Download 20.5 Strategies of Metabolism: ATP and Energy

Outline
20.1
20.2
20.3
20.4
20.5
20.6
20.7
20.8
20.9
20.10
Energy and Life
Energy and Biochemical Reactions
Cells and Their Structure
An Overview of Metabolism and Energy Production
Strategies of Metabolism: ATP and Energy Transfer
Strategies of Metabolism: Metabolic Pathways and
Coupled Reactions
Strategies of Metabolism: Oxidized and Reduced
Coenzymes
The Citric Acid Cycle
The Electron Transport Chain and ATP Production
Harmful Oxygen By-Products and Antioxidant
Vitamins
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Goals
1. What is the source of our energy, and what is its
fate in the body?
Be able to provide an overview of the sources of our
energy and how we use it, identify the cellular location
of energy generation, and explain the significance of
exergonic and endergonic reactions in metabolism.
2. How are the reactions that break down food
molecules organized?
Be able to list the stages in catabolism and describe the
role of each.
3. What are the major strategies of metabolism?
Be able to explain and give examples of the roles of
ATP, coupled reactions, and oxidized and reduced
coenzymes in metabolic pathways.
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Goals
4.
5.
6.
What is the citric acid cycle?
Be able to describe what happens in the citric acid cycle
and explain its role in energy production.
How is ATP generated in the final stage of
catabolism?
Be able to describe in general the electron-transport
chain, oxidative phosphorylation, and how they are
coupled.
What are the harmful by-products produced from
oxygen, and what protects against them?
Be able to identify the highly reactive oxygen-containing
products formed during metabolism and the enzymes
and vitamins that counteract them.
© 2013 Pearson Education, Inc.
20.1 Energy and Life
• Living things do mechanical and chemical
work, synthesizing molecules and moving
them across cell membranes.
• The energy used by all but a very few
living things on earth comes from the sun.
• Plants convert sunlight to potential energy
stored in the bonds of carbohydrates.
• Animals use this energy, and store the
excess in the bonds of fats.
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20.1 Energy and Life
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20.1 Energy and Life
• Our bodies have specific requirements for
energy.
– Energy must be released from food gradually.
– Energy must be stored in accessible forms.
– Release of energy must be finely controlled.
– Just enough energy must be released as heat
to maintain constant body temperature.
– Energy must be available to drive chemical
reactions that are not favorable at body
temperature.
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20.2 Energy and Biochemical Reactions
• Chemical reactions either release or
absorb energy according to the formula:
ΔG = ΔH – TΔS
• Reactions in living organisms are no
different from reactions in a chemistry
laboratory.
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20.2 Energy and Biochemical Reactions
• Spontaneous reactions release free
energy. Exergonic reactions are the
source of biochemical energy.
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20.2 Energy and Biochemical Reactions
• The greater the amount of free energy
released, the further a reaction proceeds
toward product formation before reaching
equilibrium.
• Reactions requiring an input of energy are
endergonic.
• Free energy changes switch sign for the
reverse of the reaction, but the value does
not change.
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20.2 Energy and Biochemical Reactions
• Living systems make use of this in
biochemical pathways.
• Energy is stored in the products of an
overall endergonic reaction pathway.
• This stored energy is released in an
overall exergonic reaction pathway that
regenerates the original reactants.
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20.2 Energy and Biochemical Reactions
• Endergonic—A nonspontaneous reaction
or process that absorbs free energy and
has a positive ΔG.
• Exergonic— A spontaneous reaction or
process that releases free energy and has
a negative ΔG.
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20.2 Energy and Biochemical Reactions
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Life without Sunlight
In 1977, hydrothermal vents—openings spewing water heated to 400 °C
deep within the earth—were found on the ocean floor. The vents were
dubbed “black smokers” because the water was black with mineral sulfides
precipitating from the hot, acidic water as it exited the vents.
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Distinctive types of bacteria form the basis for the web of life in these
locations. What replaces sunlight as their source of energy? The hot water
is rich in dissolved inorganic substances that are reducing agents and
electron donors. Life-supporting energy is set free by their oxidation.
•
Carbon dioxide dissolved in the seawater is the raw material used by the
bacteria to make their own essential carbon-containing biomolecules.
