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Enzymes: Catalysts of Biochemical Reactions

Understand the role of enzymes in biochemical reactions, including their function, activation, and inhibition. Learn about the factors affecting enzyme activity and the importance of optimal temperature and pH. Explore the concept of allosteric regulation and feedback inhibition.

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Enzymes: Catalysts of Biochemical Reactions

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  1. Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3 Product Starting molecule

  2. A diver has more potential energy on the platform than in the water. Diving converts potential energy to kinetic energy. Climbing up converts the kinetic energy of muscle movement to potential energy. A diver has less potential energy in the water than on the platform.

  3. Heat CO2 + Chemical energy H2O (a) First law of thermodynamics (b) Second law of thermodynamics

  4. More free energy (higher G) • Less stable • Greater work capacity In a spontaneous change • The free energy of the system • decreases (∆G < 0) • The system becomes more • stable • The released free energy can • be harnessed to do work • Less free energy (lower G) • More stable • Less work capacity (b) Diffusion (c) Chemical reaction (a) Gravitational motion

  5. Reactants Amount of energy released (∆G < 0) Energy Free energy Products Progress of the reaction (a) Exergonic reaction: energy released Products Amount of energy required (∆G > 0) Energy Free energy Reactants Progress of the reaction (b) Endergonic reaction: energy required

  6. ∆G < 0 ∆G = 0 (a) An isolated hydroelectric system (b) An open hydroelectric system ∆G < 0 ∆G < 0 ∆G < 0 ∆G < 0 (c) A multistep open hydroelectric system

  7. Adenine Phosphate groups Ribose

  8. P P P Adenosine triphosphate (ATP) H2O + P P P Energy + i Inorganic phosphate Adenosine diphosphate (ADP)

  9. NH2 NH3 ∆G = +3.4 kcal/mol + Glu Glu Glutamic acid Glutamine Ammonia (a) Endergonic reaction P ATP phosphorylates glutamic acid, making the amino acid less stable. 1 ADP + + ATP Glu Glu NH2 P 2 Ammonia displaces the phosphate group, forming glutamine. NH3 P + + i Glu Glu (b) Coupled with ATP hydrolysis, an exergonic reaction (c) Overall free-energy change

  10. Membrane protein P P i Solute transported Solute (a) Transport work: ATP phosphorylates transport proteins ADP + ATP P i Vesicle Cytoskeletal track ATP Protein moved Motor protein (b) Mechanical work: ATP binds noncovalently to motor proteins, then is hydrolyzed

  11. ATP + H2O Energy for cellular work (endergonic, energy-consuming processes) Energy from catabolism (exergonic, energy-releasing processes) ADP P + i

  12. Sucrose (C12H22O11) Sucrase Glucose (C6H12O6) Fructose (C6H12O6)

  13. A B C D Transition state EA A B C D Free energy Reactants A B ∆G < O C D Products Progress of the reaction

  14. Course of reaction without enzyme EA without enzyme EAwith enzyme is lower Reactants Free energy Course of reaction with enzyme ∆G is unaffected by enzyme Products Progress of the reaction

  15. Substrate Active site Enzyme Enzyme-substrate complex (a) (b)

  16. Substrates enter active site; enzyme changes shape such that its active site enfolds the substrates (induced fit). 1 2 Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. Substrates Enzyme-substrate complex Active site can lower EA and speed up a reaction. 3 6 Active site is available for two new substrate molecules. Enzyme 5 Products are released. 4 Substrates are converted to products. Products

  17. Optimal temperature for typical human enzyme Optimal temperature for enzyme of thermophilic (heat-tolerant) bacteria Rate of reaction 40 0 60 100 20 80 Temperature (ºC) (a) Optimal temperature for two enzymes Optimal pH for trypsin (intestinal enzyme) Optimal pH for pepsin (stomach enzyme) Rate of reaction 4 5 6 7 8 9 10 0 1 2 3 pH (b) Optimal pH for two enzymes

  18. Substrate Active site Competitive inhibitor Enzyme Noncompetitive inhibitor (c) Noncompetitive inhibition (b) Competitive inhibition (a) Normal binding

  19. Active site (one of four) Allosteric enyzme with four subunits Regulatory site (one of four) Activator Active form Stabilized active form Oscillation Non- functional active site Inhibitor Inactive form Stabilized inactive form (a) Allosteric activators and inhibitors Substrate Stabilized active form Inactive form (b) Cooperativity: another type of allosteric activation

  20. Initial substrate (threonine) Active site available Threonine in active site Enzyme 1 (threonine deaminase) Isoleucine used up by cell Intermediate A Feedback inhibition Enzyme 2 Active site of enzyme 1 no longer binds threonine; pathway is switched off. Intermediate B Enzyme 3 Intermediate C Isoleucine binds to allosteric site Enzyme 4 Intermediate D Enzyme 5 End product (isoleucine)

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