BIOLOGICAL SCIENCE. Lecture Presentation by Cindy S. Malone, PhD, California State University Northridge. FIFTH EDITION Freeman Quillin Allison

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1 BIOLOGICAL SCIENCE FIFTH EDITION Freeman Quillin Allison 8 Lecture Presentation by Cindy S. Malone, PhD, California State University Northridge

2 Roadmap 8 In this chapter you will learn how Enzymes use energy to drive the chemistry of life looking at energy, asking looking at enzymes, asking What happens to energy in chemical reactions? 8.1 How do enzymes help speed chemical reaction rates? 8.3 Can chemical energy drive nonspontaneous reactions? 8.2 What factors affect enzyme function? How do enzymes work together in metabolic pathways?

3 Two types of energy exist Kinetic energy Energy of motion Molecular level is thermal energy Potential energy Energy of position or configuration Molecular level chemical energy is stored The free energy of a reaction is the amount of energy available to do work

4 Figure 8.1 E p (higher) E k Mechanical energy Heat Sound 1. Potential energy 2. Kinetic energy 3. Other forms of energy Conclusion: Energy is neither created nor destroyed; it simply changes form. E p (lower)

5 Chemical potential energy In cells Electrons are the most important source Amount of potential energy in an electron is based on Its position relative to positive and negative charges Electrons closer to negative charges and farther from positive charges Have higher potential energy Molecular potential energy Is a function of electron configuration and position

6 Figure 8.2 (a) The potential energy of an electron is related to its position. Electrons have the greatest potential energy in the outermost electron shells Nucleus 1st 2nd 3rd Electron shells (b) E p (higher) E k Heat or light E p (lower) 1. Potential energy 2. Kinetic energy 3. Other forms of energy Conclusion: Energy is neither created nor destroyed; it simply changes form.

7 Energy is conserved Energy cannot be created or destroyed Energy can only be transferred and transformed Enthalpy (H) includes The potential energy of the molecule (heat content) Effect of the molecule on surrounding pressure and volume Changes in enthalpy are represented by H Difference in heat content

8 Exothermic reaction Releases heat energy G 0 Products have less potential energy than reactants Endothermic reaction Heat energy is taken up G 0 Products have higher potential energy than reactants

9 Amount of disorder When the products of a chemical reaction become less ordered than the reactant molecules Entropy increases S 0 Second law of thermodynamics Total entropy always increases in isolated systems

10 Determines whether a reaction is spontaneous or requires added energy to proceed G H T S G Gibbs free energy change H change in enthalpy A measure of chemical potential energy S change in entropy A measure of disorder T temperature in degrees Kelvin

11 G 0 a spontaneous reaction an exergonic reaction G 0 a reaction that requires energy input to occur and is not spontaneous an endergonic reaction G 0 a reaction that is at equilibrium

12 For most reactions to proceed One or more chemical bonds have to break Others have to form Substances must collide in a specific orientation that brings the electrons involved near each other When the concentration of reactants is high More collisions should occur Reactions should proceed more quickly

13 Figure 8.5 Exergonic reaction (releases energy) Energy Higher energy reactants Lower energy products Energy Lower energy reactants Higher energy products Energy Endergonic reaction (requires energy)

14 Reduction oxidation reactions (redox reactions) Are chemical reactions that involve electron transfer When an atom or molecule gains an electron It is reduced Reduction gain of one or more e and a hydrogen ion (H + )

15 ATP adenosine triphosphate ATP is the cellular currency for energy It provides the fuel for most cellular activities ATP Has high potential energy Allows cells to do work ATP works by Phosphorylating target molecules Transferring a phosphate group

16 Hydrolysis of ATP is exergonic because The entropy of the product molecules is much higher than that of the reactants Energy released during ATP hydrolysis Is transferred to a protein during phosphorylation Usually causes a change in the protein s shape

17 Figure 8.8 (a) ATP stores a large amount of potential energy. Phosphate groups Adenine Clustered negative charges raise the potential energy of linked phosphate groups Ribose (b) Energy is released when ATP is hydrolyzed. ATP Water ADP Inorganic phosphate Energy

18 Enzymes Are protein catalysts Typically catalyze only one reaction Most biological chemical reactions occur at meaningful rates only in the presence of an enzyme Enzymes Bring reactants together in precise orientations Stabilize transition states Protein catalysts are important Because they speed up the chemical reactions that are required for life

19 Enzymes bring substrates together In specific positions that facilitate reactions Are very specific in which reactions they catalyze Substrates bind to the enzyme s active site Many enzymes undergo a conformational change When the substrates are bound to the active site This change is called an induced fit

20 Figure 8.10 Substrate (glucose) Substrate (ATP) Enzyme (hexokinase) When the ATP and glucose bind to the active site, the enzyme changes shape. This induced fit reorients the substrates and binds them tighter to the active site.