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In 1991, scientists discovered a volcano erupting underneath the ocean.
Initially, all life in the vicinity was wiped out, yet soon afterward, the area
was thriving with bacteria. Could it be that a thriving population of bacteria
has been living in the hot interior of the earth ever since it formed? Were
these anaerobic bacteria earth’s first inhabitants, and could they exist
beneath the surface of other planets? Research will eventually answer
these questions.
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20.3 Cells and Their Structure
• There are two main categories of cells:
prokaryotic and eukaryotic.
• Prokaryotic cells are usually found in
single-celled organisms (bacteria, bluegreen algae).
• Eukaryotic cells are found in single-celled
yeast, and in all plants and animals.
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20.3 Cells and Their Structure
• Eukaryotic cells are about 1000 times
larger than prokaryotic cells.
• Features include:
– Membrane-enclosed nucleus
– Organelles are small, functional units that
perform specific tasks.
– Cytoplasm is the region between the cell and
nuclear membranes.
– Cytosol is the fluid part of the cytoplasm, with
electrolytes, nutrients and enzymes in
solution.
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20.3 Cells and Their Structure
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20.3 Cells and Their Structure
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20.3 Cells and Their Structure
• Mitochondria, the cell’s “power plants,”
are the most important of the organelles
for energy production.
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20.3 Cells and Their Structure
• The citric acid cycle takes place in the
matrix.
• Electron transport and ATP production
take place at the inner surface of the inner
membrane.
• The numerous folds in the inner
membrane—known as cristae—increase
the surface area over which these
pathways can take place.
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20.4 An Overview of Metabolism and Energy Production
• All the chemical reactions that take place in an
organism constitute its metabolism.
• Most reactions occur in metabolic pathways.
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20.4 An Overview of Metabolism and Energy Production
• Catabolism—Metabolic reaction pathways that break
down food molecules and release biochemical energy.
• Anabolism—Metabolic reactions that build larger
biological molecules from smaller pieces.
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20.4 An Overview of Metabolism and Energy Production
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20.4 An Overview of Metabolism and Energy Production
Stage 1: Digestion
• Enzymes in saliva, the stomach, and the small
intestine convert large molecules to smaller
molecules.
• Carbohydrates are broken down to glucose and
other sugars.
• Proteins are broken down to amino acids, and
triacylglycerols.
• Lipids are broken down to glycerol plus long-chain
carboxylic acids, termed fatty acids.
• These smaller molecules are transferred into the
blood for transport to cells throughout the body.
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20.4 An Overview of Metabolism and Energy Production
Stage 2: Acetyl-Coenzyme A
production
• The small molecules from
digestion follow pathways
that move their carbon
atoms into two-carbon acetyl
groups.
• The acetyl groups are
attached to coenzyme A by
a bond between the sulfur of
the thiol group on coenzyme
A and the carbonyl carbon
atom of the acetyl group.
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20.4 An Overview of Metabolism and Energy Production
Stage 2: Acetyl-Coenzyme A
production
• The small molecules from
digestion follow pathways that
move their carbon atoms into
two-carbon acetyl groups.
• Acetyl groups are attached to
coenzyme A by a bond
between sulfur of the thiol
group on coenzyme A and the
carbonyl carbon of the acetyl.
• Acetyl-CoA is an intermediate
in the metabolism of all food
molecules.
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20.4 An Overview of Metabolism and Energy Production
Stage 3: Citric acid cycle
• Within mitochondria, the acetyl-group
carbon atoms are oxidized to the carbon
dioxide that we exhale.
• Most of the energy released in the
oxidation leaves the citric acid cycle in the
chemical bonds of reduced coenzymes
(NADH, FADH2).
• Some energy leaves the cycle stored in the
chemical bonds of adenosine triphosphate
(ATP) or a related triphosphate.
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20.4 An Overview of Metabolism and Energy Production
Stage 4: ATP production
• Electrons from the reduced coenzymes are passed
from molecule to molecule down an electrontransport chain.
• Their energy is harnessed to produce ATP.
• At the end of the process, these electrons—along
with hydrogen ions from the reduced coenzymes—
combine with oxygen to produce water.
