Chapter Six The Behavior of Proteins: Enzymes

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Chapter SixThe Behavior of Proteins Enzymes
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Enzyme Catalysis

• Enzyme a _____________________
• with the exception of some __________ that
  catalyze their own splicing (Section 10.4), all
  enzymes are proteins (???)
• enzymes can increase the rate of a rxn by a
  factor of up to 1020 over an uncatalyzed rxn
• some enzymes are so specific that they catalyze
  the rsn of only one stereoisomer others catalyze
  a family of similar rxns
• The rate of a reaction depends on its activation
  energy, DG
• an enzyme provides an alternative pathway with a
  ______________________________

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Enzyme Catalysis (Contd)

• For a reaction taking place at constant
  temperature and pressure, e.g., in the body
• the change in __________________________ is
• Difference in energies between initial state and
  final state
• The change in free energy is related to the
  equilibrium constant, Keq, for the reaction by

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Enzyme Catalysis (Contd)

• Consider the reaction
• H2O2 ? H2O O2

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Temperature dependence of catalysis

• Temperature can also catalyze reaction
  (increase rate)
• This is dangerous, why?
• Increasing temperature will lead to
  _______________
• ______________________

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Enzyme Kinetics

• For the reaction
• The rate of reaction is given by rate equation
• Where k is a proportionality constant called the
  ___________________________________________
• ______________________________ the sum of the
  exponents in the rate equation fg

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Enzyme Kinetics (Contd)

• Consider the reaction
• Whose rate equation is given by the expression
• Determined experimentally, not from
  _______________
• The reaction is said to be first order in A,
  first order in B, and second order overall
• Consider this reaction of glycogen with phosphate

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How Enzymes bind to Substrate

• In an enzyme-catalyzed reaction
• ____________________________, S a reactant
• ______________________ the small portion of the
  enzyme surface where the substrate(s) becomes
  bound by noncovalent forces, e.g., hydrogen
  bonding, electrostatic attractions, van der Waals
  attractions

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Binding Models

• Two models have been developed to describe
  formation of the_____________________ complex
• __________________ model substrate binds to that
  portion of the enzyme with a complementary shape
• _________________ model binding of the substrate
  induces a change in the conformation of the
  enzyme that results in a complementary fit

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Two Modes of E-S Complex Formation
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Formation of Product
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An Example of Enzyme Catalysis

• ____________________ catalyzes
• The selective hydrolysis of ___________________
  where the ________ is contributed by _____ and
  ____
• It also catalyzes hydrolysis of the ____________
  bonds

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An Example of Enzyme Catalysis (Contd)
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Non-Allosteric Enzyme Behavior

• Point at which the rate of reaction does not
  change, enzyme is __________________, maximum
  rate of reaction is reached

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ATCase An Example of Allosteric Behavior

• ____________ shape - characteristic of
  __________
• Again max velocity reached, but different
  mechanism

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Michaelis-Menten Kinetics

• Initial rate of an enzyme-catalyzed rxn vs S

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Michaelis-Menten Model

• For an enzyme-catalyzed reaction
• The rates of formation and breakdown of ES are
  given by these equations
• At steady state

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Michaelis-Menten Model (Contd)

• When ______________is reached, the concentration
  of free enzyme is the total minus that bound in
  ES
• Substituting for the concentration of free enzyme
  and collecting all rate constants in one term
  gives
• KM is called the ____________________________

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Michaelis-Menten Model (Contd)

• It is now possible to solve for the concentration
  of the enzyme-substrate complex, ES
• Or alternately

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Michaelis-Menten Model (Contd)

• In the initial stages, formation of product
  depends only on the ______________________________
  _________________
• If substrate concentration is ____________________
  ____ is _______________________ ES ET
• Substituting k2ET Vmax into the top equation
  gives

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Michaelis-Menten Model (Contd)

• When _______________ the equation reduces to

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Linearizing The Michaelis-Menten Equation

• Vmax is difficult to _____________________________
  ______
• The equation for a hyperbola
• Can be transformed into the equation for a
  ________ by taking _______________________________
  ___

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Lineweaver-Burk Plot

• The _______________________ plot has the form y
  mx b, and is the formula for a straight line
• a plot of 1/V versus 1/S will give a straight
  line with slope of _______________ and y
  intercept of _______________
• known as a _______________________________________
  ___

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Lineweaver-Burk Plot (Contd)

• KM is the ________________________________________
  
• the greater the value of KM, the ________ tightly
  S is bound to E
• Vmax is the ___________________________________

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Turnover Numbers

• Vmax is related to the _________________________
  __ of enzyme also called kcat
• Number of moles of substrate that react to form
  product _________________________________________
  ____

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Enzyme Inhibition

• ____________ inhibitor a substance that binds to
  an enzyme to inhibit it, but can be released
• ____________________________ inhibitor binds to
  the active (catalytic) site and blocks access to
  it by substrate
• _____________________ inhibitor binds to a site
  other than the active site inhibits the enzyme
  by changing its conformation
• ________________________inhibitor a substance
  that causes inhibition that cannot be reversed
• usually involves formation or breaking of
  covalent bonds to or on the enzyme

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Competitive Inhibition

• Substrate competes with inhibitor for the active
  site more substrate is required to reach a given
  reaction velocity
• We can write a dissociation constant, KI for EI

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Competitive Inhibition
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Competitive Inhibition

• In a Lineweaver-Burk plot of 1/V vs 1/S, the
  __________________ (and the x intercept) changes
  but the ______________________ does not change

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A Lineweaver-Burke Plot, Competitive Inhibition
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Noncompetitive Inhibition (Contd)

• Several equilibria are involved
• The maximum velocity Vmax has the form

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Noncompetitive Inhibition (Contd)
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Lineweaver-Burke Plot, Noncompetitive Inhibition

• Because the inhibitor does not interfere with
  ______________ to the active site, KM is
  ______________________
• Increasing substrate concentration
  ____________________ noncompetitive inhibition

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Lineweaver-Burke Plot, Noncompetitive Inhibition
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Other Types of Inhibition

• _____________________ - inhibitor can bind to the
  ES complex but not to free enzyme Vmax decreases
  and KM decreases.
• __________________ - Similar to noncompetitive,
  but binding of I affects binding of S and vice
  versa.