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Concepts in Biochemistry
Third Edition
Chapter 5:
Enzymes I: Kinetics, Mechanism, andInhibition
Copyright 2006 by John Wiley & Sons, Inc.
Rodney Boyer
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Enzymes
Biological catalyst
Protein in nature
Reaction specific Nomenclature: often suffix ase (urease,
amylase)
Ribozymes also biological catalyst; RNA
Provide an alternative pathway that will lower
the activation energy of reaction
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Enzymes and cofactorsSome enzymes are active in their native state (protein
enzyme is active)
Some enzymes require another chemical entity for
activity called cofactorProtein + cofactor = holoenzyme (active)
Protein component = apoenzyme (inactive)
Cofactors metals (Zn,Cu, Mn, Ca, Mg)
- organic molecules (coenzymes: NAD, FAD,Biotin, pyridoxal phosphate refer to Table 6.1)
Prosthetic group cofactor covalently bonded to the
protein
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In an enzyme-catalyzed reaction
Substrate, S: a reactant
Active site: 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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Two models of the formation of ES
Lock-and-key model: substrate binds to that
portion of the enzyme with a complementary
shape
Induced fit model: binding of the substrate
induces a change in the conformation of the
enzyme that results in a complementary fit
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Point at which the rate of
reaction does not change,
enzyme is saturated, maximum
rate of reaction is reached
Vmax [S]V =
KM + [S]
(an equation for a hyperbola)
Michaelis-Menten Equation
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Vmax = K2[E]total
K2 or Kcat = turn over number
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Transform hyperbolic equation to a
linear equationVmax [S]
Vinit =KM + [S]
Michaelis-Mentenequation
V1 =
KM + [S]
Vmax [S]=
KM [S]
Vmax [S] Vmax [S]+
V1 =
KM
Vmax [S] Vmax
+1
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Enzyme inhibition
Inhibitors
Types of E inhibition
Irreversible
Reversible
Competitive
Non competitive (pure and mixed)
Uncompetitive
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Mixed non competitive inhibitor affects the binding
of the substrate
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