CO20

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CO20
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Enzymes

• Catalyst Speeds up rate of reaction but does
  not change equilibrium. The catalyst itself is
  not changed.
• Enzymes are protein molecules which catalyze a
  chemical reaction

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• Enzymes catalyze specific reactions on specific
  compounds (single isomers)
• Business part of enzyme is the active site
• Active site binds the substrate (compound which
  undergoes a reaction)

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Enzyme Names

• Generally come from the name of the reaction that
  enzyme catalyzes.
• Frequently end in ase
• Name of substrate - ending
• ase
• Lactose ose ase lactase

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Categories of Enzymes

• Oxidoreductasescatalyze redox reaction
• Transferasescatalyze transfer of functional
  group to a different molecule
• Kinasetransfer of phosphate group
• Transaminasetransfer of amino group
• Hydrolaseshydrolysis reactions (add
  water and break bond

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• Lyasesadd group to double bond OR remove group
  to make double bond
• Isomerasesrearrange to make isomer
• Ligasesmake or break several kinds of bonds
  C-C C-S C-O C-N

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How do enzymes catalyze a reaction?

• Enzymes lower activation energy for a reaction
• They do not change the equilibrium constant,
  only the RATE of the reaction

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• Initial rate of reaction will double when you
  double amount of substrate
• Rate increases to a maximum velocity
• when all of the active sites on an enzyme are
  full
• Vmax

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Formation of Enzyme-Substrate Complex

• E S ? ES ? ES ? EP ? E P
• Enzyme Enzyme Transition
  Enzyme Enzyme
• substrate state product
  
• Substrate complex complex Product
• Overall Reaction S ? P

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• Substrate fits into the active site and then
  undergoes a reaction
• Enzyme-substrate complex formed
• Enzyme-substrate complex is an intermediate
  species

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• Initial binding of substrate relatively fast
• Conversion of substrate to product (and release
  of product) is slower
• This is Rate Limiting Step

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Characteristics of Active Site

• Site where substrate binds to enzyme
• Generally have groups that extend into the active
  site to help catalyze the reaction
• Substrate fits into site.
• Substrate held by weak, noncovalent interactions
  in binding site
• Site very specificonly substrate that fits into
  site will undergo reaction

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Models for Enzyme Activity

• Lock and Key Model
• Emil Fischer 1894
• Substrate fits into active site just as a key
  fits into a lock
• Induced Fit Model
• Daniel Koshland 1958
• Enzyme modifies its shape to accommodate the
  substrate

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Lock and Key Model
Induced Fit Model
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Enzyme Specificity

• Enzyme specificity is the ability of an enzyme to
  bind only one (or a very few) substrates and thus
  catalyze only one reaction

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Levels of Specificity

• Absolute One substrate only
• Group Similar compounds (hexoses)
• Linkage Recognize bond (linkage) types
• Stereochemical D- or L- isomer

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Look at enzyme-substrate complex againFocus on
steps in forming transition state and product
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Formation of Enzyme-Substrate Complex

• E S ? ES ? ES ? EP ? E P
• Enzyme Enzyme Transition
  Enzyme Enzyme
• substrate state product
  
• Substrate complex complex Product
• Overall Reaction S ? P

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How transition state helps reaction to proceed
more rapidly

• Put stress on bond in substrate
• Bring reactants closer together and in correct
  orientation
• Provide different pH environment in active site

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Stress Bond
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Correct Orientation
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Effect of pH on Enzyme Rate

• Enzymes have different reaction rates at
  different pHs
• pH Optimum pH at which rate is highest
• Near pH 7 for many enzymes
• Some have optima at very high or low pH
• At pH higher or lower than optimum, rate falls
  off
• At extreme pH, enzyme will be denatured

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Fig. 20.9
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Effect of Temperature on Enzyme Rate

• Uncatalyzed reaction rates increase as
  temperature increases
• Enzymes have temperature optimum, at which the
  enzyme has its highest rate
• Generally about 37oC for many enzymes
• Above the temperature optimum, enzyme rates fall
• At high temperatures, enzyme is denatured

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Fig. 20.10
Uncatalyzed Reaction
Catalyzed Reaction
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Activating Enzymes

• Apoenzyme Protein portion of enzyme
• Cofactor Non-protein prosthetic group
• Holoenzyme Active enzyme

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Cofactor binds and changes active site
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Activating enzymes

• Coenzyme Bind temporarily to catalytic site to
  help catalyze reaction (often have vitamin
  component)
• Coenzyme binds to apoenzyme first.
• Substrate binds second
• Both product and coenzyme are released after
  reaction

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2
3
1
4
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Table 20.1
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Regulation of Enzyme Activity

• Allosteric Enzymes
• More than one binding site
• Shape of active site is changed by binding of
  molecules to another part of enzyme
• Positive Allosterism Changes to active form
• Negative Allosterism Changes to inactive form

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Fig. 20.11
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Feedback Inhibition

• Enzyme pathway
• A ? B ? C ? D ? E ? F
• If enough F is present, it can bind to an enzyme
  earlier in sequence and inactivate it. This
  stops synthesis of all subsequent products.

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Regulation of Enzyme Activity

• Zymogen (Proenzyme) Enzyme originally made in
  inactive form. Part of it must be removed before
  it is active.
• Protein Modification Group can be bound or
  removed to activate or inactivate an enzyme.
  Easily reversed.

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Enzyme Inhibitors

• Bind to enzymes and eliminate or reduce catalytic
  activity.

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Types of Inhibitors

• Irreversible Inhibitors
• Reversible, Competitive Inhibitors
• Structural Analogs
• Reversible, Noncompetitive Inhibitors

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Fig. 20.12
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Fig. 20.13
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Table 20.2
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Fig. 20.8