• The reduced coenzymes are oxidized by
atmospheric oxygen, and the energy that they carried
is stored in the chemical bonds of ATP molecules.
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20.5 Strategies of Metabolism: ATP and Energy Transfer
• ATP has three phosphate groups.
• Removal of one of the –PO4 groups by
hydrolysis gives adenosine diphosphate –
ADP.
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20.5 Strategies of Metabolism: ATP and Energy Transfer
ATP + H2O ➝ ADP + Pi
ΔG = –7.3 kcal/mol
ADP + Pi ➝ ATP + H2O
ΔG = +7.3 kcal/mol
• ATP is an energy transporter because its
production from ADP requires an input of
energy that is then released wherever the
reverse reaction occurs.
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20.5 Strategies of Metabolism: ATP and Energy Transfer
• The hydrolysis of ATP to give ADP and its
reverse, the phosphorylation of ADP, are
reactions perfectly suited to their role in
metabolism.
• The stored energy is released only in the
presence of the appropriate enzymes.
• A useful amount of energy is released when
a phosphoryl group is removed from it by
hydrolysis.
• If too much energy was involved,
interconversion would be more difficult.
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20.6 Strategies of Metabolism:
Metabolic Pathways and Coupled Reactions
• The overall free-energy change for any series of
reactions can be found by summing up the freeenergy changes for the individual steps.
• Not every step in a metabolic pathway is downhill.
• Energetically unfavorable reactions are coupled to
energetically favorable reactions so that the overall
energy change is favorable.
• Coupling allows the energy stored in one chemical
compound be transferred to other compounds.
• Excess energy is released as heat and contributes
to maintaining body temperature.
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20.6 Strategies of Metabolism:
Metabolic Pathways and Coupled Reactions
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20.6 Strategies of Metabolism:
Metabolic Pathways and Coupled Reactions
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Basal Metabolism
The minimum amount of energy expenditure required per unit of time to stay
alive is basal metabolic rate. It can be measured by finding the rate of
oxygen consumption, which is proportional to the energy used.
An average basal metabolic rate is 70 kcal/hr (293 kJ/hr), or about 1700
kcal/day (7100 kJ/day): 1 kcal/hr (4.2 kJ/hr) per kilogram of body weight by
a male and 0.95 kcal/hr (4 kJ/hr) per kilogram of body weight by a female.
The total calories a person needs each day is determined by basal
requirements plus energy used in additional physical activities.
A relatively inactive person requires about 30% above basal requirements
per day, a lightly active person requires about 50% above basal, and a very
active person can use 100% above basal requirements in a day.
Each day that you consume food with more calories than you use, the
excess calories are stored as potential energy in the chemical bonds of fats
in your body and your weight rises. Each day that you consume food with
fewer calories than you burn, some chemical energy in your body is taken
out of storage to make up the deficit.
© 2013 Pearson Education, Inc.
20.7 Strategies of Metabolism:
Oxidized and Reduced Coenzymes
• Many metabolic reactions are oxidation–reduction
reactions.
• A steady supply of oxidizing and reducing agents must
be available.
• A few coenzymes cycle continuously between their
oxidized and reduced forms.
© 2013 Pearson Education, Inc.
20.7 Strategies of Metabolism:
Oxidized and Reduced Coenzymes
• Oxidation can be loss of electrons, loss of
hydrogen, or addition of oxygen.
• Reduction can be gain of electrons, gain of
hydrogen, or loss of oxygen.
• Oxidation and reduction always occur together.
• Each increase in the number of carbon–oxygen
bonds is an oxidation, and each decrease in the
number of carbon–hydrogen bonds is a
reduction.
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20.7 Strategies of Metabolism:
Oxidized and Reduced Coenzymes
• Nicotinamide adenine dinucleotide and its phosphate are
coenzymes that enter and leave enzyme active sites in
which they are required for redox reactions.
• As oxidizing agents they remove hydrogen from a
substrate, and as reducing agents (NADH and NADPH)
they provide hydrogen that adds to a substrate.
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20.7 Strategies of Metabolism:
Oxidized and Reduced Coenzymes
• The oxidation of malate to oxaloacetate requires the
removal of two hydrogen atoms to convert a secondary
alcohol to a ketone.
