provided by a Cofactors or coenzyme Cofactors or coenzyme. 23 small molecules called coenzymes Derived from vitamins Play a variety of roles that the enzyme alone cannot play. They are recycled and only small amounts are required. metal ions Act as Lewis acids Can coordinate with the enzyme’s R groups, causing them to align in an arrangement that facilitates reaction. Apoenzyme- An enzyme that requires a cofactor but does not have one bound. It is an inactive enzyme, activation of the enzyme occurs upon binding of an organic or inorganic cofactor. Holoenzyme- An apoenzyme together with its cofactor. A holoenzyme is complete and catalytically active. Enzyme Cofactors and Vitamins

24. Carboxypeptidase A cleaves the C terminal AA of a protein when that AA has a bulky hydrophobic or aromatic side chain. Zn 2+ a cofactor for carboxypeptidase A, stabilizes the carbonyl oxygen during the hydrolysis of a peptide bond. 24

25. After the water molecule loses H +, the remaining OH - remains bonded to Zn 2+. A CO 2 molecule is held in the active site close to the Zn 2+ where it can react with the OH - to form HCO 3 -, which is then released from the active site. The active forms of carbonic anhydrase requires Zn 2+ as a cofactor. carbonic anhydrase CO 2 + H 2 O HCO 3 - + H + 25 This reactions maintains the proper pH in blood and tissues.

26. A coenzyme prepares the active site for catalytic activity. An organic molecule bound to the enzyme by weak interactions / Hydrogen bonds Most coenzymes carry electrons or small groups Many have modified vitamins in their structure 26

27. Coenzymes are small organic molecules. Coenzymes generally are complex organic molecules that can not be synthesized by some organisms – mammals in particular. Nicotinamide adenine dinucleotide (NAD + ) Nicotinamide adenine dinucleotide (NAD + ) is synthesized from the vitamin niacin.

28. The Nicotinamide portion is the part of NAD+ responsible for its metabolic functions because it is capable of being reduced and thus can serve as an oxidizing agent to which two electrons and a proton can be added: A typical reaction in which NAD+ acts as oxidizing agent is the conversion of alcohols to aldehydes or ketones. Here is the conversion of ethanol to acetaldehyde by the alcohol dehydrogenase of liver: CH 3 CH 2 OH + NAD + → CH 3 CHO + NADH + H + LADH (Liver Alcohol Dehydrogenase) uses the coenzyme nicotinamide adenine dinucleotide (NAD + ).

29. Vitamins In the early 20 th century, scientists detected minute amounts of substances found to be essential in preventing disease and maintaining health. The substances were called vitamines by Casimir Funk to indicate they were vital for survival and contained nitrogen -that is, they were amines. The e was later dropped when scientists found that some were not amines. To date, scientists have identified 13 vitamins. There are 9 water soluble (8 B vitamins and vitamin C) and 4 fat soluble. Among the most common are: FatAD EK Water B CF H Folic acidBiotin All get DEposited in fatty tissue, acKumulates Be CareFul with your money H $ 29

30. 30 9 Water-Soluble Vitamins = B vitamins and vitamin C Have polar groups –OH and -COOH and soluble in aqueous solutions. Cofactors for many enzymes. Not stored in the body.

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