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Introduction to Metabolism - Chapter 6
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: The Energy of Life
• The living cell is a miniature chemical factory
where thousands of reactions occur
• The cell extracts energy and applies energy to
perform work
• Some organisms even convert energy to light, as
in bioluminescence
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 6.1: An organism’s metabolism transforms matter
and energy, subject to the laws of thermodynamics
• Metabolism is the totality of an organism’s
chemical reactions
• Metabolism is an emergent property of life that
arises from interactions between molecules within
the cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Organization of the Chemistry of Life into
Metabolic Pathways
• A metabolic pathway begins with a specific
molecule and ends with a product
• Each step is catalyzed by a specific enzyme
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Enzyme 1
A
B
Reaction 1
Starting
molecule
Enzyme 2
Enzyme 3
D
C
Reaction 2
Reaction 3
Product
• Catabolic pathways release energy by breaking
down complex molecules into simpler compounds
• Anabolic pathways consume energy to build
complex molecules from simpler ones
• Bioenergetics is the study of how organisms
manage their energy resources
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Kinetic energy is energy associated with motion
– Heat (thermal energy) is kinetic energy associated
with random movement of atoms or molecules
• Potential energy is energy that matter possesses
because of its location or structure
– Chemical energy is potential energy available for
release in a chemical reaction
• Energy can be converted from one form to another
Animation: Energy Concepts
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
On the platform,
the diver has
more potential
energy.
Diving converts
potential
energy to
kinetic energy.
Climbing up converts
kinetic energy of
muscle movement to
potential energy.
In the water, the
diver has less
potential energy.
Concept 6.2: The free-energy change of a reaction tells
us whether the reaction occurs spontaneously
• Biologists want to know which reactions occur
spontaneously and which require input of energy
• To do so, they need to determine energy changes
that occur in chemical reactions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The change in free energy (∆G) during a process
is related to the change in enthalpy, or change in
total energy (∆H), and change in entropy (T∆S):
∆G = ∆H - T∆S
• Only processes with a negative ∆G are
spontaneous
• Spontaneous processes can be harnessed to
perform work
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Free Energy, Stability, and Equilibrium
• Free energy is a measure of a system’s instability,
its tendency to change to a more stable state
• During a spontaneous change, free energy
decreases and the stability of a system increases
• Equilibrium is a state of maximum stability
• A process is spontaneous and can perform work
only when it is moving toward equilibrium
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gravitational motion
Diffusion
Chemical reaction
Concept 6.3: ATP powers cellular work by coupling
exergonic reactions to endergonic reactions
• A cell does three main kinds of work:
– Mechanical
– Transport
– Chemical
• To do work, cells manage energy resources by
energy coupling, the use of an exergonic process
to drive an endergonic one
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Adenine
Phosphate groups
Ribose
P
P
P
Adenosine triphosphate (ATP)
H2O
Pi
+
Inorganic phosphate
P
P
Adenosine diphosphate (ADP)
+
Energy
Pi
P
Motor protein
Protein moved
Mechanical work: ATP phosphorylates motor proteins
Membrane
protein
ADP
+
Pi
ATP
Pi
P
Solute transported
Solute
Transport work: ATP phosphorylates transport proteins
P
NH2
Glu
+
NH3
+
Pi
Glu
Reactants: Glutamic acid
and ammonia
Product (glutamine)
made
Chemical work: ATP phosphorylates key reactants
ATP
Energy for cellular work
(endergonic, energyconsuming processes)
Energy from catabolism
(energonic, energyyielding processes)
ADP +
P
i
Concept 6.4: Enzymes speed up metabolic
reactions by lowering energy barriers
• A catalyst is a chemical agent that speeds up a
reaction without being consumed by the reaction
• An enzyme is a catalytic protein
• Hydrolysis of sucrose by the enzyme sucrase is an
example of an enzyme-catalyzed reaction
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sucrose
C12H22O11
Glucose
C6H12O6
Fructose
C6H12O6
The Activation Energy Barrier
• Every chemical reaction between molecules
involves bond breaking and bond forming
• The initial energy needed to start a chemical
reaction is called the free energy of activation, or
activation energy (EA)
• Activation energy is often supplied in the form of
heat from the surroundings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A
B
C
D
Free energy
Transition state
A
B
C
D
EA
Reactants
A
B
DG < O
C
D
Products
Progress of the reaction
Free energy
Course of
reaction
without
enzyme
EA
without
enzyme
EA with
enzyme
is lower
Reactants
Course of
reaction
with enzyme
DG is unaffected
by enzyme
Products
Progress of the reaction
Substrate Specificity of Enzymes
• The reactant that an enzyme acts on is called the
enzyme’s substrate
• The enzyme binds to its substrate, forming an
enzyme-substrate complex
• The active site is the region on the enzyme where
the substrate binds
• Induced fit of a substrate brings chemical groups
of the active site into positions that enhance their
ability to catalyze the reaction
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Substrate
Active site
Enzyme
Enzyme-substrate
complex
Substrates enter active site; enzyme
changes shape so its active site
embraces the substrates (induced fit).
Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
Substrates
Enzyme-substrate
complex
Active
site is
available
for two new
substrate
molecules.
Enzyme
Products are
released.
Substrates are
converted into
products.
Products
Active site (and R groups of
its amino acids) can lower EA
and speed up a reaction by
• acting as a template for
substrate orientation,
• stressing the substrates
and stabilizing the
transition state,
• providing a favorable
microenvironment,
• participating directly in the
catalytic reaction.
Effects of Local Conditions on Enzyme Activity
• An enzyme’s activity can be affected by:
– General environmental factors, such as
temperature and pH
– Chemicals that specifically influence the
enzyme
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Effects of Temperature and pH
• Each enzyme has an optimal temperature in which
it can function
• Each enzyme has an optimal pH in which it can
function
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Optimal temperature for
typical human enzyme
0
Optimal temperature for
enzyme of thermophilic
(heat-tolerant
bacteria)
40
60
Temperature (°C)
20
80
100
Optimal temperature for two enzymes
Optimal pH for pepsin
(stomach enzyme)
0
1
2
3
4
Optimal pH
for trypsin
(intestinal
enzyme)
5
pH
Optimal pH for two enzymes
6
7
8
9
10
Cofactors
• Cofactors are nonprotein enzyme helpers
• Coenzymes are organic cofactors
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Enzyme Inhibitors
• Competitive inhibitors bind to the active site of an
enzyme, competing with the substrate
• Noncompetitive inhibitors bind to another part of
an enzyme, causing the enzyme to change shape
and making the active site less effective
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Active site
Enzyme
Normal binding
A competitive
inhibitor mimics the
substrate, competing
for the active site.
Competitive
inhibitor
Competitive inhibition
A noncompetitive
inhibitor binds to the
enzyme away from the
active site, altering the
conformation of the
enzyme so that its
active site no longer
functions.
Noncompetitive inhibitor
Noncompetitive inhibition
Concept 6.5: Regulation of enzyme activity helps
control metabolism
• Chemical chaos would result if a cell’s metabolic
pathways were not tightly regulated
• To regulate metabolic pathways, the cell switches
on or off the genes that encode specific enzymes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Initial substrate
(threonine)
Active site
available
Isoleucine
used up by
cell
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Intermediate A
Feedback
inhibition
Enzyme 2
Active site of
enzyme 1 can’t
bind
Intermediate B
theonine
pathway off
Enzyme 3
Isoleucine
binds to
allosteric
site
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)