1. An Introduction to Metabolism
Chapter 8:
An Introduction to Metabolism

2. Marie Maynard Daly – American biochemist who did extensive research into the metabolism of the arterial wall in the 1960s. Her worked helped to uncover several processes related to aging.

3. 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
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4. Bioluminescence by a fungus

5. Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics
Metabolism is the totality of an organism’s chemical reactions
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6. 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
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7. Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3
Figure 8.UN01
Enzyme 1
Enzyme 2
Enzyme 3 A B C D
Reaction 1
Reaction 2
Reaction 3
Starting molecule
Product
Figure 8.UN01 In-text figure, p. 142 7

8. Catabolic pathways release energy by breaking down complex molecules into simpler compounds
Cellular respiration, the breakdown of glucose in the presence of oxygen, is an example of a pathway of catabolism
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9. The synthesis of protein from amino acids is an example of anabolism
Anabolic pathways consume energy to build complex molecules from simpler ones
The synthesis of protein from amino acids is an example of anabolism
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10. Forms of Energy Energy is the capacity to cause change
Energy exists in various forms, some of which can perform work
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
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11. Figure 8.2 Transformations between kinetic and potential energy
On the platform, a 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, a diver has
less potential energy.

12. The two laws of thermodynamics
Figure 8.3
The two laws of thermodynamics
Heat
Chemical energy
Figure 8.3 The two laws of thermodynamics.
(a) First law of thermodynamics
(b) Second law of thermodynamics 12

13. The First Law of Thermodynamics
According to the first law of thermodynamics, the energy of the universe is constant
Energy can be transferred and transformed, but it cannot be created or destroyed
The first law is also called the principle of conservation of energy
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14. The Second Law of Thermodynamics
During every energy transfer or transformation, some energy is unusable, and is often lost as heat
According to the second law of thermodynamics
Every energy transfer or transformation increases the entropy (disorder) of the universe
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15. Exergonic and Endergonic Reactions in Metabolism
An exergonic reaction proceeds with a net release of free energy and is spontaneous
An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous
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16. Figure 8.6 Free energy changes (G) in exergonic and endergonic reactions
(a) Exergonic reaction: energy released
Reactants
Products
Energy
Progress of the reaction
Amount of
energy
released (∆G<0)
Free energy
released (∆G>0)
(b) Endergonic reaction: energy required

17. Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions
A cell does three main kinds of work
Chemical
Transport
Mechanical
To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one
Most energy coupling in cells is mediated by ATP
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18. Figure 8.8 The structure of adenosine triphosphate (ATP)CH –O O
CH2 H OH N C HC
NH2
Adenine
Ribose O– P
Phosphate groups

19. Figure 8.9 The hydrolysis of ATP
Adenosine triphosphate (ATP)
H2O +
Energy
Inorganic phosphate
Adenosine diphosphate (ADP)
P i

20. Figure 8.11 How ATP drives cellular work
Motor protein
P i
Protein moved
(a) Mechanical work: ATP phosphorylates motor proteins
Membrane
protein
ATP
Solute
P i
ADP +
Solute transported
(b) Transport work: ATP phosphorylates transport proteins
Glu
NH3
NH2
(c) Chemical work: ATP phosphorylates key reactants
Reactants: Glutamic acid
and ammonia
Product (glutamine)
made +

21. Figure 8.12 The ATP cycle ATP synthesis from ADP + P i requires energy
Energy for cellular work
(endergonic, energy-
consuming processes)
Energy from catabolism
(exergonic, energy yielding
processes)
ATP hydrolysis to
ADP + P i yields energy

22. Concept 8.4: 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
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23. Figure 8.13 The effect of enzymes on reaction rate.
Progress of the reaction
Products
Course of
reaction
without
enzyme
Reactants
with enzyme EA
EA with
is lower
∆G is unaffected
by enzyme
Free energy

24. Figure 8.14 Induced fit between an enzyme and its substrate
Active site
Enzyme
(a)
(b)
Enzyme- substrate
complex

25. Figure 8.15 The active site and catalytic cycle of an enzyme
1 Substrates enter active site; enzyme
changes shape so its active site
embraces the substrates (induced fit).
Substrates
Products
Enzyme
Enzyme-substrate
complex
5 Products are
Released.
2 Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
3 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.
4 Substrates are
Converted into
Products.
6 Active site
is available for
two new substrate
molecules.

26. Figure 8.16 Environmental factors affecting enzyme activity
Optimal pH for two enzymes
Rate of reaction 20 40 60 80
100
Temperature (Cº)
(a) Optimal temperature for two enzymes
(b) Optimal pH for two enzymes pH
Optimal temperature for
typical human enzyme
enzyme of thermophilic
Optimal pH for pepsin
(stomach enzyme)
Optimal pH
for trypsin
(intestinal
enzyme) 1 2 3 4 5 6 7 8 9 10
(heat-tolerant)
bacteria

27. Cofactors & Coenzymes Cofactors are nonprotein enzyme helpers
Cofactors may be inorganic (such as a mineral in ionic form) or organic
An organic cofactor is called a coenzyme
Coenzymes include vitamins
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28. 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
Examples of inhibitors include toxins, poisons, pesticides, and antibiotics
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29. Figure 8.17 Inhibition of enzyme activity
Normal binding
(b) Competitive inhibition
A substrate can
bind normally to the
active site of an
enzyme.
A competitive
inhibitor mimics the
substrate, competing
for the active site.
Substrate
Active site
Enzyme
Noncompetitive inhibitor
(c) Noncompetitive inhibition
Competitive
inhibitor
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.

30. Allosteric Regulation of Enzymes
Allosteric regulation may either inhibit or stimulate an enzyme’s activity
Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site
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31. Figure 8.19 Allosteric regulation of enzyme activity
Stabilized inactive form
Allosteric activater stabilizes active from
Allosteric enyzme with four subunits
Active site (one of four)
Regulatory site (one of four)
Active form
Activator
Stabilized active form
Allosteric activater stabilizes active form
Inhibitor
Inactive form
Non- functional active site
(a) Allosteric activators and inhibitors. In the cell, activators and inhibitors dissociate when at low concentrations. The enzyme can then oscillate again.
Oscillation

32. Stabilized active form
Binding of one substrate molecule to active site of one subunit locks all subunits in active conformation.
Substrate
Inactive form
Stabilized active form
(b) Cooperativity: another type of allosteric activation. Note that the inactive form shown on the left oscillates back and forth with the active form when the active form is not stabilized by substrate.