Enzymes

1. Enzymes UNIT I: Protein Structure and Function

2. Overview • Virtually all reactions in body mediated by enzymes, which are protein catalysts that increase rate of reactions without being changed • Enzymes direct all metabolic events

3. Nomenclature • Each enzyme has two names: • Short, recommended name, convenient for use • More complete, systematic name used when enz must be identified without ambiguity A. Recommended name • Most commonly used enz’s end with “-ase” attached to substrate (e.g., glucosidase, sucrase, urease), or to description of action performed (e.g., lactate dehydrogenase, adenylyl cyclase) • Some enz’s retain original trivial names e.g., trypsin, pepsin

4. B. Systematic name • The international union of biochem & mol biol (IUBMB) set a system in which enzymes are divided into 6 major classes, each with numerous subgroups • Suffix –ase is attached to description of chemical reaction catalyzed e.g., D-glyceraldehyde 3-phosphate: NAD oxidoreductase. • IUBMB names unambiguous & informative, but cumbersome in general use

6. Figure 5.1. Examples of the six major classes of the international classification of enzymes (THF is tetrahydrofolate).

7. III. Properties of enzymes • Enzymes are protein catalysts, increase velocity of a chemical reaction, and not consumed • Some types of RNA can act like enzymes, usually catalyzing cleavage & synthesis of phosphodiester bonds. RNAs with catalytic activity = ribozymes, less common than protein catalysts • Active sites • A special pocket, contains aa side chains that create a 3D surface complementary to S • Active site binds S  ES complex that is converted to EP  dissociates to E + P

8. Figure 5.2. Schematic representation of an enzyme with one active site binding a substrate molecule.

9. B. Catalytic efficiency - Most E catalyzed reactions are highly efficient, 103-108 x faster than uncatalyzed. • Typically an E molecule  transforms ~ 100-1000 S molecules  P each second • Number of S molecules  P per sec is “turnover number” C. Specificity - E’s are highly specific, interacting with one or few S & catalyze only one type of chemical reaction

10. D. Cofactors - Some E’s associate with a non-protein cofactor for activity • e.g., metal ions (Zn2+ or Fe2+), and organic molecules a.k.a co-enzymes, that are often derivatives of vitamins e.g., NAD+ contains niacin, FAD contains riboflavin, coenz A contain pantothenic acid • Holoenzymes = E + its cofactor • Apoenzyme = protein portion of the holoenzyme • In absence of appropriate cofactor, apoenz. typically show no biologic activity - Prosthetic group = tightly bound coenz that does not dissociate from enz (e.g., biotin bound to carboxylase)

11. E. Regulation - E activity can be regulated i.e., E can be activated or inhibited, i.e., rate of P formation responds to needs of cell F. Location within the cell - Many E’s localized in specific organelles - Compartmentalization isolates reaction S or P from other competing reactions. This provides favorable environ for reaction, & organizes the 1000’s of E’s in a cell into purposeful pathways

12. Figure 5.3. The intracellular location of some important biochemical pathways.

13. How enzymes work Mechanism of E action can be viewed from 2 different perspectives: - Catalysis in terms of energy changes that occur during reaction, i.e., E’s provide an alternate, energetically favorable reaction pathway different from uncatalyzed one - How active site chemically facilitates catalysis

14. A. Energy changes occurring during the reaction - All chemical reactions have an energy barrier separating reactants and products = free energy of activation  energy difference b/w reactants and high energy intermediate that occurs during formation of product A ↔ T* ↔ B - T* is the transition state = high energy intermediate

15. Figure 5.4 Effect of an enzyme on the activation energy of a reaction

16. Free energy of activation: • - Difference in free energy b/w reactant and T*, because of high free energy of activation, rates of uncatalyzed chemical reactions are often slow • 2. Rate of reaction: • For molecules to react, they must contain sufficient energy to overcome energy barrier of transition state • In absence of E, only a small proportion of molecules may possess enough energy to achieve T*. Rate of reaction is determined by number of energized molecules • The lower free energy to pass through T*, & the faster the reaction.

17. 3. Alternate reaction pathway: - An E allows a reaction to proceed under conditions prevailing in cell by providing a pathway with a lower free energy of activation -E does not change free energies of R’s or P’s, and so does not change equilibrium of reaction

18. B. Chemistry of active site - Active site is a complex molecular machine employing a diversity of chemical mechanisms to facilitate R  P. A number of factors responsible for catalytic efficiency of E’s e.g., 1. Transition state stabilization: active site often acts as a flexible molecular template that binds S in a geometric structure resembling activated T*. By stabilizing S in T*, the E greatly increases the conc. of reactive intermediates that can be converted to P, thus, accelerates reaction

19. 2. Other mechanisms: active site can provide catalytic groups that enhance probability that T* is formed. - In some E’s, groups can participate in general acid-base catalysis e.g., aa residues provide or accept protons - In other E’s, catalysis may involve transient formation of covalent enzyme-substrate complex

20. Enzymes that form covalent intermediates

21. 3. Visualization of the transition state Figure 5.5. Schematic representation of energy changes accompanying formation of enzyme-substrate complex and subsequent formation of a transition-state complex.

