Lecture-6 Energy, Enzymes, and Metabolism.ppt
- 1. Energy, Enzymes, and Metabolism NADEEM AHMED Ph.D
- 2. Energy, Enzymes, and Metabolism • What Physical Principles Underlie Biological Energy Transformations? • What Is the Role of ATP in Biochemical Energetics? • What Are Enzymes? • How Do Enzymes Work? • How Are Enzyme Activities Regulated?
- 3. What Physical Principles Underlie Biological Energy Transformations? The transformation of energy is a hallmark of life. Energy is the capacity to do work, or the capacity for change. Energy transformations are linked to chemical transformations in cells.
- 4. 8.1 What Physical Principles Underlie Biological Energy Transformations? All forms of energy can be placed in two categories: • Potential energy is stored energy—as chemical bonds, concentration gradient, charge imbalance, etc. • Kinetic energy is the energy of movement
- 5. What Physical Principles Underlie Biological Energy Transformations? The laws of thermodynamics (thermo, “energy”; dynamics, “change”) apply to all matter and all energy transformations in the universe. They help us to understand how cells harvest and transform energy to sustain life.
- 6. What Physical Principles Underlie Biological Energy Transformations? First law of thermodynamics: Energy is neither created nor destroyed. When energy is converted from one form to another, the total energy before and after the conversion is the same.
- 7. What Physical Principles Underlie Biological Energy Transformations? Second law of thermodynamics: When energy is converted from one form to another, some of that energy becomes unavailable to do work. No energy transformation is 100 percent efficient.
- 8. Figure The Laws of Thermodynamics
- 9. What Physical Principles Underlie Biological Energy Transformations? Entropy is a measure of the disorder in a system. It takes energy to impose order on a system. Unless energy is applied to a system, it will be randomly arranged or disordered.
- 10. What Physical Principles Underlie Biological Energy Transformations? In any system: Total energy = usable energy + unusable energy enthalpy (H) = free energy (G) + entropy (S) or H = G + TS (T = absolute temperature) G = H – TS
- 11. What Physical Principles Underlie Biological Energy Transformations? Change in energy can be measured in calories or joules. Change in free energy (ΔG) in a reaction is the difference in free energy of the products and the reactants.
- 12. What Physical Principles Underlie Biological Energy Transformations? ΔG = ΔH – TΔS • If ΔG is negative, free energy is released • If ΔG is positive, free energy is consumed If free energy is not available, the reaction does not occur.
- 13. What Physical Principles Underlie Biological Energy Transformations? Magnitude of ΔG depends on: • ΔH—total energy added (ΔH > 0) or released (ΔH < 0) • ΔS—change in entropy. Large changes in entropy make ΔG more negative
- 14. What Physical Principles Underlie Biological Energy Transformations? If a chemical reaction increases entropy, the products will be more disordered. Example: In hydrolysis of a protein into its component amino acids, ΔS is positive.
- 15. What Physical Principles Underlie Biological Energy Transformations? Second law of thermodynamics: Disorder tends to increase because of energy transformations. Living organisms must have a constant supply of energy to maintain order.
- 16. What Physical Principles Underlie Biological Energy Transformations? Metabolism: Sum total of all chemical reactions in an organism. Anabolic reactions: Complex molecules are made from simple molecules; energy input is required. Catabolic reactions: Complex molecules are broken down to simpler ones and energy is released.
- 17. What Physical Principles Underlie Biological Energy Transformations? Exergonic reactions release free energy (–ΔG): Catabolism; complexity decreases (generates disorder). Endergonic reactions consume free energy (+ΔG): anabolism; complexity (order) increases.
- 18. Figure Exergonic and Endergonic Reactions
- 19. Figure 8.3 Exergonic and Endergonic Reactions (Part 2)
- 20. 8.1 What Physical Principles Underlie Biological Energy Transformations? In principle, chemical reactions can run in both directions. At chemical equilibrium, ΔG = 0 Forward and reverse reactions are balanced. The concentrations of A and B determine which direction will be favored. B A
- 21. What Physical Principles Underlie Biological Energy Transformations? Every reaction has a specific equilibrium point. ΔG is related to the point of equilibrium: The further towards completion the point of equilibrium is, the more free energy is released. ΔG values near zero are characteristic of readily reversible reactions.
