The Respiratory Chain Consists of Four Complexes Three Proton Pumps and a Physical Link to the Citric Acid Cycle

II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.2. Oxidative Phosphorylation Depends on Electron Transfer
Figure 18.7. Distance Dependence of Electron-Transfer Rate. The rate of electron transfer decreases as the electron
donor and the electron acceptor move apart. In a vacuum, the rate decreases by a factor of 10 for every increase of 0.8 Å.
In proteins, the rate decreases more gradually, by a factor of 10 for every increase of 1.7 Å. This rate is only approximate
because variations in the structure of the intervening protein medium can affect the rate.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.2. Oxidative Phosphorylation Depends on Electron Transfer
Figure 18.8. Free-Energy Dependence of Electron-Transfer Rate. The rate of an electron-transfer reaction at first
increases as the driving force for the reaction increases. The rate reaches a maximum and then decreases at very large
driving forces.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a
Physical Link to the Citric Acid Cycle
Electrons are transferred from NADH to O2 through a chain of three large protein complexes called NADH-Q
oxidoreductase, Q-cytochrome c oxido-reductase, and cytochrome c oxidase (Figure 18.9 and Table 18.2). Electron flow
within these transmembrane complexes leads to the transport of protons across the inner mitochondrial membrane.
Electrons are carried from NADH-Q oxidoreductase to Q-cytochrome c oxidoreductase, the second complex of the
chain, by the reduced form of coenzyme Q (Q), also known as ubiquinone because it is a ubiquitous quinone in biological
systems. Ubiquinone is a hydrophobic quinone that diffuses rapidly within the inner mitochondrial membrane.
Ubiquinone also carries electrons from FADH2, generated in succinate dehydrogenase in the citric acid cycle, to Qcytochrome c oxidoreductase, generated through succinate-Q reductase. Cytochrome c, a small, soluble protein, shuttles
electrons from Q-cytochrome c oxidoreductase to cytochrome c oxidase, the final component in the chain and the one
that catalyzes the reduction of O2. NADH-Q oxidoreductase, succinate-Q reductase, Q-cytochrome c oxidoreductase,
and cytochrome c oxidase are also called Complex I, II, III, and IV, respectively. Succinate-Q reductase (Complex II), in
contrast with the other complexes, does not pump protons.
Coenzyme Q is a quinone derivative with a long isoprenoid tail. The number of five-carbon isoprene units in coenzyme
Q depends on the species. The most common form in mammals contains 10 isoprene units (coenzyme Q10). For
simplicity, the subscript will be omitted from this abbreviation because all varieties function in an identical manner.
Quinones can exist in three oxidation states (Figure 18.10). In the fully oxidized state (Q), coenzyme Q has two keto
groups. The addition of one electron and one proton results in the semiquinone form (QH·). The semiquinone form is
relatively easily deprotonated to form a semiquinone radical anion (Q·-). The addition of a second electron and proton
generates ubiquinol (QH2), the fully reduced form of coenzyme Q, which holds its protons more tightly. Thus, for
quinones, electron-transfer reactions are coupled to proton binding and release, a property that is key to transmembrane
proton transport.
18.3.1. The High-Potential Electrons of NADH Enter the Respiratory Chain at NADHQ Oxidoreductase
The electrons of NADH enter the chain at NADH-Q oxidoreductase (also called NADH dehydrogenase), an enormous
enzyme (880 kd) consisting of at least 34 polypeptide chains. The construction of this proton pump, like that of the other
two in the respiratory chain, is a cooperative effort of genes residing in both the mitochondria and the nucleus. The
structure of this enzyme has been determined only at moderate resolution (Figure 18.11). NADH-Q oxidoreductase is Lshaped, with a horizontal arm lying in the membrane and a vertical arm that projects into the matrix. Although a detailed
understanding of the mechanism is likely to require higher-resolution structural information, some aspects of the
mechanism have been established.
The reaction catalyzed by this enzyme appears to be
The initial step is the binding of NADH and the transfer of its two high-potential electrons to the flavin mononucleotide
(FMN) prosthetic group of this complex to give the reduced form, FMNH2. Like quinones, flavins bind protons when
they are reduced. FMN can also accept one electron instead of two (or FMNH2 can donate one electron) by forming a
semiquinone radical intermediate (Figure 18.12). The electron acceptor of FMN, the isoalloxazine ring, is identical with
that of FAD. Electrons are then transferred from FMNH2 to a series of iron-sulfur clusters, the second type of prosthetic
group in NADH-Q oxidoreductase.