21 The activation energy (E a ) of a reaction Is the amount of free energy required to reach the intermediate condition, or transition state Reactions occur when Reactants have enough kinetic energy to reach the transition state The kinetic energy of molecules is a function of their temperature

22 Figure 8.11 Free energy Transition state E a Activation energy Reactants G Products Progress of reaction

23 Interactions between the enzyme and the substrate Stabilize the transition state Lower the activation energy required for the reaction to proceed

24 Figure 8.12 Free energy Transition state Activation energy with enzyme E a Reactants G does not change G Products Progress of reaction

25 Enzyme catalysis has three steps: 1. Initiation Substrates are precisely oriented as they bind to the active site 2. Transition state facilitation Interactions between the substrate and active site R- groups lower the activation energy 3. Termination Reaction products are released from the enzyme

26 Figure 8.13 Substrates Transition state Products Enzyme 1. Initiation: Reactants bind to the active site in a specific orientation, forming an enzyme-substrate complex. Shape changes 2. Transition state facilitation: Interactions between enzyme and substrate lower the activation energy required. 3. Termination: Products have lower affinity for active site and are released. Enzyme is unchanged after the reaction.

27 A catalyst Is a substance that lowers the activation energy of a reaction And increases the rate of the reaction Catalysts Lower the activation energy of a reaction by Lowering the free energy of the transition state Do not change G Are not consumed in the reaction

28 Enzymes are saturable The rate of a reaction is limited by the amounts of Substrate present Enzyme available The speed of an enzyme-catalyzed reaction Increases linearly at low substrate concentrations Slows as substrate concentration increases Reaches maximum speed at high substrate concentrations

29 Figure 8.14 Rate of product formation Maximum speed of reaction Catalyzed reaction Uncatalyzed reaction Substrate concentration

30 Enzymes are regulated by molecules that are not part of the enzyme itself 1. Cofactors are inorganic ions Such as the metal ions Zn 2+, Mg 2+, and Fe 2+ Reversibly interact with enzymes 2. Coenzymes are organic molecules That interact with enzymes Such as the electron carriers NADH or FADH 2

31 Enzymes function best At some particular temperature and ph Temperature affects The movement of the substrates and enzyme ph affects The enzyme s shape and reactivity

32 Figure 8.15 Relative chitinase activity (%) Relative chitinase activity (%) (a) Enzymes from different organisms may function best at different temperatures. From bacteria that live in a cool and neutral environment From bacteria that live in a hot and acidic environment (b) Enzymes from different organisms may function best at different phs. From bacteria that live in a hot and acidic environment From bacteria that live in a cool and neutral environment Temperature ( C) ph

33 The rate of an enzyme-catalyzed reaction depends on Substrate concentration The enzyme s intrinsic affinity for the substrate Temperature ph

34 Competitive inhibition occurs when A molecule similar in size and shape to the substrate competes with the substrate for access to the active site Allosteric regulation occurs when A molecule causes a change in enzyme shape By binding to the enzyme At a location other than the active site Allosteric regulation can activate or deactivate the enzyme

35 Figure 8.16 (a) Competitive inhibition (b) Allosteric regulation Substrates Enzyme Enzyme in absence of regulation or or Shape or changes Competitive inhibition The substrates cannot bind when a regulatory molecule binds to the enzyme s active site. Regulatory molecule Allosteric activation The active site becomes available to the substrates when a regulatory molecule binds to a different site on the enzyme. Regulatory molecule Shape changes Allosteric inhibition The active site becomes unavailable to the substrates when a regulatory molecule binds to a different site on the enzyme. Regulatory molecule