22. V. Factors affecting reaction velocity • E’s can be isolated and their properties studied in vitro. Different E’s show different responses to [S], Temp, pH A. Substrate concentration 1. Maximal velocity: rate or velocity of a reaction (v) is the # S molecules  P per unit time; it is usually expressed as μmol of P formed per minute. Rate of E-catalyzed reaction increases with S conc. until a maximal velocity (Vmax) is reached. Leveling off of reaction rate at high [S] reflects saturation with S of all available binding sites on E molecules present

23. 2. Hyperbolic shape of the enzyme kinetics curve - Most E’s show Michaelis-Menten kinetics, in which plot of initial velocity, v0, against [S] is hyperbolic - In contrast, allosteric E’s frequently show sigmoidal curve

24. Figure 5.6. Effect of substrate conc. on reaction velocity.

25. B. Temperature • Increase of velocity with temperature. As a result of increased # of molecules having sufficient energy to pass over energy barrier and form P’s • Decrease of velocity with higher temperatures. As a result of temp-induced denaturation of E.

26. Figure 5.7 Effect of temperature on an enzyme-catalyzed reaction.

27. C. pH • Effect of pH on ionization of active site: conc. of H+ affects reaction velocity in several ways. 1st, catalytic process usually requires E and S have specific chemical groups in ionized or unionized state in order to interact e.g., amino group of E be in protonated form (-NH3+). At alkaline pH, this group is deprotonated and rate of reaction declines • Effect of pH on E denaturation. Extremes of pH can  denaturation, because structure of catalytically active protein molecule depends on ionic character of aa side chains

28. 3. The pH optimum varies for different enzymes: the pH at which maximal E activity is achieved is different for different E’s, & often reflects [H+] at which E functions in body e.g., pepsin, a digestive E in stomach, is maximally active at pH 2, whereas other E’s, designed to work at neutral pH are denatured by such an acidic environ

29. Figure 5.8 Effect of pH on enzyme-catalyzed reactions.

30. VI. Michaelis-Menten equation • Reaction model - E reversibly combines with S to form ES complex that subsequently breaks down to P, regenerating free E. The model, involving one S molecule: E + S ↔ ES → E + P B. Michaelis-Menten equation - Describes how reaction velocity varies with [S] k1 k2 K-1

31. Vmax [S] v0 = Km + [S] V0 = initial velocity Vmax = maximal velocity Km = Michaelis constant = (k-1 + k2)/k1 [S] = substrate conc • Assumptions made in deriving Michaelis-Menten eq: • Relative concentrations of E and S: [S] is much greater than [E], so % of total S bound by E at any one time is small • Steady-state assumption: [ES] does not change with time i.e., rate of formation of ES = breakdown of ES (to E + S & to E + P) • Initial velocity: only v0’s are used in analysis of E reactions. i.e., rate of reaction is measured as soon as E and S are mixed. At that time conc of P is very small and so, rate of back reaction (P  S) can be ignored

32. C. Important conclusions about Michaelis-Menten kinetics • Characteristics of Km: - Km is characteristic of an E and its particular S, and reflects affinity of E for that S. - Km is numerically = [S] at which reaction velocity is ½ Vmax. Km does not vary with conc of E • Small Km: reflects a high affinity of E for S, as low conc of S is needed to half-saturate the E- i.e., reach a velocity that is ½ Vmax • Large Km: reflects a low affinity of E for S. As high [S] is needed to half saturate the E.

33. Figure 5.9 Effect of substrate concentration on reaction velocities for two enzymes: enzyme 1 with a small Km, and enzyme 2 with a large Km.

34. 2. Relationship of velocity to enzyme concentration - Rate of reaction α [E] at all S conc’s. e.g., if [E] is halved, initial rate of reaction (v0), & that of Vmax, are reduced to ½ that of the original. • Order of reaction: • When [S] is much less than Km, velocity of reaction is ~ proportional to [S]. Rate of reaction is said to be 1st order wrt S. • When [S] is much greater than Km, velocity is constant and = Vmax. Rate of reaction is then independent of [S], and is said to be zero order wrt [S].