Fe-S clusters in iron-sulfur proteins (also called nonheme iron proteins) play a critical role in a wide range of reduction
reactions in biological systems. Several types of Fe-S clusters are known (Figure 18.13). In the simplest kind, a single
iron ion is tetrahedrally coordinated to the sulfhydryl groups of four cysteine residues of the protein. A second kind,
denoted by 2Fe-2S, contains two iron ions and two inorganic sulfides. Such clusters are usually coordinated by four
cysteine residues, although exceptions exist, as we shall see when we consider Q-cytochrome c oxidoreductase. A third
type, designated 4Fe-4S, contains four iron ions, four inorganic sulfides, and four cysteine residues. We encountered a
variation of this type of cluster in aconitase in Section 17.1.4. NADH-Q oxidoreductase contains both 2Fe-2S and 4Fe4S clusters. Iron ions in these Fe-S complexes cycle between Fe2+ (reduced) or Fe3+(oxidized) states. Unlike quinones
and flavins, iron-sulfur clusters generally undergo oxidation-reduction reactions without releasing or binding protons.
Electrons in the iron-sulfur clusters of NADH-Q oxidoreductase are shuttled to coenzyme Q. The flow of two electrons
from NADH to coenzyme Q through NADH-Q oxidoreductase leads to the pumping of four hydrogen ions out of the
matrix of the mitochondrion. The details of this process remain the subject of active investigation. However, the coupled
electron- proton transfer reactions of Q are crucial. NADH binds to a site on the vertical arm and transfers its electrons to
FMN. These electrons flow within the vertical unit to three 4Fe-4S centers and then to a bound Q. The reduction of Q to
QH results in the uptake of two protons from the matrix (Figure 18.14). The pair of electrons on bound QH2 are
2
transferred to a 4Fe-4S center and the protons are released on the cytosolic side. Finally, these elections are transferred to
a mobile Q in the hydrophobic core of the membrane, resulting in the uptake of two additional protons from the matrix.
The challenge is to delineate the binding events and conformational changes induced by these electron transfers and
learn how the uptake and release of protons from the appropriate sides of the membrane is facilitated.
18.3.2. Ubiquinol Is the Entry Point for Electrons from FADH2 of Flavoproteins
The citric acid cycle enzyme succinate dehydrogenase, which generates FADH2 with the oxidation of succinate to
fumarate (Section 17.1.8), is part of the succinate-Q reductase complex (Complex II), an integral membrane protein of
the inner mitochondrial membrane. FADH2 does not leave the complex. Rather, its electrons are transferred to Fe-S
centers and then to Q for entry into the electron-transport chain. Two other enzymes that we will encounter later,
glycerol phosphate dehydrogenase (Section 18.5.1) and fatty acyl CoA dehydrogenase (Section 22.2.4), likewise transfer
their highpotential electrons from FADH2 to Q to form ubiquinol (QH2), the reduced state of ubiquinone. The succinateQ reductase complex and other enzymes that transfer electrons from FADH2 to Q, in contrast with NADH-Q
oxidoreductase, do not transport protons. Consequently, less ATP is formed from the oxidation of FADH2 than from
NADH.
18.3.3. Electrons Flow from Ubiquinol to Cytochrome c Through Q-Cytochrome c
Oxidoreductase
The second of the three proton pumps in the respiratory chain is Q-cytochrome c oxidoreductase (also known as
Complex III and cytochrome reductase). A cytochrome is an electron-transferring protein that contains a heme prosthetic
group. The iron ion of a cytochrome alternates between a reduced ferrous (+2) state and an oxidized ferric (+3) state
during electron transport. The function of Q-cytochrome c oxidoreductase is to catalyze the transfer of electrons from
QH2 to oxidized cytochrome c (cyt c), a water-soluble protein, and concomitantly pump protons out of the mitochondrial
matrix.
