Biology 107 Introduction to Metabolism II

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Biology 107Introduction to Metabolism II

• September 15, 2003

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Introduction to Metabolism II

• Student Objectives As a result of this lecture
  and the assigned reading, you should understand
  the following
• Enzymes act as catalysts (i.e., they participate
  in the reaction but are not reactants enzymes
  are not consumed or transformed chemically in
  reactions they catalyze).
• Enzymes are proteins that increase the speed of
  the reaction by lowering the activation energy
  necessary for the reaction.

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Introduction to Metabolism II

• Properties of enzymes
• a. Enzymes combine briefly with reactants during
  enzyme- catalyzed reactions.
• b. Enzymes are relatively unchanged after
  catalyzing the conversion of reactants to
  products.
• c. Enzymes are specific in their activity each
  enzyme catalyzes the reaction of a single type
  of molecule or a group of closely related
  molecules.
• d. Enzymes are saturated by high substrate
  concentrations.
• e. Many enzymes require non-protein groups,
  cofactors. Inorganic cofactors are metallic
  ions. Organic cofactors, coenzymes, are complex
  groups derived from vitamins.

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Introduction to Metabolism II

• Conditions affecting enzyme activity include 1)
  substrate concentration 2) temperature 3) pH
  4) cofactor concentrations.
• Enzyme inhibitors can interfere with the activity
  of enzymes the inhibitors may be of two types
  1) competitive inhibitors or 2) noncompetitive
  inhibitors.
• Oxidation-reduction reactions (redox reactions)
  - Oxidation the loss of electrons by a
  molecule, while reduction gain of electrons.
  In living systems, the energy-capturing reactions
  (photosynthesis) and energy-releasing reactions
  (glycolysis and respiration) are
  oxidation-reduction reactions.

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Introduction to Metabolism II

1. In an endergonic biosynthetic reaction, the
   electrons forming the chemical bonds of the
   product are at a higher energy level than the
   electrons of the reactants (i.e., the reaction
   requires input of energy).
2. Cells supply the energy for endergonic reactions
   through coupled reactions in which endergonic
   reactions are linked to exergonic reactions.

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Enzymes Lower Activation Energy But They Do Not
Change the Overall Energy Profile
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The Enzyme Active Site Binds the Substrate(s)
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The Reaction Environment Will Influence Reaction
Rates
Factors that may influence enzymatic reaction
rates include Temperature pH Substrate
concentration Cofactor concentration Enzyme
concentration
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The Enzyme Active Site Binds the Substrate(s)
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Example of Enzyme-catalyzed Reaction
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Competitive Inhibitors Bind to the Active Site of
an Enzyme
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Non-competitive (Allosteric) Inhibitors Bind to
Enzyme Sites Other Than the Active Site
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Comparison of Competitive and Non-competitive
Inhibition of Enzymes
Inhibitors are specific for single enzymes or
closely related (structurally) enzymes. An
enzyme may have sites for both competitive and
non-competitive inhibitors
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Allosteric Binding by Some Molecules Activates or
Stabilize Enzymes while Other Molecules May
Inhibit Enzymes
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Inhibitors for a Pathway May Be End-products That
Feedback to Turn Off the Pathway
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