Download Ch. 5: Energy and Enzymes

Energy and Chemical Reactions
Ch. 5
Pre-AP Biology
Ms. Haut
Laws of Thermodynamics
• The study of energy
• First Law of Thermodynamics
(Conservation of Energy)
– Energy cannot be created or destroyed
However, Energy can be changed from one
form to another
Figure 5.2A
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Laws of Thermodynamics
• Second Law of Thermodynamics
– Energy changes are not 100% efficient
– Energy conversions increase disorder, or
entropy
– Some energy is always lost as heat
Figure 5.2B
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Energy
• Energy—the capacity to
do work
– Kinetic energy—
energy of motion
(heat)
– Potential energy—
stored capacity to
perform work
(chemical energy
stored in chemical
bonds)
Figure 5.1x3
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Chemical Reactions
• Cells carry out thousands of chemical reactions
– The sum of these reactions constitutes cellular
metabolism
• Chemical reactions either store or release energy
– Endergonic reactions—absorb and store energy in a
reaction
– Exergonic reactions—energy is released from
reaction
– Metabolism = endergonic rxns + exergonic rxns
Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings
Energy in Cells
• ATP is used to shuttle chemical energy within
the cell
• ATP—energy-rich covalent bonds b/w
outer 2 phosphate bonds
• ATP, molecular model
Figure 5.4Ax
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
•When ATP gives up its energy, it forms ADP and
an energy shuttle, the phosphate group.
•Energy coupling—using energy released from
exergonic reactions to drive endergonic reactions
Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings
• For a chemical reaction to begin, reactants must
absorb some energy
Enzyme
– This energy is called the energy of activation (EA)
– This represents the energy barrier that prevents
molecules from breaking down spontaneously
EA
barrier
Reactants
1
Products
2
Figure 5.5A
•A protein catalyst called an enzyme can decrease the energy
barrier
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Enzyme—catalytic protein that speeds up
the chemical reactions by lowering the
activation energy (end with –ase)
Activation energy (EA)—amount of energy
needed to start a chemical reaction
before the reaction will proceed
Energy Profile
Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings
Enzymes are substrate-specific—has
unique 3-D shape which determines
what the enzyme works on (substrate)
• The 3-D shape of the enzyme creates
a “pocket” called the Active Site in
which the substrate binds
• Active site has
particular amino
acid side-chains
that match up
with side-chains
of substrate
http://www.pickens.k12.sc.us/phsteachers/edmunds/Enzyme%20Active%
20Site.gif
”Induced Fit Model”
• How an enzyme
works
• The enzyme
is unchanged and
can repeat the
process
Copyright © 2001 Pearson Education,
Inc. publishing Benjamin Cummings
Affecting Enzyme Function
Pre-AP Biology
Ms. Haut
•Enzymes speed up the rate of chemical
reactions because they lower the energy of
activation.
•Enzymes lower the energy of activation by
forming an enzyme-substrate complex.
Enzyme Activity
• Enzyme activity is influenced by
– temperature
– salt concentration
– pH
– Presence of other molecules
Enzyme Activity
• A cell’s physical and chemical
environment affects enzyme activity
• Each enzyme has optimal
environmental conditions that favor the
most active enzyme conformation
(shape)
Effects of Temperature
• Optimal temp. allows greatest number
of molecular collisions without
denaturing the enzyme
Reaction rate  when temperature 
Kinetic energy increases and collisions
increase
Beyond optimal temperature, reaction
rate slows
Too low, collisions b/w substrate and active
site don’t occur fast enough
Too high, agitation disrupts weak bonds of the
tertiary structure of enzyme (enzyme unfolds)
Effects of pH
• Optimal pH range for most enzymes
is pH 6 - 8
Beyond optimal pH, reaction rate slows
Too low (acidic) H+ ions interact with
amino acid side-chains and disrupt weak
bonds of the tertiary structure of enzyme
Too high (basic) base interacts with H+
ions on amino acid side-chains and disrupt
weak bonds of the tertiary structure of
enzyme
Enzyme Inhibitors
• Certain chemicals selectively inhibit
enzyme activity
– Irreversible—inhibitor attaches by covalent
bonds
– Reversible—inhibitor attaches by weak
bonds
Effects of Enzyme Inhibitors
• Competitive inhibitors—
chemicals that resemble
an enzyme’s normal
substrate and compete
with it for the active site
• Blocks active site from
substrate
• If reversible, can be
overcome by
increasing substrate
concentration
Competitive Inhibitor
Effects of Enzyme Inhibitors
• Noncompetitive inhibitors—
chemicals that bind to another
part of the enzyme molecule
(allosteric site)
• Causes enzyme to change
shape so active site can’t
bind substrate
• May act as metabolic
poison (DDT, many
antibiotics)
• Essential mechanism in
cell’s regulating metabolic
reactions
Negative Feedback
Metabolic Control often Depends on
Allosteric Regulation
• Allosteric enzymes have 2 shapes,
catalytically active and inactive
• Binding of an activator to the allosteric
site stabilizes active shape
• Binding of an inhibitor (noncompetitive)
to the allosteric site stabilizes inactive
shape
Connection: Some pesticides and
antibiotics inhibit enzymes
• Certain pesticides are toxic to insects
because they inhibit key enzymes in the
nervous system
• Many antibiotics inhibit enzymes that are
essential to the survival of disease-causing
bacteria
– Penicillin inhibits an enzyme that bacteria use
in making cell walls
Control of Metabolism
• Feedback inhibition—regulation of a
metabolic pathway by its end product,
which inhibits an enzyme within the
pathway
Prevents cell from wasting chemical
resources synthesizing more product
than is needed
Threonine
Enzyme
1
A
Enzyme
2
Enzyme
3
B
Enzyme
4
C
Enzyme
5
D
isoleucine
Negative Feedback inhibition
Acknowledgements
• Unless otherwise noted, illustrations are credited to Pearson Education
which have been borrowed from BIOLOGY: CONCEPTS AND
CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and
Taylor, ©2003. These images have been produced from the originals
by permission of the publisher. These illustrations may not be
reproduced in any format for any purpose without express written
permission from the publisher.
• BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by
Campbell, Reece, Mitchell, and Taylor, ©2001. These images have
been produced from the originals by permission of the publisher. These
illustrations may not be reproduced in any format for any purpose
without express written permission from the publisher.
•