Biochemistry, 4th Edition P65 docx

Complex III Mediates Electron Transport from Coenzyme Q to Cytochrome c In the third complex of the electron-transport chain, reduced coenzyme Q UQH2 passes its electrons to cytochrome

electrons to UQ, including mitochondrial sn -glycerophosphate dehydrogenase, an

inner membrane-bound shuttle enzyme, and the fatty acyl-CoA dehydrogenases,

three soluble matrix enzymes involved in fatty acid oxidation (Figure 20.8; also see

Chapter 23) The path of electrons from succinate to UQ is shown in Figure 20.7

Complex III Mediates Electron Transport from Coenzyme Q

to Cytochrome c

In the third complex of the electron-transport chain, reduced coenzyme Q (UQH2)

passes its electrons to cytochrome c via a unique redox pathway known as the Q cycle.

UQ–cytochrome c reductase (UQ–cyt c reductase), as this complex is known, involves

three different cytochromes and an Fe-S protein In the cytochromes of these and

sim-ilar complexes, the iron atom at the center of the porphyrin ring cycles between the

reduced Fe2(ferrous) and oxidized Fe3(ferric) states

Cytochromes were first named and classified on the basis of their absorption

spec-tra (Figure 20.9), which depend upon the structure and environment of their heme

groups The b cytochromes contain iron protoporphyrin IX (Figure 20.10), the same

heme found in hemoglobin and myoglobin The c cytochromes contain heme c, derived

from iron protoporphyrin IX by the covalent attachment to cysteine residues from the

associated protein (One other heme variation, heme a, contains a 15-carbon

iso-prenoid chain on a modified vinyl group and a formyl group in place of one of the

methyls [see Figure 20.10] Cytochrome a is found in two forms in Complex IV of the

electron-transport chain, as we shall see.) UQ–cyt c reductase (Figure 20.11) contains

a b -type cytochrome, of 30 to 40 kD, with two different heme sites and one c -type

cyto-chrome The two hemes on the b cytochrome polypeptide in UQ–cyt c reductase are

distinguished by their reduction potentials and the wavelength (max) of the so-called

H3C

O C SCoA [FAD]

[FADH2] UQ

H3C

O C SCoA

FIGURE 20.8 The fatty acyl-CoA dehydrogenase reaction, emphasizing that the reaction involves reduction of

enzyme-bound FAD (indicated by brackets).

β

(a)

(b)

Wavelength (nm)

(a) Cytochrome c: reduced spectrum

α

(b) Cytochrome c: oxidized spectrum

FIGURE 20.9 Visible absorption spectra of cytochrome c.

N

N

CH3

CH2CH2COO_

CH2CH2COO_

H3C

H3C

H3C

N

N

CHCH3

CH3

CH2CH2COO_

CH2CH2COO_

H3C

H3C

H3C

S

S

N

N

CH3

CH2CH2COO_

CH2CH2COO_

H3C

CH2CH

H3C

OH

CH3CH

Iron protoporphyrin IX

(found in cytochrome b,

myoglobin, and hemoglobin)

Heme c

(found in cytochrome c)

Heme a

(found in cytochrome a)

FIGURE 20.10 The structures of iron protoporphyrin IX,

heme c, and heme a.

-band One of these hemes, known as b L or b 566, has a standard reduction potential,

Ᏹo, of 0.100 V and a wavelength of maximal absorbance (max) of 566 nm The other,

known as b H or b 562, has a standard reduction potential of 0.050 V and a maxof 562

nm (H and L here refer to high and low reduction potential.)

