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Chapter 23 Enzymes

EnzymesRibbon diagram of cytochrome c oxidase, the enzyme that directly uses oxygen during respiration.

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Enzyme CatalysisEnzyme: A biological catalyst.• With the exception of some RNAs that catalyze their own

self-cleavage, all enzymes are proteins.• Enzymes can increase the rate of a reaction by a factor of 109 to

1020 over an uncatalyzed reaction.• Some catalyze the reaction of only one compound.

• Others are stereoselective; for example, enzymes that catalyze the reactions of only L-amino acids.

• Others catalyze reactions of specific types of compounds or bonds; for example, trypsin catalyzes hydrolysis of peptide bonds formed by the carboxyl groups of Lys and Arg.

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Enzyme CatalysisFigure 23-1 Trypsin catalyzes the hydrolysis of peptide bonds formed by the carboxyl group of lysine and arginine.

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Classification of EnzymesEnzymes are commonly named after the reaction or reactions they catalyze.• Example: lactate dehydrogenase, acid phosphatase.

Enzymes are classified into six major groups according to the type of reaction catalyzed:• Oxidoreductases: Oxidation-reduction reactions.• Transferases: Group transfer reactions.• Hydrolases: Hydrolysis reactions.• Lyases: Addition of two groups to a C-C double bond,

or removal of two groups to create a C-C double bond.• Isomerases: Isomerization reactions.• Ligases: The joining to two molecules.

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Classification of Enzymes1. Oxidoreductase:

2. Transferase:

3. Hydrolase:

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Classification of Enzymes4. Lyase:

5. Isomerase:

6. Ligase:

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Enzyme TerminologyApoenzyme: The protein part of an enzyme.Cofactor: A nonprotein portion of an enzyme that is necessary for catalytic function; examples are metallic ions such as Zn2+ and Mg2+.Coenzyme: A nonprotein organic molecule, frequently a B vitamin, that acts as a cofactor.Substrate: The compound or compounds whose reaction an enzyme catalyzes.Active site: The specific portion of the enzyme to which a substrate binds during reaction.

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Schematic of an Active SiteFigure 23.2 Schematic diagram of the active site of an enzyme and the participating components.

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Terms in Enzyme ChemistryActivation: Any process that initiates or increases the

activity of an enzyme.Inhibition: Any process that makes an active enzyme less

active or inactive.Competitive inhibitor: A substance that binds to the

active site of an enzyme thereby preventing binding of substrate.

Noncompetitive inhibitor: Any substance that binds to a portion of the enzyme other than the active site and thereby inhibits the activity of the enzyme.

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Enzyme ActivityEnzyme activity: A measure of how much a reaction rate is increased.We examine how the rate of an enzyme-catalyzed reaction is affected by:• Enzyme concentration.• Substrate concentration. • Temperature.• pH.

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Enzyme ActivityFigure 23-3 The effect of enzyme concentration on the rate of an enzyme-catalyzed reaction. Substrate concentration, temperature, and pH are constant.

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Enzyme ActivityFigure 23-4 The effect of substrate concentration on the rate of an enzyme-catalyzed reaction. Enzyme concentration, temperature, and pH are constant.

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Enzyme ActivityFigure 23-5 The effect of temperature on the rate of an enzyme-catalyzed reaction. Substrate and enzyme concentrations and pH are constant.

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Enzyme ActivityFigure 23-6 The effect of pH on the rate of an enzyme-catalyzed reaction. Substrate and enzyme concentrations and temperature are constant.

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Mechanism of ActionFigure 23-7 Lock-and-key model of enzyme mechanism.• The enzyme is a rigid three-dimensional body.• The enzyme surface contains the active site.

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Mechanism of ActionFigure 23-8 The Induced-fit model of an enzyme mechanism.

• The active site becomes modified to accommodate the substrate.

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Mechanism of ActionFigure 23-9 The mechanism of competitive inhibition.When a competitive inhibitor enters the active site, the substrate cannot enter.

