Download Chapter 27 Bioenergetics: How the Body Converts Food to Energy

Chapter 27 Bioenergetics: How the Body Converts Food to Energy
27.1
Chemical energy present in food molecules is extracted and converted to a usable form
by the process of catabolism. The energy derived from the degradation of different types
of molecules is collected in the form of the energy-rich molecule, ATP.
27.2
(a) The common catabolic pathway is a series of chemical reactions through which most
small molecules obtained from food are degraded to produce carbon dioxide, water, and
energy in the form of ATP. The process can be divided into two sequences.
(b) The two parts of the common catabolic pathway are 1) the citric acid cycle, and 2)
oxidative phosphorylation.
27.3
(a) Mitochondria have two membranes, a highly-folded inner membrane, and an outer
membrane (see Figure 27.3).
(b) The outer membrane is permeable to the diffusion of small ions and molecules.
Special transport processes are required to move molecules through the inner membrane.
27.4
Enzymes needed for the common catabolic pathway, which occurs in the mitochondria,
are synthesized in the cytoplasm and transported into the mitochondria. They cross the
outer membrane through translocator outer membrane channels and are accepted in the
intermembrane space by chaperone-like translocator inner membrane complexes.
27.5
Cristae are the folds that are present in the inner mitochondrial membrane. The folds
provide extensive surface area for the concentration of enzymes and other components
required for metabolism.
27.6
(a) The enzymes for the citric acid cycle are found in the mitochondrial matrix.
(b) The enzymes of oxidative phosphorylation are located on the cristae, the foldings
of the inner membrane of the mitochondria.
27.7
Each ATP molecule has two phosphate anhydride bonds that release a substantial amount
of energy when they are hydrolyzed during metabolic processes (Figure 27.5):
phosphate
anhydride
bonds
phosphate
ester
bond
N
NH2
N
O
O
O
N
HO P O P O P O CH2 O
O- O- OH
H
H
H
OH OH
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27.8
The products of acid-catalyzed hydrolysis of AMP are adenosine and phosphate. The
ester bond between the phosphate and ribose is cleaved.
27.9
In reactions (a) and (b) the same type of anhydride bond is hydrolyzed. When the
reactions are measured under standard conditions, the energy yield is about the same, 7.3
kcal/mole. In the cell, however, the concentration of ADP is very low, compared to ATP,
and ADP is rarely used for energy.
27.10 It is estimated that we need about 40 kg of ATP each day.
27.11 The chemical bond between the ribitol and flavin in FAD is an amine (see Figure 27.6).
27.12 The two electrons from the reduction reaction are used to form the bond between the
the nicotinamide ring and the entering hydrogen.
27.13 Two nitrogen atoms in the flavin ring that are linked to carbon (N=C) are reduced to
yield FADH2.
27.14 Ribose is a monosaccharide with five carbons that is able to form a cyclic acetal ring.
Ribitol is derived from ribose by reduction of the aldehyde carbon to an alcohol.
Ribitol is an open chain, not a ring like ribose.
27.15 (a) The most important carrier of phosphate groups is ATP.
(b) The most important carriers of hydrogen ions and electrons from redox reactions are
NADH and FADH2.
(c) Coenzyme A carries acetyl groups.
27.16 The ribitol-phosphate linkage is an ester bond similar to the phosphate ester bond in
AMP. It is expected to yield about 3.4 kcal/mole upon hydrolysis, just like AMP.
27.17 An amide linkage is formed to bring together the amine group of mercaptoethylamine
and the carboxyl group of pantothenic acid (Figure 27.7).
27.18 (a) The vitamin B portion of NAD+ is called nicotinamide or niacin.
(b) The vitamin B portion of FAD is riboflavin.
(c) The vitamin B portion of Coenzyme A is pantothenic acid.
27.19 No, the reactive part of CoA is the thiol group (-SH) at the end of the molecule.
27.20 The resulting compound is a thioester formed between the –SH group of the CoA and
the carboxyl group of acetate.
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27.21 Most fats and carbohydrates are degraded in catabolism to the compound acetyl CoA.
27.22 (a) The letter C refers to the carbon atom. The numbers refer to a number of carbon
atoms. A compound with two carbons reacts with a compound with four carbons to yield
a product containing six carbons.
(b) Acetyl CoA + oxaloacetate = citric acid
27.23 α-Ketoglutarate is the only C-5 intermediate in the citric acid cycle.
27.24 The reaction numbers as shown in Figure 27.8 are used here. The following reactions in
the citric acid cycle are not redox reactions: 1, 2, 5, and 7. These reactions can easily be
identified by noting that the redox reactions use NAD+ or FAD as coenzymes.