The structure of the UQ–cyt c reductase, also known as the cytochrome bc1

was a co-recipient of the Nobel Prize in Chemistry for his work on the structure of

a photosynthetic reaction center; see Chapter 21) The complex is a dimer, with each monomer consisting of 11 protein subunits and 2165 amino acid residues (monomer mass, 248 kD) The dimeric structure is pear-shaped and consists of a large domain that extends 75 Å into the mitochondrial matrix, a transmembrane domain consisting of 13 transmembrane -helices in each monomer and a small

do-main that extends 38 Å into the intermembrane space (Figure 20.11) Most of the

Rieske protein(an Fe-S protein named for its discoverer) is mobile in the crystal (only 62 of its 196 residues are shown in the structure in Figure 20.11), and Deisen-hofer has postulated that mobility of this subunit could be required for electron transfer in the function of this complex

Complex III Drives Proton Transport As with Complex I, passage of electrons through the Q cycle of Complex III is accompanied by proton transport across the in-ner mitochondrial membrane The postulated pathway for electrons in this system is shown in Figure 20.12 A large pool of UQ and UQH2exists in the inner mitochon-drial membrane The Q cycle is initiated when a molecule of UQH2from this pool

diffuses to a site (called Qp) on Complex III near the cytosolic face of the membrane Oxidation of this UQH2occurs in two steps First, an electron from UQH2is

trans-ferred to the Rieske protein and then to cytochrome c1 This releases two Hto the cytosol and leaves UQ, a semiquinone anion form of UQ, at the Qpsite The

sec-ond electron is then transferred to the b L heme, converting UQ to UQ The

Rieske protein and cytochrome c1are similar in structure; each has a globular do-main and is anchored to the inner mitochondrial membrane by a hydrophobic seg-ment However, the hydrophobic segment is N-terminal in the Rieske protein and

C-terminal in cytochrome c

FIGURE 20.11 The structure of UQ–cyt c reductase, also

known as the cytochrome bc1 complex The -helical

bundle near the top of the structure defines the

trans-membrane domain of the protein (pdb id  1BE3).

The electron on the b Lheme facing the cytosolic side of the membrane is now

passed to the b Hheme on the matrix side of the membrane This electron

trans-fer occurs against a membrane potential of 0.15 V and is driven by the loss of

redox potential as the electron moves from b L (Ᏹo  0.100 V) to b H (Ᏹo 

0.050 V) The electron is then passed from b Hto a molecule of UQ at a second

quinone-binding site, Qn, converting this UQ to UQ The resulting UQ

remains firmly bound to the Qnsite This completes the first half of the Q cycle

(Figure 20.12a)

The second half of the cycle (Figure 20.12b) is similar to the first half, with a

sec-ond molecule of UQH2oxidized at the Qpsite, one electron being passed to

cyto-chrome c1and the other transferred to heme b L and then to heme b H In this latter

half of the Q cycle, however, the b Helectron is transferred to the semiquinone anion,

UQ, at the Qnsite With the addition of two Hfrom the mitochondrial matrix,

this produces a molecule of UQH2, which is released from the Qnsite and returns to

the coenzyme Q pool, completing the Q cycle

The Q Cycle Is an Unbalanced Proton Pump Why has nature chosen this rather

convoluted path for electrons in Complex III? First of all, Complex III takes up two

protons on the matrix side of the inner membrane and releases four protons on

the cytoplasmic side for each pair of electrons that passes through the Q cycle The

other significant feature of this mechanism is that it offers a convenient way for a

two-electron carrier, UQH2, to interact with the b L and b Hhemes, the Rieske

pro-tein Fe-S cluster, and cytochrome c , all of which are one-electron carriers

e–

e–

e–

e–

e–

e–

(a) First half of Q cycle

Intermembrane

space (P-phase)

Matrix (N-phase)

Matrix (N-phase)

UQH2

UQ

Pool

UQH2 UQ–

UQ

UQ UQ–

Qp site H

+

2

FeS

Cyt c1 Cyt c

Cyt b L

Cyt b H

Qn site

First UQH2 from pool

UQ to pool

2 e– oxidation

at Qp site

2 H+ out

UQ

at Qn site

Cyt c

Synopsis

(b) Second half of Q cycle

Intermembrane

space (P-phase)

UQH2 UQ–

UQ

Qp site

H +

2

FeS

Cyt c1 Cyt c

Cyt b L

Cyt b H

Qn site UQH2 UQ–

H +

2

1 e–

1 e–

Second UQH2 from pool

UQH2

to pool

UQ

to pool

2 e– oxidation

at Qp site

2 H+ out

2 H+

UQ.–

at Qn site

Cyt c

Synopsis

1 e–

1 e–

Net

UQH2 + 2 H+ in + 2 Cyt cox 2 e– 4 H+out + 2 Cyt cred+ UQ

UQH2

UQ

UQH2 UQ

UQH2 UQ UQ

ACTIVE FIGURE 20.12 The Q cycle in

mitochondria (a) The electron-transfer pathway

follow-ing oxidation of the first UQH 2 at the Qpsite near the

cytosolic face of the membrane (b) The pathway

fol-lowing oxidation of a second UQH 2 Test yourself on

the concepts in this figure at www.cengage.com/ login.