• The oxidizing agent, which will be reduced during the
reaction, is NAD+ functioning as a coenzyme for the
enzyme malate dehydrogenase.
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20.7 Strategies of Metabolism:
Oxidized and Reduced Coenzymes
• A hydrogen atom is equivalent to a hydrogen ion, H+ plus
an electron, e–.
• Flavin adenine dinucleotide (FAD), another common
oxidizing agent, is reduced by the formation of covalent
bonds to two hydrogen atoms to give FADH2.
© 2013 Pearson Education, Inc.
20.7 Strategies of Metabolism:
Oxidized and Reduced Coenzymes
• Because reduced coenzymes have picked up
electrons (in their bonds to hydrogen) that are
passed along in subsequent reactions, they
are often referred to as electron carriers.
• As coenzymes cycle through their oxidized
and reduced forms, they also carry energy
along from reaction to reaction.
• Ultimately, this energy is passed on to the
bonds in ATP.
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20.8 The Citric Acid Cycle
• The acetyl groups in acetyl-SCoA
molecules are readily removed in an
energy-releasing hydrolysis reaction.
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20.8 The Citric Acid Cycle
• Oxidation of 2 carbons to give CO2 and
transfer of energy to reduced coenzymes
occurs in the citric acid cycle.
• This is also known as the tricarboxylic acid
cycle (TCA) or Krebs cycle.
• The citric acid cycle is a closed loop of
reactions in which the product of the final
step, oxaloacetate, is the reactant in the
first step.
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20.8 The Citric Acid Cycle
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20.8 The Citric Acid Cycle
STEPS 1 and 2:
– Acetyl groups enter the cycle at Step 1
by addition to 4-carbon oxaloacetate to
give citrate, a 6-carbon intermediate.
– Citrate is a tertiary alcohol and cannot
be oxidized; it is converted in Step 2 to
its isomer, isocitrate, a secondary
alcohol that can be oxidized to a ketone.
The isomerization is catalyzed by
aconitase.
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20.8 The Citric Acid Cycle
STEPS 3 and 4:
– Both steps are oxidations that rely on NAD+
as the oxidizing agent.
– One CO2 leaves at Step 3 as the —OH group
of isocitrate is oxidized to a keto group.
– A second CO2 leaves at Step 4, and the
resulting succinyl group is added to coenzyme
A. In both steps, electrons and energy are
transferred.
– Succinyl-CoA carries four carbon atoms along
to the next step.
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20.8 The Citric Acid Cycle
STEP 5:
– The 4-carbon oxaloacetate must be restored
for Step 1 of the next cycle.
– The exergonic conversion of succinyl-CoA to
succinate is coupled with phosphorylation of
GDP to give GTP. GTP is similar in structure
to ATP and, like ATP, carries energy.
– In many cells, GTP is directly converted to
ATP. Step 5 is the only step in the cycle that
generates an energy-rich triphosphate.
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20.8 The Citric Acid Cycle
STEP 6:
– Succinate from Step 5 is oxidized by removal
of two hydrogen atoms to give fumarate.
– The enzyme for this reaction, succinate
dehydrogenase, is part of the inner
mitochondrial membrane.
– The reaction requires FAD, which is
covalently bound to its enzyme.
– Succinate dehydrogenase and FAD pass
electrons directly into electron transport.
– Step 6 neither uses nor releases energy.
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20.8 The Citric Acid Cycle
STEPS 7 and 8:
– Water is added across the double bond of
fumarate to give malate (Step 7).
– Oxidation of malate, a secondary alcohol,
gives oxaloacetate (Step 8).
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20.8 The Citric Acid Cycle
• The rate of the citric acid cycle is controlled by the
body’s need for ATP and coenzymes.
• When energy is being used at a high rate, ADP
accumulates and activates isocitrate dehydrogenase,
the enzyme for Step 3.
• When the body’s supply of energy is abundant,
NADH is present in excess and acts as an inhibitor of
isocitrate dehydrogenase.
• The cycle is activated when energy is needed and
inhibited when energy is in good supply.