27.25 FAD is used in the citric acid cycle as a coenzyme to oxidize succinate to fumarate.
27.26 Steps 3 and 5 are decarboxylation reactions, thus yield the one carbon fragment, carbon
dioxide. Carbon dioxide is carried by hemoglobin in the blood to the lungs where it
is exhaled.
27.27 Fumarase is classified as a lyase because it catalyzes the addition of water to a double
bond.
27.28 Oxidoreductases in the citric acid cycle include: isocitrate dehydrogenase, α-ketoglutarate
dehydrogenase complex, succinate dehydrogenase, and malate dehydrogenase.
27.29 ATP is not produced directly by the citric acid cycle. GTP, a reactive molecule with the
same amount of energy as ATP, is produced in Step 5 and may be used for some energyrequiring processes.
27.30 The four dicarboxylic acids containing four carbons are: succinate, fumarate, malate,
and oxaloacetate.
(a) The least oxidized is succinate as the two center carbons are saturated with hydrogen
atoms.
(b) The most oxidized is oxaloacetate as one carbon is a carbonyl (keto) group.
27.31 The stepwise degradation and oxidation of acetyl CoA in the citric acid cycle is very
efficient in the extraction and collection of energy. Rather than occurring in one single
burst, the energy is released in small increments and collected in the form of reduced
cofactors, NADH and FADH2.
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27.32 No, the two carbons from acetyl after one turn end up as the top two carbons in citrate,
which then become the top two carbons in α-ketoglutarate. The two carbon dioxides
come from the middle carboxyl of isocitrate and the bottom carboxyl of α-ketoglutarate.
Both carbons leaving as carbon dioxide come from oxaloacetate.
27.33 Carbon-carbon double bonds are present in the citric acid cycle intermediates fumarate
and cis-aconitate.
27.34 Regulation of the citric acid cycle is controlled by a feedback mechanism. When the
important energy products of catabolism, ATP and NADH, reach sufficient quantities,
they act as inhibitors of some enzymes. Steps 1, 3, and 4 are inhibited by ATP and/or
NADH.
27.35 When α-ketoglutarate is oxidized in Step 4 of the citric acid cycle, the electrons are
transferred to NAD+ to make the reduced form, NADH.
27.36 Catabolic processes lead to the production of energy in the molecular form of ATP
and reduced cofactors, NADH and FADH2. ATP may be used directly for energy, but
the cofactors must be oxidized to recover their energy. The reduced cofactors are
oxidized by the electron transport chain that transfers the electrons to molecular oxygen.
The energy released by the oxidation is used to make energy-rich ATP. In summary,
oxidative phosphorylation is two linked processes: oxidation of the reduced coenzymes,
and synthesis of ATP.
27.37 The mobile carriers of electrons in the electron transport chain are coenzyme Q and
cytochrome c.
27.38 (a) Complexes that contain iron centers: Fe-S clusters (I and III), cytochromes c, b, and a
(III and IV). Cytochrome oxidase (IV) contains iron and copper.
(b) Compounds that do not contain iron: CoQ (II) and FMN (I).
27.39 The proton translocator ATPase is a complex that resembles a rotor engine. It rotates
every time a proton passes through the inner membrane (Figure 27.10).
27.40 (a) The forward reaction, oxidation of NADH, occurs during electron transport: At
Complex I, the electrons from NADH are used to reduce CoQ.
(b) The reverse reaction, reduction of NAD+ in catabolism: three reactions in the citric
acid cycle (Steps 3, 4, and 8; see Figure 27.8).
27.41 During oxidative phosphorylation water is formed from protons, electrons, and oxygen
on the matrix side of the inner membrane. This occurs when electrons are shuttled
through complex IV (Figure 27.10).
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27.42 Hydrogen ions and electrons are separated at three points in the electron transport chain.
At Complexes I, III, and IV, protons are pumped across the membrane from the matrix
to the intermembrane space as electrons are transported from carrier to carrier and finally
to oxygen (Figure 27.10). This process of proton pumping is important because this sets
up the high-energy gradient that is used to make ATP.
27.43 (a) For each pair of protons translocated through the ATPase complex, one molecule of
ATP is generated. Each pair of electrons that enters oxidative phosphorylation at complex
I yields three ATP.
(b) Each C-2 fragment, which represents the carbons in acetyl CoA, yields 12 ATP.
27.44 Because proton concentration is increasing in the intermembrane space, it is expected that
pH is decreased (more acidic) compared to the matrix.
27.45 Protons are translocated through a proton channel formed by the Fo part of the ATPase
which has 12 protein subunits embedded in the inner membrane (Figure 26.10).
