CHAPTER 6 LECTURE SLIDES

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Energy and Metabolism SWBAT:

• Explain enzyme function by their lowering activation energy.

• Demonstrate the catalytic cycle of an enzyme: how the enzyme is not permanently altered and how a product is produced from substrate(s).

• Report how various factors that affect enzyme function: enzyme concentration, substrate concentration, temperature, pH, salinity, competitive and non-competitive inhibitors, activators.

• Explain how enzymes can be regulated through feedback inhibition.

• Illustrate the structure of the ATP (or GTP) molecule.

• Explain how the structure of ATP enables it to transfer energy within the cell.

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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?

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Weeeeee!

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

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Laws of thermodynamics

• First law of thermodynamics – Energy cannot be created or destroyed – 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 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

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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 13

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.

2.

Increasing energy of reacting molecules (heating) Lowering activation energy 14

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 15

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Animation: How Enzymes Work

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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 18

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 19

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Enzymes

• 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 21

Active site

• Pockets or clefts for substrate binding • Forms enzyme-substare complex • Precise fit of substrate into active site • Applies stress to distort particular bond to lower activation energy – Induced fit 22

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• 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 an be controlled as a unit 25

Enzyme Action

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Metabolic pathways

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Series of enzyme-controlled reactions leading to formation of a

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product Each new substrate is the product of the previous reaction Substrate 1 Enzyme A

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Substrate 2 Enzyme B Substrate 3 Enzyme C Substrate 4 Enzyme D Product

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Enzyme names commonly:

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Reflect the substrate

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Have the suffix – ase Examples: sucrase, lactase, protease, lipase

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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 27

Enzyme function

• Rate of enzyme-catlyzed reaction depends on concentrations of substrate and enzyme • Any chemical or physical condition that affects the enzyme’s 3 dimensional shape can change rate – Optimum temperature – Optimum pH 28

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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 30

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Allostery

• 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 32

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 33

Biochemical pathways

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

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Feedback inhibition

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

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