Cytochrome c Is a Mobile Electron Carrier Electrons traversing Complex III are

passed through cytochrome c1to cytochrome c Cytochrome c is the only one of the

mitochondrial cytochromes that is water soluble Its structure (Figure 20.13) is glob-ular; the planar heme group lies near the center of the protein, surrounded pre-dominantly by hydrophobic amino acid residues The iron in the porphyrin ring is coordinated both to a histidine nitrogen and to the sulfur atom of a methionine residue Coordination with ligands in this manner on both sides of the porphyrin plane precludes the binding of oxygen and other ligands, a feature that distinguishes

cytochrome c from hemoglobin (see Chapter 15).

Cytochrome c, like UQ, is a mobile electron carrier It associates loosely with the

inner mitochondrial membrane (in the intermembrane space on the cytosolic side

of the inner membrane) to acquire electrons from the Fe-S–cyt c1 aggregate of Complex III, and then it migrates along the membrane surface in the reduced state,

carrying electrons to cytochrome c oxidase, the fourth complex of the

electron-transport chain

Complex IV Transfers Electrons from Cytochrome c to Reduce Oxygen

on the Matrix Side

Complex IV is called cytochrome c oxidase because it accepts electrons from cyto-chrome c and directs them to the four-electron reduction of O2to form H2O:

4 cyt c (Fe2) 4 H O2⎯⎯→ 4 cyt c (Fe3) 2 H2O (20.21)

Thus, cytochrome c oxidase and O2are the final destination for the electrons de-rived from the oxidation of food materials In concert with this process, cytochrome

c oxidase also drives transport of protons across the inner mitochondrial

mem-brane The combined processes of oxygen reduction and proton transport involve

a total of 8Hin each catalytic cycle—four Hfor O2reduction and four H trans-ported from the matrix to the intermembrane space

The total number of subunits in cytochrome c oxidase varies from 2–4 (in

bacte-ria) to 13 (in mammals) Three subunits (I, II, and III) are common to most or-ganisms (Figure 20.14) This minimal complex, which contains two hemes (termed

a and a3) and three copper ions (two in the CuAcenter and one in the CuBsite), is sufficient to carry out both oxygen reduction and proton transport

The total mass of the protein in mammalian Complex IV (Figure 20.15) is 204 kD

In mammals, subunits I through III, the largest ones, are encoded by mitochondrial DNA, synthesized in the mitochondrion, and inserted into the inner membrane from the matrix side The 10 smaller subunits are coded by nuclear DNA, are synthesized in the cytosol, and are presumed to play regulatory roles in the complex

FIGURE 20.13 The structure of mitochondrial

cyto-chrome c The heme is shown at the center of the

struc-ture It is covalently linked to the protein via two sulfur

atoms A third sulfur from a methionine residue

coordi-nates the iron (pdb id  2B4Z).

FIGURE 20.14 Bovine cytochrome c oxidase consists of

13 subunits The 3 largest subunits—I (purple), II (yellow),

and III (blue)—contain the proton channels and the

redox centers (pdb id  2EIJ).

FIGURE 20.15 The complete structure of bovine

cyto-chrome c oxidase (pdb id  2EIJ).