- 22. Figure 8.4 Chemical Reactions Run to Equilibrium
- 23. 8.2 What Is the Role of ATP in Biochemical Energetics? ATP (adenosine triphosphate) captures and transfers free energy. ATP releases a large amount of energy when hydrolyzed. ATP can phosphorylate, or donate phosphate groups to other molecules.
- 24. 8.2 What Is the Role of ATP in Biochemical Energetics? ATP is a nucleotide. Hydrolysis of ATP yields free energy. ΔG = –7.3 to –14 kcal/mol (exergonic) energy free P ADP O H ATP i 2
- 25. Figure 8.5 ATP (Part 1)
- 26. 8.2 What Is the Role of ATP in Biochemical Energetics? Bioluminescence is an endergonic reaction driven by ATP hydrolysis: light PP AMP in oxylucifer ATP O luciferin i luciferase 2
- 27. Figure 8.5 ATP (Part 2)
- 28. What Is the Role of ATP in Biochemical Energetics? The formation of ATP is endergonic: Formation and hydrolysis of ATP couples exergonic and endergonic reactions. O H ATP energy free P ADP i 2
- 29. Energy Coupling • Living organisms have the ability to couple exergonic and endergonic reactions: • Energy released by exergonic reactions is captured and used to make ATP from ADP and Pi • ATP can be broken back down to ADP and Pi, releasing energy to power the cell’s endergonic reactions.
- 30. Figure Coupling of Reactions Exergonic and endergonic reactions are coupled.
- 31. Activation Energy • All reactions, both endergonic and exergonic, require an input of energy to get started. This energy is called activation energy • The activation energy, EA Is the initial amount of energy needed to start a chemical reaction Activation energy is needed to bring the reactants close together and weaken existing bonds to initiate a chemical reaction. Is often supplied in the form of heat from the surroundings in a system. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Free energy Progress of the reaction ∆G < O EA A B C D Reactants A C D B Transition state A B C D Products
- 32. What Are Enzymes? Catalysts speed up the rate of a reaction. The catalyst is not altered by the reactions. Most biological catalysts are enzymes (proteins) that act as a framework in which reactions can take place.
- 33. What Are Enzymes? Some reactions are slow because of an energy barrier—the amount of energy required to start the reaction, called activation energy (Ea).
- 34. Figure Activation Energy Initiates Reactions (Part 1)
- 35. Figure Activation Energy Initiates Reactions (Part 2)
- 36. What Are Enzymes? Activation energy changes the reactants into unstable forms with higher free energy—transition state intermediates. Activation energy can come from heating the system—the reactants have more kinetic energy. Enzymes and ribozymes lower the energy barrier by bringing the reactants together.
- 37. What Are Enzymes? Biological catalysts (enzymes and ribozymes) are highly specific. Reactants are called substrates. Substrate molecules bind to the active site of the enzyme. The three-dimensional shape of the enzyme determines the specificity.
- 38. Figure Enzyme and Substrate
- 39. What Are Enzymes? The enzyme-substrate complex (ES) is held together by hydrogen bonds, electrical attraction, or covalent bonds. E + S → ES → E + P The enzyme may change when bound to the substrate, but returns to its original form.
- 40. What Are Enzymes? Enzymes lower the energy barrier for reactions. The final equilibrium doesn’t change, and ΔG doesn’t change.
- 41. Figure Enzymes Lower the Energy Barrier
- 42. How Do Enzymes Work? In catalyzing a reaction, an enzyme may use one or more mechanisms.
- 43. Figure Life at the Active Site (A) Enzymes orient substrate molecules, bringing together the atoms that will bond.
- 44. Figure Life at the Active Site (B) Enzymes can stretch the bonds in substrate molecules, making them unstable.
- 45. Figure Life at the Active Site (C) Enzymes can temporarily add chemical groups to substrates.
- 46. How Do Enzymes Work? Acid-base catalysis: Enzyme side chains transfer H+ to or from the substrate, causing a covalent bond to break. Covalent catalysis: A functional group in a side chain bonds covalently with the substrate. Metal ion catalysis: Metals on side chains loose or gain electrons.
