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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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