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CHAPTER 6 AN INTRODUCTION TO METABOLISM Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Section B: Enzymes 1. Enzymes speed up metabolic reactions by lowering energy barriers 2. Enzymes are substrate specific 3. The active site in an enzyme’s catalytic center 4. A cell’s physical and chemical environment affects enzyme activity

CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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Page 1: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

CHAPTER 6 AN INTRODUCTION TO

METABOLISM

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Section B: Enzymes1. Enzymes speed up metabolic reactions by lowering energy barriers2. Enzymes are substrate specific3. The active site in an enzyme’s catalytic center4. A cell’s physical and chemical environment affects enzyme activity

Page 2: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• A catalyst is a chemical agent that changes the rateof a reaction without being consumed by the reaction.• An enzyme is a catalytic protein.

• Enzymes regulate the movement of moleculesthrough metabolic pathways.

1. Enzymes speed up metabolic reactions bylowering energy barriers

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 3: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Chemical reactions between molecules involveboth bond breaking and bond forming.• To hydrolyze sucrose, the bond between glucose and

fructose must be broken and then new bonds formedwith a hydrogen ion and hydroxyl group from water.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.11

Page 4: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Even in an exergonic reaction, the reactants mustabsorb energy from their surroundings, the freeenergy of activation or activation energy (EA), tobreak the bonds.• This energy makes the reactants unstable, increases the

speed of the reactant molecules, and creates morepowerful collisions.

• In exergonic reactions, not only is the activationenergy released back to the surroundings, but evenmore energy is released with the formation of newbonds.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 5: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Activation energy is the amount of energynecessary to push the reactants over an energybarrier.• At the summit the

molecules are atan unstable point,the transition state.

• The difference betweenfree energy of theproducts and the freeenergy of the reactantsis the delta G.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.12

Page 6: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• For some processes, the barrier is not high and thethermal energy provided by room temperature issufficient to reach the transition state.

• In most cases, EA is higher and a significant inputof energy is required.• A spark plug provides the energy to energize gasoline.

• Without activation energy, the hydrocarbons of gasolineare too stable to react with oxygen.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 7: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• The laws of thermodynamics would seem to favorthe breakdown of proteins, DNA, and othercomplex molecules.• However, in the temperatures typical of the cell there is

not enough energy for a vast majority of molecules tomake it over the hump of activation energy.

• Yet, a cell must be metabolically active.

• Heat would speed reactions, but it would also denatureproteins and kill cells.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 8: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Enzyme speed reactions by lowering EA.• The transition state can then be reached even at

moderate temperatures.

• Enzymes do not change delta G.• It hastens reactions that would occur eventually.

• Because enzymesare so selective,they determinewhich chemicalprocesses willoccur at any time.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.13

Page 9: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• A substrate is a reactant which binds to an enzyme.

• When a substrate or substrates binds to an enzyme,the enzyme catalyzes the conversion of the substrateto the product.• Sucrase is an enzyme that binds to sucrose and breaks the

disaccharide into fructose and glucose.

2. Enzymes are substrate specific

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 10: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• The active site of an enzymes is typically a pocketor groove on the surface of the protein into whichthe substrate fits.

• The specificity of an enzyme is due to the fitbetween the active site and that of the substrate.

• As the substrate binds, the enzyme changes shapeleading to a tighter induced fit, bringing chemicalgroups in position to catalyze the reaction.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.14

Page 11: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• In most cases substrates are held in the active site byweak interactions, such as hydrogen bonds and ionicbonds.• R groups of a few amino acids on the active site catalyze

the conversion of substrate to product.

3. The active site is an enzyme’s catalyticcenter

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 12: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.15

Page 13: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• A single enzyme molecule can catalyze thousandsor more reactions a second.

• Enzymes are unaffected by the reaction and arereusable.

• Most metabolic enzymes can catalyze a reaction inboth the forward and reverse direction.• The actual direction depends on the relative

concentrations of products and reactants.

• Enzymes catalyze reactions in the direction ofequilibrium.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 14: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Enzymes use a variety of mechanisms to loweractivation energy and speed a reaction.• The active site orients substrates in the correct

orientation for the reaction.

• As the active site binds the substrate, it may put stresson bonds that must be broken, making it easier to reachthe transition state.

• R groups at the active site may create a conducivemicroenvironment for a specific reaction.

• Enzymes may even bind covalently to substrates in anintermediate step before returning to normal.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 15: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• The rate that a specific number of enzymes convertssubstrates to products depends in part on substrateconcentrations.

• At low substrate concentrations, an increase insubstrate speeds binding to available active sites.

• However, there is a limit to how fast a reaction canoccur.

• At some substrate concentrations, the active sites onall enzymes are engaged, called enzyme saturation.

• The only way to increase productivity at this pointis to add more enzyme molecules.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 16: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• The three-dimensional structures of enzymes (almostall proteins) depend on environmental conditions.

• Changes in shape influence the reaction rate.

• Some conditions lead to the most activeconformation and lead to optimal rate of reaction.

4. A cell’s physical and chemicalenvironment affects enzyme activity

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 17: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Temperature has a major impact on reaction rate.• As temperature increases, collisions between substrates

and active sites occur more frequently as molecules movefaster.

• However, at some point thermal agitation begins todisrupt the weak bonds that stabilize the protein’s activeconformation and the protein denatures.

• Each enzyme has an optimal temperature.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.16a

Page 18: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Because pH also influences shape and thereforereaction rate, each enzyme has an optimal pH too.

• This falls between pH 6 - 8 for most enzymes.

• However, digestive enzymes in the stomach aredesigned to work best at pH 2 while those in theintestine are optimal at pH 8, both matching theirworking environments.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.16b

Page 19: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Many enzymes require nonprotein helpers,cofactors, for catalytic activity.• They bind permanently to the enzyme or reversibly.

• Some inorganic cofactors include zinc, iron, andcopper.

• Organic cofactors, coenzymes, include vitamins ormolecules derived from vitamins.

• The manners by which cofactors assist catalysisare diverse.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 20: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Binding by some molecules, inhibitors, preventenzymes from catalyzing reactions.• If binding involves covalent bonds, then inhibition is

often irreversible.• If binding is weak, inhibition may be reversible.

• If the inhibitor binds to the same site as thesubstrate, then it blocks substrate binding viacompetitive inhibition.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 6.17a, b

Page 21: CHAPTER 6 AN INTRODUCTION TO METABOLISM Section …lhsteacher.lexingtonma.org/Pohlman/06B-Enzymes.pdf · Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• If the inhibitor binds somewhere other than theactive site, it blocks substrate binding vianoncompetitive inhibition.

• Binding by the inhibitor causes the enzyme tochange shape, rendering the active site unreceptiveat worst or less effective at catalyzing the reaction.

• Reversible inhibition of enzymes is a natural partof the regulation of metabolism.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.17c