27.46 Osmosis is the process when molecules move, through some kind of barrier, from one
area to another. This causes the buildup of a concentration difference (a gradient) and
increases osmotic pressure. In this case the molecules, protons, are chemical entities,
hence the overall process is “chemiosmotic”.
27.47 The catalytic unit of ATPase is composed of α and β subunits (Figure 27.10). This part of
the ATPase catalyzes the formation of ATP:
ADP + Pi
ATP + H2O
27.48 The δ subunits, called the stator, stabilize the whole complex.
27.49 The energy, in kcal, from the oxidation of 1 g of acetate by the citric acid cycle:
Molecular wt. acetate = 59 g/mole. 1 g of acetate = 1 ÷ 59 = 0.017 mole. Each mole of
acetate produces 12 moles of ATP (See Problem 27.43(b)).
0.017 mole x 12 = 0.204 mole of ATP.
0.204 mole of ATP x 7.3 kcal/mole = 1.5 kcal.
27.50 (a) In this problem, we will not consider the ATPs produced during the conversion of the
hexose to the two C-2 fragments. According to Section 27.7 of your text, the degradation
of the C-2 fragment yields 12 ATPs when it is degraded by the common pathway.
Because there are two of the fragments, the hexose yields a total of 24 ATPs.
(b) According to the reaction in Section 27.7, two oxygen molecules are required for the
oxidation of each C-2 fragment. The hexose thus will require four oxygen molecules.
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27.51 (a) Muscle contraction takes place when thick protein filaments called myosin slide past
thin protein filaments called actin. The hydrolysis of ATP by myosin, an ATPase, drives
the alternating association and dissociation of actin and myosin. This causes the
contraction and relaxation of muscles (Section 27.8C).
(b) The energy in muscle contraction comes from ATP.
27.52 An example of the energy in ATP being converted to electrical energy is the sodium/
potassium ATPase (see Chemical Connections 21C). When ATP is hydrolyzed, sodium
ions are pumped out of a cell and potassium ions are pumped in. Because the charge
concentration is now different outside and inside the cell there is an electrical gradient.
This process is important in neurotransmission.
27.53 Glycogen phosphorylase is activated by phosphorylation of serine residues in the
protein subunits. The phosphoryl groups are transferred from ATP (see Chemical
Connection 23E). Phosphorylase is also activated by allosteric effectors.
27.54 A protonophore is a compound that transports protons through a membrane without
the expenditure of energy. An example of a protonophore is the uncoupling agent
2,4-dinitrophenol.
27.55 The antibiotic oligomycin inhibits the catalytic subunits of ATPase, thus it stops
phosphorylation of ADP. Although it acts as an effective antibiotic, its toxic effect
on ATPase does not allow its use in humans.
27.56 Superoxide dismutase, an enzyme that degrades the superoxide ion, plays an important
defensive role in strokes. As a result of oxygen deprivation of the brain when arteries
are blocked, a condition known as ischemia-reperfusion may occur. This means that
when these tissues suddenly become oxygenated, reactive superoxide ion can form. The
enzyme superoxide dismutase breaks down the excess activated oxygen.
27.57 (a) Cytochrome P-450 is an enzyme that catalyzes the hydroxylation of various natural
and synthetic substrates. The hydroxyl group is derived from molecular oxygen.
(b) The source of the reactant oxygen is the air we breathe. Most (90%) of the oxygen
we breathe is used for respiration.
27.58 (a) The only difference between ATP and GTP is the structure of the nitrogen base,
adenine vs. guanine (Figure 25.1).
(b) Both ATP and GTP have the same kind of phosphate anhydride bonds so it is
expected their energy levels are the same. The same amount of energy is released
when ATP or GTP are hydrolyzed.
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27.59 Number of g of acetic acid that must be metabolized to yield 87.6 kcal of energy:
87.6 kcal ÷ 7.3 kcal/mole ATP = 12 moles of ATP. 12 moles of ATP are released from
the oxidation of 1 mole of acetate. 1 mole of acetate (MW = 60) = 60 g of acetic acid.
27.60 The hydroxyl group in citrate is a tertiary alcohol. In isocitrate, the hydroxyl group
is a secondary alcohol.
27.61 The mechanical energy generated from the translocation of protons in oxidative
phosphorylation is first displayed in the rotating ion channel of ATPase.
27.62 This is a reference to step 3 in figure 27.8, conversion of isocitrate to α-ketoglutarate. The
reaction is clearly one of decarboxylation, loss of carbon dioxide. But the presence of the
coenzyme NAD+ indicates a second kind of reaction, a redox. The coenzyme is reduced
to NADH and the hydroxyl group of isocitrate is oxidized to a keto group. This complex
reaction is called oxidative decarboxylation.