Q-cytochrome c oxidoreductase is a dimer with each monomer containing 11 subunits (Figure 18.15). Q-cytochrome c
oxidoreductase itself contains a total of three hemes, contained within two cytochrome subunits: two b-type hemes,
termed heme b L (L for low affinity) and heme b H (H for high affinity), within cytochrome b, and one c-type heme
within cytochrome c 1. The prosthetic group of the heme in cytochromes b, c 1, and c is iron- protoporphyrin IX, the
same heme as in myoglobin and hemoglobin (Section 10.2.1). The hemes in cytochromes c and c 1, in contrast with
those in cytochrome b, are covalently attached to the protein (Figure 18.16). The linkages are thioethers formed by the
addition of the sulfhydryl groups of two cysteine residues to the vinyl groups of the heme. Because of these groups, this
enzyme is also known as cytochrome bc 1. In addition to the hemes, the enzyme also contains an iron-sulfur protein with
an 2Fe-2S center. This center, termed the Rieske center, is unusual in that one of the iron ions is coordinated by two
histidine residues rather than two cysteine residues. This coordination stabilizes the center in its reduced form, raising its
reduction potential. Finally, Q-cytochrome c oxidoreductase contains two distinct binding sites for ubiquinone termed Qo
and Qi, with the Qi site lying closer to the inside of the matrix.
18.3.4. Transmembrane Proton Transport: The Q Cycle
The mechanism for the coupling of electron transfer from Q to cytochrome c to transmembrane proton transport is
known as the Q cycle (Figure 18.17). The Q cycle also facilitates the switch from the two-electron carrier ubiquinol to
the one-electron carrier cytochrome c. The cycle begins as ubiquinol (QH2) binds in the Qo site. Ubiquinol transfers its
electrons, one at a time. One electron flows first to the Rieske 2Fe-2S cluster, then to cytochrome c 1, and finally to a
molecule of oxidized cytochrome c, converting it into its reduced form. The reduced cytochrome c molecule is free to
diffuse away from the enzyme. The second electron is transferred first to cytochrome b L, then to cytochrome b H, and
finally to an oxidized uniquinone bound in the Qi site. This quinone (Q) molecule is reduced to a semiquinone anion (Q ·
-).
Importantly, as the QH2 in the Qo site is oxidized to Q, its protons are released to the cytosolic side of the membrane.
This Q molecule in the Qo site is free to diffuse out into the ubiquinone pool.
At this point, Q · - resides in the Qi site. A second molecule of QH2 binds to the Qo site and reacts in the same way as the
first. One of its electrons is transferred through the Rieske center and cytochrome c 1 to reduce a second molecule of
cytochrome c. The other electron goes through cytochromes b L and b H to Q · - bound in the Qi site. On the addition of
the second electron, this quinone radical anion takes up two protons from the matrix side to form QH2. The removal of
these two protons from the matrix contributes to the formation of the proton gradient. At the end of the Q cycle, two
molecules of QH2 are oxidized to form two molecules of Q, and one molecule of Q is reduced to QH2, two molecules of
cytochrome c are reduced, four protons are released on the cytoplasmic side, and two protons are removed from the
mitochondrial matrix.
18.3.5. Cytochrome c Oxidase Catalyzes the Reduction of Molecular Oxygen to Water
The final stage of the electron-transport chain is the oxidation of the reduced cytochrome c generated by Complex III,
which is coupled to the reduction of O2 to two molecules of H2O. This reaction is catalyzed by cytochrome c oxidase
(Complex IV). The four-electron reduction of oxygen directly to water without the release of intermediates poses a
challenge. Nevertheless, this reaction is quite thermodynamically favorable. From the reduction potentials in Table 18.1,
the standard free-energy change for this reaction is calculated to be ∆ G°´ = -55.4 kcal mol-1 (-231.8 kJ mol-1). As much
of this free energy as possible must be captured in the form of a proton gradient for subsequent use in ATP synthesis.
Bovine cytochrome c oxidase is reasonably well understood at the structural level (Figure 18.18). It consists of 13
subunits, of which 3 (called subunits I, II, and III) are encoded by the mitochondrial genome. Cytochrome c oxidase
contains two heme A groups and three copper ions, arranged as two copper centers, designated A and B. One center, Cu
/Cu , contains two copper ions linked by two bridging cysteine residues. This center initially accepts electrons from
A
A
reduced cytochrome c. The remaining copper ion, Cu , is coordinated by three histidine residues, one of which is
B
modified by covalent linkage to a tyrosine residue. Heme A differs from the heme in cytochrome c and c 1 in three ways:
(1) a formyl group replaces a methyl group, (2) a C15 hydrocarbon chain replaces one of the vinyl groups, and (3) the
heme is not covalently attached to the protein.