In the bovine structure, subunit I is cylindrical in shape and consists of 12

trans-membrane helices, without any significant extratrans-membrane parts Hemes a and a3,

which lie perpendicular to the membrane plane, and CuBare cradled by the helices

of subunit I (Figure 20.16) Subunits II and III lie on opposite sides of subunit I and

do not contact each other (see Figure 20.14) Subunit II has an extramembrane

do-main on the outer face of the inner mitochondrial membrane This dodo-main

con-sists of a 10-strand -barrel that holds the two copper ions of the CuAsite 7 Å from

the nearest surface atom of the subunit Subunit III consists of seven

transmem-brane helices with no significant extramemtransmem-brane domains

Electron Transfer in Complex IV Involves Two Hemes and Two Copper Sites

Electron transfer through Complex IV begins with binding of cytochrome c to the

-barrel of subunit II Four electrons are transferred sequentially (one each from

four molecules of cytochrome c) first to the CuAcenter, next to heme a, and finally

to the CuB/heme a3active site, where O2is reduced to H2O (Figure 20.16):

Cyt c⎯⎯→ CuA⎯⎯→ heme a ⎯⎯→ CuB/heme a3⎯⎯→ O2 (20.22)

A tryptophan residue, which lies 5Å above the CuAsite (Figure 20.17a), is the entry

point for electrons from cytochrome c It lies in a hydrophobic patch on subunit II,

surrounded by a ring of negatively charged Asp and Glu residues Electrons flow

rapidly from CuAto heme a, which is coordinated by a pair of His residues (Figure

20.17b), and then to the CuB/heme a3complex The Fe atom in heme a3is five

co-ordinate (Figure 20.17c), with four ligands from the heme plane and one from

His376 This leaves a sixth position free, and this is the catalytic site where O2binds

and is reduced CuBis about 5Å from the Fe atom of heme a3and is coordinated by

three histidine ligands, including His240, His290, and His291(Figure 20.17c) An

un-usual crosslink between His240and Tyr244lowers the pKaof the Tyr hydroxyl so that

it can participate in proton transport across the membrane

2 H+

2 H+

H2O

Cyt a3 Cyt a

O2+ 2 H + 2

2 Cyt c

CuB

CuA

e–

e–

e–

2 

2 

2 

ACTIVE FIGURE 20.16 The

electron-transfer pathway for cytochrome oxidase Cytochrome c

binds on the cytosolic side, transferring electrons through the copper and heme centers to reduce O 2 on the matrix side of the membrane (pdb id  2EIJ).Test yourself on

the concepts in this figure at www.cengage.com/ login.

Proton Transport Across Cytochrome c Oxidase Is Coupled

to Oxygen Reduction

Proton transport in R sphaeroides cytochrome c oxidase takes place via two channels

denoted the D- and K-pathways (Figure 20.18a) Both these channels contain water molecules, and they are lined with polar residues that can either protonate and de-protonate or form hydrogen bonds The D-pathway is named for Asp132at the chan-nel opening, and the K-pathway is named for Lys362, a critical residue located mid-way in the channel These two channels converge at the binuclear CuB/heme a3site midway across the complex and the membrane Here, Glu286 serves as a branch point, shuttling protons either to the catalytic site for O2reduction (to form H2O)

or to the exit channel (residues 320 to 340) that leads protons to the intermembrane space (Figure 20.18a) In each catalytic cycle, two Hpass through the K-pathway and six Htraverse the D-pathway The K-pathway protons and two of the D-pathway protons participate in the reduction of one O2to two H2O, and the remaining four D-pathway protons are passed across the membrane and released to the intermem-brane space

M207

H 61

H 240

H 290

H 291

C 196

C 200

H161

H 204

E198

FIGURE 20.17 Structures of the redox centers of bovine cytochrome c oxidase (a) The CuAsite, (b) the heme a

site, and (c) the binuclear CuB/heme a3 site (pdb id  2EIJ).

D 229

K  227

T337

H 334

CuB/

heme a3

E  101

T359

Y 288

K 362

S365

H333

D132

N 139

S201

W 172

E 286

E286

Heme a

R481

R 482

N 121

O

(b)

O

C

H H H

H

C

H+

H+

FIGURE 20.18 (a) The proton channels of cytochrome c

oxidase from R sphaeroides Functional residues in the

D- and K-pathways are indicated The D- and

K-pathways converge at the Cu B/heme a3 center The

proton exit channel is lined by residues 320 to 340 of

subunit I (pdb id  1M56) (b) Protons are presumed to

“hop” along arrays of water molecules in the proton

transport channels of cytochrome c oxidase Such a

chain of protonation and deprotonation events means

that the proton eventually released from the exit

chan-nel is far removed from the proton that entered the

D-pathway and initiated the cascade.