27.63 Citrate and malate, intermediates in the citric acid cycle, both have carboxyl groups
and a hydroxyl group.
27.64 The two keto acids are α-ketoglutarate and oxaloacetate. They are both produced by
redox reactions:
CH2 -COO-
CH2 -COO-
CH-COO- + NAD+
CH2
HO CH-COOIsocitrate
HO CH-COOCH2 -COO-
+ CO2 + NADH + H+
O C-COOα-Ketoglutarate
O C-COO
+ NADH + H+
CH2 -COO-
+ NAD +
Malate
Oxaloacetate
27.65 Myosin, the thick filament in muscle, is an enzyme that acts as an ATPase.
27.66 For each pair of electrons in NADH and FADH2 that enters electron transport, there is
one molecule of water produced. Therefore the answer for (a) and (b) is one. The
difference between the two molecules is the number of ATP produced, not the number of
water molecules.
27.67 Isocitrate has two stereocenters.
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27.68 No, the carbon that is released as carbon dioxide comes from the middle carboxyl of
isocitrate, which originally came from oxaloacetate.
27.69 The proton channel is located in the Fo unit of ATPase that is embedded in the inner
membrane.
27.70 The passage of a proton through the membrane channel does not directly, in one step,
lead to the production of ATP. Rather the proton passage causes channel protein
movement that causes the rotor of the ATPase to turn and produce ATP from ADP and
phosphate. The mechanical energy of the rotation is converted to the chemical energy of
ATP.
27.71 All the sources of energy used for ATP synthesis are not completely known at this time.
Most of the energy comes from proton translocation. Some energy for proton pumping
comes also from breaking the covalent bond of oxygen (reduction of oxygen to water).
27.72 (a) There are two decarboxylation steps in the citric acid cycle, steps 3 and 4 (see figure
27.8).
(b) In both steps, the oxidizing agent is NAD+.
27.73 Succinate dehydrogenase catalyzes the oxidation of succinate to fumarate. FAD becomes
reduced in the reaction and then transfers its electrons directly to the electron transport
chain at complex II.
27.74 Malate has only one stereocenter. That tetrahedral carbon has a hydrogen, carboxyl,
hydroxyl, and a carbon bound to it.
27.75 Citrate isomerizes to isocitrate to convert a tertiary alcohol to a secondary alcohol.
Tertiary alcohols cannot be oxidized, but secondary alcohols can be oxidized to produce a
keto group, which is necessary to continue the pathway.
27.76 All pathways converge to the common one, thus the name. Many of the other pathways
produce products that are intermediates of this common pathway.
27.77 Iron is found in iron-sulfur clusters in proteins and is also part of the heme group of
cytochromes.
27.78 The proteins of the electron transport chain must move protons from one side of the
membrane to the other. Also, some substrates approach the electron transport chain from
one side of the membrane while others approach from the other side.
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27.79 Mobile electron carriers transfer electrons on their path from one large, less mobile,
protein complex to another.
27.80 The CO2 lost in the citric acid cycle quickly forms bicarbonate and carbonic acid and is
carried away in the blood and exhaled. By removing the product of the reaction, the
reaction becomes irreversible.
27.81 ATP and reducing agents such as NADH and FADH2, generated by the citric acid cycle,
are needed for biosynthetic pathways. Also, many of the intermediates of the citric acid
cycle are drawn off as part of biosynthetic pathways.
27.82 One fewer ATP is produced for each pair of electrons that enter the pathway if FAD,
rather than NAD+, is used as an electron carrier.
27.83 Biosynthetic pathways are likely to be ones of reduction because their net effect is to
reverse catabolism, which is oxidative.
27.84 Biosynthetic pathways require energy because their net effect is to reverse catabolism,
which produces energy.
27.85 ATP is not stored in the body. It is hydrolyzed to provide energy for many different kinds
of processes and thus turns over rapidly. While the body produces kilograms of ATP
every day, it also uses kilograms every day. The average life span of an individual
molecule of ATP is less than a second.
27.86 Inhibitors of electron transport help establish the order of carriers. If electron transport is
blocked, the electron donor accumulates and the electron acceptor is depleted. The
change in the levels of the two carriers establishes their position in the chain.
27.87 The citric acid cycle generates NADH and FADH2, which are linked to oxygen by the
electron transport chain.
27.88 We are going to see some important molecules, ATP, NADH, and others, many times in
the upcoming chapters.
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