Biology 107 Introduction to Metabolism II September 15, 2003

Introduction to Metabolism II Student Objectives:As a result of this lecture and the assigned reading, you should understand the following: 1.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). 2.Enzymes are proteins that increase the speed of the reaction by lowering the activation energy necessary for the reaction.

Introduction to Metabolism II 3.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.

Introduction to Metabolism II 4.Conditions affecting enzyme activity include: 1) substrate concentration; 2) temperature; 3) pH; 4) cofactor concentrations. 5.Enzyme inhibitors can interfere with the activity of enzymes; the inhibitors may be of two types: 1) competitive inhibitors or 2) noncompetitive inhibitors. 6.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.

Introduction 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 enzymatic reaction rates include: TemperaturepH Substrate concentration Cofactor concentration Enzyme concentration

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 single enzymes or closely related (structurally) enzymes. An enzyme may have sites for both competitive and non-competitive inhibitors

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