How are protons driven across cytochrome c oxidase? The mechanism involves

three key features:

• The pKavalues of protein side chains in the proton channels are shifted (by the

local environment) to make them effective proton donors or acceptors during

transport For example, the pKaof Glu286is unusually high at 9.4 (This is

simi-lar to the behavior of Asp85and Asp96in bacteriorhodopsin; see pages 285–286,

Chapter 9.)

• Electron transfer events induce conformation changes that control proton

trans-port For example, redox events at the CuB/heme a3site are sensed by Glu286and

an adjacent proton-gating loop (residues 169 to 175), controlling H binding

and release by Glu286and proton movement through the exit channel

• Protons are “transported” via chains of hydrogen-bonded water molecules in the

proton channels (Figure 20.18b) Sequential hopping of protons along these

“proton wires” essentially transfers a “positive charge” between distant residues in

the channel (Note that the Hthat arrives at an accepting residue is not the

same proton that left the donating residue.)

The Four Electron-Transport Complexes Are Independent

It should be emphasized here that the four major complexes of the

electron-transport chain operate quite independently in the inner mitochondrial

mem-brane Each is a multiprotein aggregate maintained by numerous strong

associa-tions between peptides of the complex, but there is no evidence that the complexes

associate with one another in the membrane Measurements of the lateral diffusion

rates of the four complexes, of coenzyme Q , and of cytochrome c in the inner

mito-chondrial membrane show that the rates differ considerably, indicating that these

complexes do not move together in the membrane Kinetic studies with

reconsti-tuted systems show that electron transport does not operate by means of connected

sets of the four complexes

The Model of Electron Transport Is a Dynamic One The model that emerges for

electron transport is shown in Figure 20.19 The four complexes are independently

mobile in the membrane Coenzyme Q collects electrons from NADH–UQ

reduc-tase and succinate–UQ reducreduc-tase and delivers them (by diffusion through the

mem-brane core) to UQ–cyt c reductase Cytochrome c is water soluble and moves freely

in the intermembrane space, carrying electrons from UQ–cyt c reductase to

cyto-chrome c oxidase In the process of these electron transfers, protons are driven

across the inner membrane (from the matrix side to the intermembrane space)

The proton gradient generated by electron transport represents an enormous

source of potential energy As seen in the next section, this potential energy is used

to synthesize ATP as protons flow back into the matrix

Electron Transfer Energy Stored in a Proton Gradient:

The Mitchell Hypothesis

In 1961, Peter Mitchell, a British biochemist, proposed that the energy stored in a

pro-ton gradient across the inner mitochondrial membrane by electron transport drives

the synthesis of ATP in cells The proposal became known as Mitchell’s chemiosmotic

matrix to the intermembrane space and cytosol by the events of electron transport

This mechanism stores the energy of electron transport in an electrochemical

potential As protons are driven out of the matrix, the pH rises and the matrix

becomes negatively charged with respect to the cytosol (Figure 20.20) Electron

transport-driven proton pumping thus creates a pH gradient and an electrical

gra-dient across the inner membrane, both of which tend to attract protons back into

the matrix from the cytoplasm Flow of protons down this electrochemical gradient,

an energetically favorable process, drives the synthesis of ATP

Succinate Fumarate

Intermembrane

space

Matrix

III

Cyt cox

Cyt cox

Cyt cred

Cyt cred

UQH2 UQH2

H2O

O2+ 2 H+ 2

FIGURE 20.19 A model for the electron-transport pathway in the mitochondrial inner membrane UQ/UQH 2

and cytochrome c are mobile electron carriers and function by transferring electrons between the complexes.

The proton transport driven by Complexes I, III, and IV is indicated.

Cytosol

Intermembrane space (high [H+], low pH)

Matrix (low [H+], high pH) H+

H+ H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

I

F 0

F 1

IV

FIGURE 20.20 The proton and electrochemical gradients existing across the inner mitochondrial membrane The electrochemical gradient is generated by the transport of protons across the membrane by Complexes I, III, and IV in the inner mitochondrial membrane.

