1. 1 Energy and Metabolism Chapter 6

2. 2 Flow of Energy Thermodynamics – Branch of chemistry concerned with energy changes Cells are governed by the laws of physics and chemistry

3. Energy – capacity to do work – 2 states 1.Kinetic – energy of motion 2.Potential – stored energy – Many forms – mechanical, heat, sound, electric current, light, or radioactivity – Heat the most convenient way of measuring energy 1 calorie = heat required to raise 1 gram of water 1ºC calorie or Calorie? 3

4. 4 a. Potential energy b. Kinetic energy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

5. Energy flows into the biological world from the sun Photosynthetic organisms capture this energy Stored as potential energy in chemical bonds 5

6. 6 Redox reactions Oxidation – Atom or molecule loses an electron Reduction – Atom or molecule gains an electron – Higher level of energy than oxidized form Oxidation-reduction reactions (redox) – Reactions always paired

7. 7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. e – AB A + B + A + B – Loss of electron (oxidation) Gain of electron (reduction) Lower energyHigher energy

8. Laws of thermodynamics First law of thermodynamics – Energy cannot be created or destroyed – Energy can only change from one form to another – Total amount of energy in the universe remains constant – During each conversion, some energy is lost as heat 8

9. 9 Second law of thermodynamics – Entropy (disorder) is continuously increasing – Energy transformations proceed spontaneously to convert matter from a more ordered/less stable form to a less ordered/ more stable form

10. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Disorder happens spontaneously Organization requires energy © Jill Braaten

11. 11 Free energy G = Energy available to do work G = H – TS  H = enthalpy, energy in a molecule’s chemical bonds  T = absolute temperature  S = entropy, unavailable energy

12. ΔG = ΔH – TS ΔG = change in free energy Positive ΔG – Products have more free energy than reactants – H is higher or S is lower – Not spontaneous, requires input of energy – Endergonic Negative ΔG – Products have less free energy than reactants – H is lower or S is higher or both – Spontaneous (may not be instantaneous) – Exergonic 12

13. 13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a.a. b.b. 0 0 Course of Reaction Products  G > 0  G < 0 Reactants Course of Reaction Products Free Energy (G) Energy Released Energy Supplied Free Energy (G) Energy Released Energy Supplied Energy is released Energy must be supplied Exergonic Endergonic

14. Activation energy Extra energy required to destabilize existing bonds and initiate a chemical reaction Exergonic reaction’s rate depends on the activation energy required – Larger activation energy proceeds more slowly Rate can be increased 2 ways 1.Increasing energy of reacting molecules (heating) 2.Lowering activation energy 14

15. 15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ΔGΔG Energy Released Energy Supplied Free Energy (G) Activation energy Activation energy 0 uncatalyzed catalyzed Course of Reaction Product Reactant

16. Catalysts Substances that influence chemical bonds in a way that lowers activation energy Cannot violate laws of thermodynamics – Cannot make an endergonic reaction spontaneous Do not alter the proportion of reactant turned into product 16

17. 17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ΔGΔG Energy Released Energy Supplied Free Energy (G) Activation energy Activation energy 0 uncatalyzed catalyzed Course of Reaction Product Reactant

18. 18 ATP Adenosine triphosphate Chief “currency” all cells use Composed of – Ribose – 5 carbon sugar – Adenine – Chain of 3 phosphates Key to energy storage Bonds are unstable ADP – 2 phosphates AMP – 1 phosphate – lowest energy form

19. 19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. AMP CORE O O–O– O O O H HH H O CC N N N C N C CH H PO–O– OP O PO ADP ATP Triphosphate group O–O– CH 2 High-energy bonds a.a. Adenine NH 2 Ribose OH b.b.