The two heme A molecules, termed heme a and heme a , have distinct properties because they are located in different
3
environments within cytochrome c oxidase. Heme a functions to carry electrons from CuA/CuA, whereas heme a 3
passes electrons to CuB, to which it is directly adjacent. Together, heme a and Cu form the active center at which O
3
B
2
is reduced to H O.
2
The catalytic cycle begins with the enzyme in its fully oxidized form (Figure 18.19). One molecule of reduced
cytochrome c transfers an electron, initially to CuA/CuA. From there, the electron moves to heme a, then to heme a 3,
and finally to CuB, which is reduced from the Cu2+ (cupric) form to the Cu+ (cuprous) form. A second molecule of
cytochrome c introduces a second electron that flows down the same path, stopping at heme a 3, which is reduced to the
Fe2+ form. Recall that the iron in hemoglobin is in the Fe2+ form when it binds oxygen (Section 10.2.1). Thus, at this
stage, cytochrome c oxidase is poised to bind oxygen and does so. The proximity of CuB in its reduced form to the heme
a 3-oxygen complex allows the oxygen to be reduced to peroxide (O2 2-), which forms a bridge between the Fe3+ in
heme a 3 and CuB 2+ (Figure 18.20). The addition of a third electron from cytochrome c as well as a proton results in the
cleavage of the O-O bond, yielding a ferryl group, Fe4+ = O, at heme a 3 and CuB 2+-OH. The addition of the final
electron from cytochrome c and a second proton reduces the ferryl group to Fe3+-OH. Reaction with two additional
protons allows the release of two molecules of water and resets the enzyme to its initial, fully oxidized form.
This reaction can be summarized as
The four protons in this reaction come exclusively from the matrix. Thus, the consumption of these four protons
contributes directly to the proton gradient. Recall that each proton contributes 5.2 kcal mol-1 (21.8 kJ mol-1) to the free
energy associated with the proton gradient; so these four protons contribute 20.8 kcal mol-1 (87.2 kJ mol-1), an amount
substantially less than the free energy available from the reduction of oxygen to water. Remarkably, cytochrome c
oxidase evolved to pump four additional protons from the matrix to the cytoplasmic side of the membrane in the course
of each reaction cycle for a total of eight protons removed from the matrix (Figure 18.21). The details of how these
protons are transported through the protein is still under study. However, two effects contribute to the mechanism. First,
charge neutrality tends to be maintained in the interior of proteins. Thus, the addition of an electron to a site inside a
protein tends to favor the binding of a proton to a nearby site. Second, conformational changes take place, particularly
around the heme a 3-CuB center, in the course of the reaction cycle, and these changes must be used to allow protons to
enter the protein exclusively from the matrix side and to exit exclusively to the cytosolic side. Thus, the overall process
catalyzed by cytochrome c oxidase is
As discussed earlier, molecular oxygen is an ideal terminal electron acceptor, because its high affinity for electrons
provides a large thermodynamic driving force. However, danger lurks in the reduction of O . The transfer of four
2
electrons leads to safe products (two molecules of H2O), but partial reduction generates hazardous compounds. In
particular, the transfer of a single electron to O forms superoxide anion, whereas the transfer of two electrons yields
2
peroxide.
These compounds and, particularly, their reaction products can be quite harmful to a variety of cell components. The
strategy for the safe reduction of O2 is clear from the discussion of the reaction cycle: the catalyst does not release partly
reduced intermediates. Cytochrome c oxidase meets this crucial criterion by holding O2 tightly between Fe and Cu ions.
18.3.6. Toxic Derivatives of Molecular Oxygen Such as Superoxide Radical Are
Scavenged by Protective Enzymes
Although cytochrome c oxidase and other proteins that reduce O2 are remarkably successful in not releasing
intermediates, small amounts of superoxide anion and hydrogen peroxide are unavoidably formed. Superoxide, hydrogen
peroxide, and species that can be generated from them such as OH· are collectively referred to as reactive oxygen species
or ROS.
What are the cellular defense strategies against oxidative damage by ROS? Chief among them is the enzyme superoxide
dismutase. This enzyme scavenges superoxide radicals by catalyzing the conversion of two of these radicals into
hydrogen peroxide and molecular oxygen.
Dismutation
A reaction in which a single reactant is converted into two different
products.
