Biology 107Biology 107

Introduction to Metabolism IIIntroduction to Metabolism II

September 15, 2003September 15, 2003

Introduction to Metabolism IIIntroduction to Metabolism II

Student Objectives:Student Objectives: As a result of this lecture and the assigned As a result of this lecture and the assigned reading, you should understand the following:reading, you should understand the following:

1.1. Enzymes act as Enzymes act as catalystscatalysts (i.e., they participate in the reaction but (i.e., they participate in the reaction but are not reactants; enzymes are not consumed or transformed are not reactants; enzymes are not consumed or transformed chemically in reactions they catalyze). chemically in reactions they catalyze).

2.2. Enzymes are proteins that increase the speed of the reaction by Enzymes are proteins that increase the speed of the reaction by lowering the activation energy necessary for the reactionlowering the activation energy necessary for the reaction..

Introduction to Metabolism IIIntroduction to Metabolism II

3.3. Properties of enzymes:Properties of enzymes:

a.a. Enzymes combine briefly with reactants during enzyme-Enzymes combine briefly with reactants during enzyme-catalyzed reactions.catalyzed reactions.

b.b. Enzymes are relatively unchanged after catalyzing the Enzymes are relatively unchanged after catalyzing the conversion of reactants to products.conversion of reactants to products.

c.c. Enzymes are specific in their activity; each enzyme Enzymes are specific in their activity; each enzyme catalyzes catalyzes the reaction of a single type of molecule or a the reaction of a single type of molecule or a group of closely group of closely related molecules.related molecules.

d.d. Enzymes are saturated by high substrate concentrations.Enzymes are saturated by high substrate concentrations.

e.e. Many enzymes require non-protein groups, Many enzymes require non-protein groups, cofactorscofactors. . Inorganic cofactors are metallic ions. Organic cofactors, Inorganic cofactors are metallic ions. Organic cofactors, coenzymescoenzymes, are complex groups derived from vitamins., are complex groups derived from vitamins.

Introduction to Metabolism IIIntroduction to Metabolism II

4.4. Conditions affecting enzyme activity include: 1) substrate Conditions affecting enzyme activity include: 1) substrate concentration; 2) temperature; 3) pH; 4) cofactor concentrations.concentration; 2) temperature; 3) pH; 4) cofactor concentrations.

5.5. Enzyme inhibitors can interfere with the activity of enzymes; the Enzyme inhibitors can interfere with the activity of enzymes; the inhibitors may be of two types: 1) inhibitors may be of two types: 1) competitive inhibitorscompetitive inhibitors or 2) or 2) noncompetitive inhibitorsnoncompetitive inhibitors..

6.6. Oxidation-reduction reactions (redox reactions) - Oxidation-reduction reactions (redox reactions) - OxidationOxidation = the = the loss of electrons by a molecule, while loss of electrons by a molecule, while reductionreduction = gain of = gain of electrons. In living systems, the energy-capturing reactions electrons. In living systems, the energy-capturing reactions (photosynthesis) and energy-releasing reactions (glycolysis and (photosynthesis) and energy-releasing reactions (glycolysis and respiration) are oxidation-reduction reactions.respiration) are oxidation-reduction reactions.

Introduction to Metabolism IIIntroduction to Metabolism II

9. 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).

10. Cells supply the energy for endergonic reactions through coupled reactions in which endergonic reactions are linked to exergonic reactions.

Enzymes Lower Activation Energy But They Do Not Change the Overall Energy Profile

The Enzyme Active Site Binds the Substrate(s)

The Reaction Environment Will Influence

Reaction Rates

Factors that may influence Factors that may influence enzymatic reaction rates enzymatic reaction rates include:include:

TemperatureTemperature

pHpH

Substrate Substrate concentrationconcentration

Cofactor Cofactor concentrationconcentration

Enzyme Enzyme concentrationconcentration

The Enzyme Active Site Binds the Substrate(s)

Example of Enzyme-catalyzed Reaction

Competitive Inhibitors Bind to the Active Site of an Enzyme

Non-competitive (Allosteric) Inhibitors Bind to Enzyme Sites Other Than the Active Site

Comparison of Competitive and Non-competitive

Inhibition of Enzymes

Inhibitors are specific for Inhibitors are specific for single enzymes or closely single enzymes or closely related (structurally) related (structurally) enzymes.enzymes.

An enzyme may have An enzyme may have sites for both competitive sites for both competitive and non-competitive and non-competitive inhibitorsinhibitors

Allosteric Binding by Some Molecules Activates or Stabilize Enzymes while Other Molecules May

Inhibit Enzymes

Inhibitors for a Pathway May Be

End-products That “Feedback” to Turn

Off the Pathway