20. 20 ATP cycle ATP hydrolysis drives endergonic reactions – Coupled reaction results in net –ΔG (exergonic and spontaneous) ATP not suitable for long-term energy storage – Fats and carbohydrates better – Cells store only a few seconds worth of ATP

21. 21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. + + PiPi Energy from exergonic cellular reactions ATP H2OH2O ADP Energy for endergonic cellular processes

22. 22 Enzymes: Biological Catalysts Most enzymes are protein – Some are RNA Shape of enzyme stabilizes a temporary association between substrates Enzyme not changed or consumed in reaction Carbonic anhydrase – 200 molecules of carbonic acid per hour made without enzyme – 600,000 molecules formed per second with enzyme

23. 23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Active site a.b. EnzymeEnzyme–substrate complex Substrate

24. Active site Pockets or clefts for substrate binding Forms enzyme–substrate complex Precise fit of substrate into active site Applies stress to distort particular bond to lower activation energy – Induced fit 24

25. 25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1. The substrate, sucrose, consists of glucose and fructose bonded together. 2. The substrate binds to the active site of the enzyme, forming an enzyme– substrate complex. 3. The binding of the substrate and enzyme places stress on the glucose– fructose bond, and the bond breaks. 4. Products are released, and the enzyme is free to bind other substrates. Bond Glucose Fructose Active site Enzyme sucrase H2OH2O

26. 26 Enzymes may be suspended in the cytoplasm or attached to cell membranes and organelles Multienzyme complexes – subunits work together to form molecular machine – Product can be delivered easily to next enzyme – Unwanted side reactions prevented – All reactions can be controlled as a unit

27. 27 Nonprotein enzymes Ribozymes 1981 discovery that certain reactions catalyzed in cells by RNA molecule itself 1. 2 kinds 1.Intramolecular catalysis – catalyze reaction on RNA molecule itself 2.Intermolecular catalysis – RNA acts on another molecule

28. Enzyme function Rate of enzyme-catalyzed reaction depends on concentrations of substrate and enzyme Any chemical or physical condition that affects the enzyme’s three-dimensional shape can change rate – Optimum temperature – Optimum pH 28

29. 29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. b. Rate of Reaction Optimum pH for pepsin Rate of Reaction pH of Reaction Optimum pH for trypsin 123456789 304050607080 Temperature of Reaction (˚C) Optimum temperature for human enzyme Optimum temperature for enzyme from hotsprings prokaryote

30. 30 Inhibitors Inhibitor – substance that binds to enzyme and decreases its activity Competitive inhibitor – Competes with substrate for active site Noncompetitive inhibitor – Binds to enzyme at a site other than active site – Causes shape change that makes enzyme unable to bind substrate

31. 31 a. Competitive inhibitionb. Noncompetitive inhibition Enzyme Allosteric site Active site Competitive inhibitor interferes with active site of enzyme so substrate cannot bind Allosteric inhibitor changes shape of enzyme so it cannot bind to substrate Active site Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Substrate Inhibitor

32. 32 Allosteric Enzymes Allosteric enzymes – enzymes exist in active and inactive forms Most noncompetitive inhibitors bind to allosteric site – chemical on/off switch Allosteric inhibitor – binds to allosteric site and reduces enzyme activity Allosteric activator – binds to allosteric site and increases enzyme activity

33. 33 Metabolism Total of all chemical reactions carried out by an organism Anabolic reactions/anabolism – Expend energy to build up molecules Catabolic reactions/catabolism – Harvest energy by breaking down molecules

34. 34 Biochemical pathways Reactions occur in a sequence Product of one reaction is the substrate for the next Many steps take place in organelles

35. 35 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Initial substrate Intermediate substrate A Intermediate substrate B Intermediate substrate C End product Enzyme1 Enzyme2 Enzyme3 Enzyme4

36. Feedback inhibition End-product of pathway binds to an allosteric site on enzyme that catalyzes first reaction in pathway Shuts down pathway so raw materials and energy are not wasted 36

37. 37 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a.b. Enzyme 1 Enzyme 2 Enzyme 3 Enzyme 1 Enzyme 2 Enzyme 3 End product Initial substrate Intermediate substrate A Intermediate substrate B Initial substrate