Eukaryotes contain two forms of this enzyme, a manganese-containing version located in mitochondria and a copperzinc-dependent cytosolic form. These enzymes perform the dismutation reaction by a similar mechanism (Figure 18.22).
The oxidized form of the enzyme is reduced by superoxide to form oxygen. The reduced form of the enzyme, formed in
this reaction, then reacts with a second superoxide ion to form peroxide, which takes up two protons along the reaction
path to yield hydrogen peroxide.
The hydrogen peroxide formed by superoxide dismutase and by other processes is scavenged by catalase, a ubiquitous
heme protein that catalyzes the dismutation of hydrogen peroxide into water and molecular oxygen.
Superoxide dismutase and catalase are remarkably efficient, performing their reactions at or near the diffusion-limited
rate (Section 8.4.2). Other cellular defenses against oxidative damage include the antioxidant vitamins, vitamins E and C.
Because it is lipophilic, vitamin E is especially useful in protecting membranes from lipid peroxidation.
The importance of the cell's defense against ROS is demonstrated by the presence of superoxide dismutase in all aerobic
organisms. Escherichia coli mutants lacking this enzyme are highly vulnerable to oxidative damage. Moreover, oxidative
damage is believed to cause, at least in part, a growing number of diseases (Table 18.3).
18.3.7. The Conformation of Cytochrome c Has Remained Essentially Constant for
More Than a Billion Years
Cytochrome c is present in all organisms having mitochondrial respiratory chains: plants, animals, and eukaryotic
microorganisms. This electron carrier evolved more than 1.5 billion years ago, before the divergence of plants and
animals. Its function has been conserved throughout this period, as evidenced by the fact that the cytochrome c of any
eukaryotic species reacts in vitro with the cytochrome c oxidase of any other species tested thus far. Finally, some
prokaryotic cytochromes, such as cytochrome c2 from a photosynthetic bacterium and cytochrome c 550 from a
denitrifying bacterium, closely resemble cytochrome c from tuna heart mitochondria (Figure 18.23). This evidence
attests that the structural and functional characteristics of cytochrome c present an efficient evolutionary solution to
electron transfer.
The resemblance among cytochrome c molecules extends to the level of amino acid sequence. Because of the molecule's
relatively small size and ubiquity, the amino acid sequences of cytochrome c from more than 80 widely ranging
eukaryotic species were determined by direct protein sequencing by Emil Smith, Emanuel Margoliash, and others.
Comparison of these sequences revealed that 26 of 104 residues have been invariant for more than one and a half billion
years of evolution. A phylogenetic tree, constructed from the amino acid sequences of cytochrome c, reveals the
evolutionary relationships between many animal species (Figure 18.24).
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.9. Sequence of Electron Carriers in the Respiratory Chain.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Table 18.2. Components of the mitochondrial electron-transport chain
Oxidant or reductant
Enzyme complex
NADH-Q
oxidoreductase
Mass
(kd)
880
Succinate-Q reductase
140
Q-cytochrome c
oxidoreductase
250
Subunits Prosthetic group Matrix side
34 FMN
Fe-S
4 FAD
Fe-S
10 Heme b H
Membrane core
NADH
Q
Succinate
Q
Q
Cytosolic side
Cytochrome c
Heme b L
Heme c 1
Cytochrome c oxidase
160
Fe-S
10 Heme a
Cytochrome c
Heme a 3
CuA and CuB
Sources: J. W. DePierre and L. Ernster, Annu. Rev. Biochem. 46(1977):215; Y. Hatefi, Annu Rev. Biochem. 54(1985);1015; and J.
E. Walker, Q. Rev. Biophys. 25(1992):253.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.10. Oxidation States of Quinones. The reduction of ubiquinone (Q) to ubiquinol (QH2) proceeds through a
semiquinone anion intermediate (Q
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
-).
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.11. Structure of NADH-Q Oxidoreductase (Complex I). The structure, determined by electron microscopy
at 22-Å resolution, consists of a membrane-spanning part and a long arm that extends into the matrix. NADH is oxidized
in the arm, and the electrons are transferred to reduce Q in the membrane. [After N. Grigorieff, J. Mol. Biol. 277
(1998):1033 1048.]