- 47. How Do Enzymes Work? Shape of enzyme active site allows a specific substrate to fit (lock and key). Binding of substrate to the active site depends on hydrogen bonds, attraction and repulsion of electrically charged groups, and hydrophobic interactions. Many enzymes change shape when they bind to the substrate—induced fit.
- 48. How Do Enzymes Work? Some enzymes require “partners”: • Prosthetic groups: Non-amino acid groups bound to enzymes • Cofactors: Inorganic ions • Coenzymes: Small carbon-containing molecules; not bound permanently to enzymes
- 50. How Do Enzymes Work? The rate of a catalyzed reaction depends on substrate concentration. Concentration of an enzyme is usually much lower than concentration of a substrate. At saturation, all enzyme is bound to substrate—maximum rate.
- 51. Figure Catalyzed Reactions Reach a Maximum Rate
- 52. How Do Enzymes Work? Maximum rate is used to calculate enzyme efficiency: Molecules of substrate converted to product per unit time (turnover). Ranges from 1 to 40 million molecules per second!
- 53. How Are Enzyme Activities Regulated? Inhibitors regulate enzymes: Molecules that bind to the enzyme and slow reaction rates. Naturally occurring inhibitors regulate metabolism.
- 54. How Are Enzyme Activities Regulated? Irreversible inhibition: Inhibitor covalently bonds to side chains in the active site—permanently inactivates the enzyme. Example: DIPF or nerve gas Diisopropyl fluorophosphate
- 56. How Are Enzyme Activities Regulated? Reversible inhibition: Inhibitor bonds noncovalently to the active site and prevents substrate from binding. Competitive inhibitors compete with the natural substrate for binding sites. When concentration of competitive inhibitor is reduced, it detaches from the active site.
- 57. Figure Reversible Inhibition (A)
- 58. 8.5 How Are Enzyme Activities Regulated? Noncompetitive inhibitors: Bind to the enzyme at a different site (not the active site). The enzyme changes shape and alters the active site.
- 59. Figure Reversible Inhibition (B)
- 60. How Are Enzyme Activities Regulated? Allostery (allo, “different”; stereos, “shape”) Some enzymes exist in more than one shape: • Active form—can bind substrate • Inactive form—cannot bind substrate but can bind an inhibitor
- 61. How Are Enzyme Activities Regulated? Most allosteric enzymes are proteins with quaternary structure. Active site is on the catalytic subunit. Inhibitors and activators bind to the regulatory subunits.
- 62. Figure Allosteric Regulation of Enzymes
- 63. How Are Enzyme Activities Regulated? Within a certain range, reaction rates of allosteric enzymes are sensitive to small changes in substrate concentration.
- 64. Figure 8.18 Allostery and Reaction Rate
- 65. How Are Enzyme Activities Regulated? Allosteric enzymes are very sensitive to low concentrations of inhibitors, and are important in regulating metabolic pathways.
- 66. How Are Enzyme Activities Regulated? Metabolic pathways: The first reaction is the commitment step—other reactions then happen in sequence. Feedback inhibition (end-product inhibition): The final product acts as a noncompetitive inhibitor of the first enzyme, which shuts down the pathway.
- 67. Figure 8.19 Feedback Inhibition of Metabolic Pathways
- 68. 8.5 How Are Enzyme Activities Regulated? Every enzyme is most active at a particular pH. pH influences the ionization of functional groups. Example: at low pH (high H+) —COO– may react with H+ to form —COOH which is no longer charged; this affects folding and thus enzyme function.
- 69. Figure 8.20 pH Affects Enzyme Activity
- 70. 8.5 How Are Enzyme Activities Regulated? Every enzyme has an optimal temperature. At high temperatures, noncovalent bonds begin to break. Enzyme can lose its tertiary structure and become denatured.
- 71. Figure 8.21 Temperature Affects Enzyme Activity
- 72. 8.5 How Are Enzyme Activities Regulated? Isozymes: Enzymes that catalyze the same reaction but have different properties, such as optimal temperature. Organisms can use isozymes to adjust to temperature changes. Enzymes in humans have higher optimal temperature than enzymes in most bacteria—a fever can denature the bacterial enzymes.