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20.9 The Electron-Transport Chain and ATP Production
• At the conclusion of the citric acid cycle,
the reduced coenzymes are ready to
donate their energy to making ATP.
• The energy is released in a series of
oxidation–reduction reactions that move
electrons from one carrier to the next.
• Each reaction in the series is exergonic.
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20.9 The Electron-Transport Chain and ATP Production
• The sequence of reactions is known as the electrontransport chain (also the respiratory chain).
• The enzymes and coenzymes of the chain are embedded
in the inner membrane of the mitochondrion.
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20.9 The Electron-Transport Chain and ATP Production
• As electrons move down the electron-transport
pathway, the energy is used to move hydrogen ions
from the matrix into the intermembrane space.
• Because the inner membrane is otherwise
impermeable to the H+ ion, the result is a higher H+
concentration in the intermembrane space than in
the mitochondrial matrix.
• Moving ions from a region of lower concentration to
one of higher concentration requires energy to
make it happen.
• This energy is recaptured for use in ATP synthesis.
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20.9 The Electron-Transport Chain and ATP Production
• Electron transport proceeds in four enzyme
complexes in fixed positions within the inner
membrane of mitochondria, and two electron
carriers that move from one complex to another.
• The four fixed complexes are very large
assemblages of polypeptides and electron
acceptors. Electron acceptors are generally
cytochromes or quinones.
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20.9 The Electron-Transport Chain and ATP Production
• Each of complexes I–IV contain
several electron carriers.
• Hydrogen ions and electrons
move through the pathway in
the direction of the arrow.
• Each complex is at a lower
energy level than the preceding.
• Plant cells contain mitochondria
and chloroplasts. Chloroplasts
carry out photosynthesis, a
series of reactions that also
involve an electron transport
chain.
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20.9 The Electron-Transport Chain and ATP Production
• Hydrogen ions can return to the matrix by
passing through a channel that is part of the
ATP synthase enzyme complex.
• In doing so, they release the potential energy
gained as they were moved against the
concentration gradient.
• This energy release drives the phosphorylation
of ADP.
• Recent research suggests that each NADH
molecule yields about 2.5 molecules of ATP
and that each FADH2 molecule yields
approximately 1.5 molecules of ATP.
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20.9 The Electron-Transport Chain and ATP Production
Blockers and Uncouplers of Oxidative Phosphorylation
• Cyanide and barbiturates are among a group of
substances that block respiration (oxidative
phosphorylation) at the cytochromes in the
electron-transport chain, with different blockers
acting on different cytochromes.
• Other substances allow electron transport to
occur but prevent the conversion of ADP to ATP
by ATP synthase. When ATP production is
severed from energy use, it is said that ATP
production is uncoupled from the energy of the
proton gradient.
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20.10 Harmful Oxygen By-Products and
Antioxidant Vitamins
• In oxygen-consuming redox reactions, the
product may be highly reactive species.
• The superoxide ion •O22– and the hydroxyl
free radical •OH2– react as soon as possible
to get rid of the unpaired electron.
• Hydrogen peroxide, H2O2 is a strong
oxidizing agent.
• Reactive oxygen species can break covalent
bonds in enzymes and other proteins, DNA,
and the lipids in cell membranes.
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20.10 Harmful Oxygen By-Products and
Antioxidant Vitamins
• Potentially harmful oxygen species are constantly
being generated.
• Protection is provided by superoxide dismutase
and catalase enzymes, and by the antioxidant
vitamins E, C, and A which bind to reactive
species.
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20.10 Harmful Oxygen By-Products and
Antioxidant Vitamins
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Plants and Photosynthesis
Plants can derive energy directly from sunlight.
In photosynthesis, plants use solar energy to synthesize oxygen and energyrich carbohydrates from energy-poor reactants: CO2 and water.
The energy-capturing phase of photosynthesis takes place in green leaves.
Plant cells contain chloroplasts, which resemble mitochondria. Embedded in
membranes within the chloroplasts are large groups of chlorophyll molecules
and the enzymes of an electron-transport chain.
As solar energy is absorbed, chlorophyll molecules pass it along to specialized
reaction centers, where it is used to boost the energy of electrons. The excited
electrons then give up their extra energy as they pass down a pair of electrontransport chains.