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.12. Oxidation States of Flavins. The reduction of flavin mononucleotide (FMN) to FMNH2 proceeds
through a semiquinone intermediate.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.13. Iron-Sulfur Clusters. (A) A single iron ion bound by four cysteine residues. (B) 2Fe-2S cluster with iron
ions bridged by sulfide ions. (C) 4Fe-4S cluster. Each of these clusters can undergo oxidation-reduction reactions.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.14. Coupled Electron-Proton Transfer Reactions. The reduction of a quinone (Q) to QH2 in an appropriate
site can result in the uptake of two protons from the mitochondrial matrix.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.15. Structure of Q-Cytochrome C Oxidoreductase (Cytochrome BC 1). This enzyme is a homodimer with
11 distinct polypeptide chains. The major prosthetic groups, three hemes and a 2Fe-2S cluster, mediate the
electron-transfer reactions between quinones in the membrane and cytochrome c in the intermembrane space.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.16. Attachment of C -Type Cytochromes. A heme group is covalently attached to a protein through
thioether linkages formed by the addition of sulfhydryl groups of cysteine residues to vinyl groups on protoporphyrin.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.17. Q Cycle. The two electrons of a bound QH2 are transferred, one to cytochrome c and the other to a bound
Q to form the semiquinone Q
-.
The newly formed Q dissociates and is replaced by a second QH2, which also gives up
its electrons, one to a second molecule of cytochrome c and the other to reduce Q - to QH2. This second electron
transfer results in the uptake of two protons from the matrix. Prosthetic groups are shown in their oxidized forms in blue
and in their reduced forms in red.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.18. Structure of Cytochrome C Oxidase. This enzyme consists of 13 polypeptide chains. The major
prosthetic groups include CuA/CuA, heme a, and heme a 3-CuB. Heme a 3-CuB is the site of the reduction of
oxygen to water. CO(bb) is a carbonyl group of the peptide backbone.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.19. Cytochrome Oxidase Mechanism. The cycle begins with all prosthetic groups in their oxidized forms
(shown in blue). Reduced cytochrome c introduces an electron that reduces CuB. A second reduced cytochrome c then
reduces the iron in heme a 3. This Fe2+ center then binds oxygen. Two electrons are transferred to the bound oxygen to
form peroxide, which bridges between the iron and CuB. The introduction of an additional electron by a third molecule
of reduced cytochrome c cleaves the O-O bond and results in the uptake of a proton from the matrix. The introduction of
a final electron and three more protons generates two molecules of H2O, which are released from the enzyme to
regenerate the initial state. The four protons found in the water molecules come from the matrix.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.20. Peroxide Bridge. The oxygen bound to heme a 3 is reduced to peroxide by the presence of CuB.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.21. Proton Transport by Cytochrome C Oxidase. Four "chemical" protons are taken up from the matrix
side to reduce one molecule of O2 to two molecules of H2O. Four additional "pumped" protons are transported out of the
matrix and released on the cytosolic side in the course of the reaction. The pumped protons double the efficiency of freeenergy storage in the form of a proton gradient for this final step in the electron-transport chain.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.22. Superoxide Dismutase Mechanism. The oxidized form of superoxide dismutase (Mox) reacts with one
superoxide ion to form O2 and generate the reduced form of the enzyme (Mred). The reduced form then reacts with a
second superoxide and two protons to form hydrogen peroxide and regenerate the oxidized form of the enzyme.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Table 18.3. Pathological and other conditions that may entail free-radical injury
Atherogenesis
Emphysema; bronchitis
Parkinson disease
Duchenne muscular dystrophy
Cervical cancer
Alcoholic liver disease
Diabetes
Acute renal failure
Down syndrome
Retrolental fibroplasia
Cerebrovascular disorders
Ischemia; reperfusion injury
Source: After D.B. Marks, A.D. Marks, and C.M. Smith, Basic Medical Biochemistry: A Clinical Approach (Williams & Wilkins,
1996, p. 331).
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.23. Conservation of the Three-Dimensional Structure of Cytochrome C. The side chains are shown for the
21 conserved amino acids and the heme.
II. Transducing and Storing Energy
18. Oxidative Phosphorylation
18.3. The Respiratory Chain Consists of Four Complexes: Three Proton Pumps and a Physical Link to the Citric Acid Cycle
Figure 18.24. Evolutionary Tree Constructed From Sequences of Cytochrome C. Branch lengths are proportional to
the number of amino acid changes that are believed to have occurred. This drawing is an adaptation of the work of
Walter M. Fitch and Emanuel Margoliash.