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Mechanism of ActionFigure 23-10 Mechanism of noncompetitive inhibition. The inhibitor binds itself to a site other than the active site (allosterism), thereby changing the conformation of the active site. The substrate still binds but there is no catalysis.

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Mechanism of ActionFigure 23-11 Enzyme kinetics in the presence and the absence of inhibitors.

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Mechanism of Action• Both the lock-and-key model and the induced-fit model

emphasize the shape of the active site.• However, the chemistry of the active site is the most

important.• Just five amino acids participate in the active site in

more than 65% of the enzymes studied to date.• These five are His > Cys > Asp > Arg > Glu.• Four of these amino acids have either acidic or basic

side chains; the fifth has a sulfhydryl group (-SH).

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Catalytic Power• Figure 23-12 Enzymes provide an alternative pathway for

reaction. (a) The activation energy profile for a typical reaction. (b) A comparison of the activation energy profiles for a catalyzed and uncatalyzed reactions.

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Enzyme RegulationFeedback control: An enzyme-regulation process where the product of a series of enzyme-catalyzed reactions inhibits an earlier reaction in the sequence.

• The inhibition may be competitive or noncompetitive.

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Enzyme Regulation• Proenzyme (zymogen): An inactive form of an enzyme that

must have part of its polypeptide chain hydrolyzed and removed before it becomes active.• An example is trypsin, a digestive enzyme.• It is synthesized and stored as trypsinogen, which has

no enzyme activity.• It becomes active only after a six-amino acid fragment

is hydrolyzed and removed from the N-terminal end of its chain.

• Removal of this small fragment changes not only the primary structure but also the tertiary structure, allowing the molecule to achieve its active form.

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Enzyme RegulationAllosterism: Enzyme regulation based on an event occurring at a place other than the active site but that creates a change in the active site.• An enzyme regulated by this mechanism is called an

allosteric enzyme.• Allosteric enzymes often have multiple polypeptide

chains.• Negative modulation: Inhibition of an allosteric

enzyme.• Positive modulation: Stimulation of an allosteric

enzyme. • Regulator: A substance that binds to an allosteric

enzyme.

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Enzyme Regulation• Figure 23.14 The

allosteric effect. Binding of the regulator to a site other than the active site changes the shape of the active site.

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Enzyme RegulationFigure 23.15 Effects of binding activators and inhibitors to allosteric enzymes. The enzyme has an equilibrium between the T form and the R form.

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Enzyme RegulationProtein modification: The process of affecting enzyme activity by covalently modifying it.• The best known examples of protein modification

involve phosphorylation/dephosphorylation.• Example: Pyruvate kinase (PK) is the active form of the

enzyme; it is inactivated by phosphorylation to pyruvate kinase phosphate (PKP).

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Enzyme RegulationIsoenzyme (Isozymes): An enzyme that occurs in multiple forms; each catalyzes the same reaction.• Example: lactate dehydrogenase (LDH) catalyzes the

oxidation of lactate to pyruvate.• The enzyme is a tetramer of H and M chains.• H4 is present predominately in heart muscle.• M4 is present predominantly in the liver and in skeletal

muscle.• See Figure 23-16 for the relative distribution of these

isoenzymes.

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Enzyme RegulationFigure 23-16 The isozymes of lactate dehydrogenase (LDH). The electrophoresis gel depicts the relative isozyme types found in different tissues.

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Enzymes Used in Medicine

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Transition-State Analogs• Transition state analog: A molecule whose shape mimics

the transition state of a substrate.• Figure 23.17 The proline racemase reaction. Pyrrole-2-

carboxylate mimics the planar transition state of the reaction (next screen).

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Transition-State AnalogFigure 23-17 The proline racemase reaction and a mimic for

the transition state.

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Transition-State Analogs• Abzyme: An antibody that has catalytic activity because it

was created using a transition state analog as an immunogen. (a) The molecule below is a transition analog for the reaction of an amino acid with pyridoxal-5’-phosphate. (b) The abzyme is then used to catalyze the reaction on the next screen.

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Transition-State Analogs

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Chapter 23 Enzymes

End Chapter 23

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