Light-dependent reactions produce ATP and NADPH. The water enters the
plant through the roots, and oxygen formed is released through the leaves.
ATP and NADPH enter the interior of the chloroplasts where their energy is
used to drive the synthesis of carbohydrate molecules in lightindependent
reactions.
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Chapter Summary
1. What is the source of our energy, and what is its
fate in the body?
• Endergonic reactions are unfavorable and require an
external source of free energy to occur.
• Exergonic reactions are favorable, proceed
spontaneously, and release free energy.
• We derive energy by oxidation of food molecules that
contain energy captured by plants from sunlight.
• The energy is released gradually in exergonic
reactions and is available to do work, to drive
endergonic reactions, to provide heat, or to be stored
until needed.
• Energy generation in eukaryotic cells takes place in
mitochondria.
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Chapter Summary, Continued
2. How are the reactions that break down food
molecules organized?
• Food molecules undergo catabolism (are
broken down) to provide energy in four stages.
1.
2.
3.
4.
Digestion to form smaller molecules that can be absorbed into
cells;
Decomposition (by separate pathways for lipids, carbohydrates,
and proteins) into 2-carbon acetyl groups that are bonded to
coenzyme A in acetyl coenzyme A;
Reaction of the acetyl groups via the citric acid cycle to
generate energy-rich reduced coenzymes and liberate carbon
dioxide; and
Electron transport and transfer of the energy of the reduced
coenzymes from the citric acid cycle to our principal energy
transporter, ATP.
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Chapter Summary, Continued
3. What are the major strategies of metabolism?
• Using the energy from exergonic reactions, ADP is
phosphorylated to give ATP.
• Where energy must be expended, it is released by
removal of a phosphoryl group from ATP to give back
ADP. An otherwise “uphill” reaction in a metabolic
pathway is driven by coupling with an exergonic,
“downhill” reaction that provides enough energy so that
their combined outcome is exergonic and favorable.
• The oxidizing and reducing agents needed by the
many redox reactions of metabolism are coenzymes
that constantly cycle between their oxidized and
reduced forms.
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Chapter Summary, Continued
4.
What is the citric acid cycle?
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The citric acid cycle is a cyclic pathway of eight reactions, in which
the product of the final reaction is the substrate for the first reaction.
•
The reactions of the citric acid cycle:
1.
Set the stage for oxidation of the acetyl group (Steps 1 and 2);
2.
Remove two carboxyl groups as CO2 molecules (oxidative
decarboxylation) from the tricarboxylic acid isocitrate (Steps 3 and 4);
3.
Oxidize the 4-carbon dicarboxylic acid succinate and regenerate
oxaloacetate so that the cycle can start again (Steps 5–8).
•
Along the way, four reduced coenzyme molecules and one
molecule of GTP (converted immediately to ATP) are produced for
each acetyl group oxidized. The reduced coenzymes carry energy
for the subsequent production of additional ATP.
•
The cycle is activated when energy is in short supply and inhibited
when energy is in good supply.
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Chapter Summary, Continued
5.
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How is ATP generated in the final stage of catabolism?
ATP generation is accomplished by a series of enzyme
complexes in the inner membranes of mitochondria.
Electrons and hydrogen ions enter the first two complexes
of the electron-transport chain from succinate (in the citric
acid cycle) and NADH, where they are transferred to
coenzyme Q.
Then, the electrons and hydrogen ions proceed
independently, the electrons gradually giving up their
energy to the transport of hydrogen ions across the inner
mitochondrial membrane to maintain different
concentrations on opposite sides of the membrane.
The hydrogen ions return to the matrix by passing through
ATP synthase, where the energy they release is used to
convert ADP to ATP.
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Chapter Summary, Continued
6. What are the harmful by-products produced
from oxygen, and what protects against them?
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Harmful by-products of oxygen-consuming
reactions are the hydroxyl free radical, superoxide
ion (also a free radical), and hydrogen peroxide.
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These reactive species damage other molecules
by breaking bonds.
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Superoxide dismutase and catalase are enzymes
that disarm these oxygen by-products. Vitamins E,
C, and A (or its precursor β-carotene) are also
antioxidants.
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