35. Figure 5.10 Effect of substrate concentration on reaction velocity for an enzyme catalyzed reaction.

36. D. Lineweaver-Burke plot • When v0 is plotted against S, it is not always possible to determine when Vmax has been achieved because of the gradual upward slope of the hyperbolic curve at high [S] • If 1/v0 is plotted vs. 1/[S], a straight line is obtained. This plot = Lineweaver-Burke plot (a.k.a double-reciprocal plot) can be used to calculate Km & Vmax, as well as to determine mechanism of action of E inhibitors

37. Km = 1 1/v0 + Vmax [S] Vmax 1. The eq. describing Lineweaver-Burke plot is: - Where intercept on x-axis = -1/Km, & intercept on y-axis = 1/Vmax

38. Figure 5.11. Lineweaver-Burke plot.

39. VII. Inhibition of enzyme activity • Any S that can diminish velocity of E catalyzed reaction is inhibitor (I) • Reversible inhibitors bind to E through non-covalent bonds. Dilution of E-I complex results in dissociation of reversibly bound I, and recovery of E activity • Irreversible inhibition occurs when an inhibited E does not regain activity on dilution of E-I complex • Commonly encountered types: • Competitive • Non-competitive

40. A. Competitive inhibition • I binds reversibly to same site of S, i.e., competes with S for that site • Effect on Vmax: effect of competitive I is reversed by increasing [S]. At sufficiently high [S], reaction velocity reaches Vmax observed in absence of I. • Effect on Km: competitive I increases apparent Km for a given S. i.e., in presence of competitive I, more S is needed to achieve ½ Vmax • Effect on Lineweaver-Burke plot: plots of inhbited & uninhibited reactions intersect on y-axis at 1/Vmax (Vmax is unchanged). Inhibited and uninhibited reactions show different x-axis intercepts i.e., apparent Km is increased in presence of competitive I.

41. Figure 5.12. A. Effect of a competitive inhibitor on the reaction velocity (vo) versus substrate [S] plot. B. Lineweaver-Burke plot of competitive inhibition of an enzyme.

42. 4. Statin drugs as examples of competitive inhibitors: • This group of antihyperlipidemic agents competitively inhibits 1st committed step in cholesterol synthesis • This reaction is catalyzed by hydroxymethylglutaryl CoA reductase (HMG CoA reductase) • Statin drugs e.g., atorvastatin (Lipitor) & simvastatin (Zocor) are structural analogs of natural S for this E, & compete effectively to inhibit HMG CoA reductase  inhibit de novo cholesterol synthesis, thereby lowering plasma cholesterol levels.

43. Figure 5.13. Lovastatin competes with HMG CoA for the active site of HMG CoA reductase.

44. B. Non-competitive inhibition • Occurs when I and S bind at different sites on E. The non-competitive I can bind either free E or ES complex, thereby preventing reaction from occurring • Effect on Vmax: non-competitive inhibition can not be overcome by increasing conc of S, i.e., non-competitive inhibition decreases Vmax • Effect on Km: non-competitive I’s do not interfere with binding of S to E. So, E shows same Km in presence or absence of the non-competitive inhibitor

45. 3. Effect on Lineweaver-Burke plot: non-competitive inhibition is differentiated by noting Vmax decrease, whereas Km is unchanged in presence of non-competitive inhibitor 4. Examples of non-competitive inhibitors: some I’s act by forming covalent bonds with specific groups of E’s. e.g., lead forms covalent bonds with sulfhydryl side chains of cysteine in proteins. Ferrochelatase catalyzes insertion of Fe2+ into protoporphyrin (a precursor of heme) is sensitive to inhibition by lead. Other e.g.’s are certain insecticides whose neurotoxic effects result from their irreversible binding at catalytic site of acetylcholiesterase (that cleaves the neurotransmitter acetylcholine)

46. Figure 5.14. A. Effect of a noncompetitive inhibitor on the reaction velocity (vo) versus substrate [S] plot. B. Lineweaver-Burke plot of noncompetitive inhibition of an enzyme.

47. C. Enzyme inhibitors as drugs • E.g., the widely prescribed ß-lactam antibiotics e.g., penicillin & amoxycillin inhibit enzymes involved in bacterial CW synthesis • Drugs may also act by inhibiting extracellular reactions e.g., angiotensin-converting enzyme (ACE) inhibitors. They lower blood pressure by blocking the E that cleaves angiotensin I to form the potent vasoconstrictor, angiotensin II. These drugs, e.g., captopril, enalapril, lisinopril, cause vasodilation & so reduction in blood pressure

48. Figure 5.15 A noncompetitive inhibitor binding to both free enzyme and enzyme-substrate complex.

49. VIII. Regulation of enzyme activity • Regulation of reaction velocity of E’s is essential to coordinate numerous metabolic processes • Rate of most E’s responsive to changes in [S], as intracellular level of many S’s is in range of Km. an increase in [S]  increase in reaction rate  return [S] to normal. • Some E’s with specialized regulatory functions respond to allosteric effectors or covalent modification, or show altered rates of E synthesis when physiologic conditions are changed

50. A. Allosteric binding sites • Allosteric E’s regulated by molecules = effectors (also modifiers), that bind non-covalently at a site other than active site • These E’s are composed of multiple subunits, & regulatory site may be present on a subunit that is not itself catalytic • Presence of allosteric effector can alter affinity of E to its S, or modify maximal catalytic activity of E, or both • Effectors that inhibit E activity = negative effectors, that increase E activity = positive effectors • Allosteric E’s usually contain subunits and frequently catalyze the committed step early in a pathway