Biology 107
Introduction to Metabolism II
September 12, 2005
Nutrients Are Recycled But Energy
Flows Through Biological Systems
Consequence Of Laws Of
Thermodynamics For Living
Organisms
Organisms require a constant input of
energy to maintain a high level of
organization.
Photosynthesis
(Transformation of Light Energy)
6 CO2 + 12 H2O + radiant energy
C6H12O6 + 6 H2O + 6 O2
Stores energy in chemical bonds
Respiration
(Transformation of Chemical Energy)
C6H12O6 + 6 H2O + 6 O2
6 CO2 +12 H2O + energy
Provides energy for “work”
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 enzymecatalyzed 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:
Temperature
pH
Substrate concentration
Cofactor concentration
Enzyme concentration
The Enzyme Active Site Binds the Substrate(s)
Example of Enzyme-catalyzed Reaction
Catalytic Cycle
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
Specific Localization of Enzymes
Within the Cell
Some enzymes are
soluble individual
molecules, some are
grouped into
complexes, some
are incorporated into
membranes, and
others are contained
inside organelles.