The ratio of protons transported per pair of electrons passed through the chain—

the so-called Hⴙ/2eⴚratio—has been an object of great interest for many years

Nev-ertheless, the ratio has remained extremely difficult to determine The consensus

esti-mate for the electron-transport pathway from succinate to O2is 6H/2e The ratio for

Complex I by itself remains uncertain, but recent best estimates place it as high as

4H/2e On the basis of this value, the stoichiometry of transport for the pathway

from NADH to O2is 10H/2e Although this is the value assumed in Figure 20.19, it

is important to realize that this represents a consensus drawn from many experiments

20.4 What Are the Thermodynamic Implications

of Chemiosmotic Coupling?

Mitchell’s chemiosmotic hypothesis revolutionized our thinking about the energy

coupling that drives ATP synthesis by means of an electrochemical gradient How

much energy is stored in this electrochemical gradient? For the transmembrane

flow of protons across the inner membrane (from inside [matrix] to outside), we

could write

Hin⎯⎯→ H

The free energy difference for protons across the inner mitochondrial membrane

includes a term for the concentration difference and a term for the electrical

po-tential This is expressed as

where c1and c2are the proton concentrations on the two sides of the membrane,

Z is the charge on a proton, Ᏺ is Faraday’s constant, and  is the potential

dif-ference across the membrane For the case at hand, this equation becomes

In terms of the matrix and cytoplasm pH values, the free energy difference is

G  2.303 RT(pHout pHin) Ᏺ (20.26) Reported values for  and pH vary, but the membrane potential is always found

to be positive outside and negative inside, and the pH is always more acidic outside

and more basic inside Taking typical values of   0.18 V and pH  1 unit, the

free energy change associated with the movement of one mole of protons from

in-side to outin-side is

WithᏲ  96.485 kJ/V  mol, the value of G at 37°C is

G  5.9 kJ  17.4 kJ  23.3 kJ (20.28) which is the free energy change for movement of a mole of protons across an inner

membrane Note that the free energy terms for both the pH difference and the

po-tential difference are unfavorable for the outward transport of protons, with the latter

term making the greater contribution On the other hand, the G for inward flow of

protons is 23.3 kJ/mol It is this energy that drives the synthesis of ATP, in accord with

Mitchell’s model Peter Mitchell was awarded the Nobel Prize in Chemistry in 1978

20.5 How Does a Proton Gradient Drive the Synthesis of ATP?

The great French chemist Antoine Lavoisier showed in 1777 that foods undergo

combustion in the body Since then, chemists and biochemists have wondered how

energy from food oxidation is captured by living things Mitchell paved the way by

[Hout] [Hin]

[c2]

[c1]

suggesting that a proton gradient across the inner mitochondrial membrane could drive the synthesis of ATP But how could the proton gradient be coupled to ATP production? The answer lies in a mitochondrial complex called ATP synthase, or sometimes F1F0–ATPase (for the reverse reaction it catalyzes) The F1portion of the ATP synthase was first identified in early electron micrographs of mitochondrial preparations as spherical, 8.5-nm projections or particles on the inner membrane The purified particles catalyze ATP hydrolysis, the reverse reaction of the ATP syn-thase Stripped of these particles, the membranes can still carry out electron transfer but cannot synthesize ATP In one of the first reconstitution experiments with mem-brane proteins, Efraim Racker showed that adding the particles back to stripped membranes restored electron transfer-dependent ATP synthesis

ATP Synthase Is Composed of F1and F0

ATP synthase is a remarkable molecular machine It is an enzyme, a proton pump, and

a rotating molecular motor Nearly all the ATP that fuels our cellular processes is made

by this multifaceted molecular superstar The spheres observed in electron

micro-graphs make up the F 1 unit,which catalyzes ATP synthesis (Figure 20.21) These F1

spheres are attached to an integral membrane protein aggregate called the F 0 unit.F1 consists of five polypeptide chains named

try 33 0includes three hydrophobic subunits

de-noted by a, b, and c, with an apparent stoichiometry of a1b2c10–15 F0forms the trans-membrane pore or channel through which protons move to drive ATP synthesis

The a and b subunits of F0 form part of the stator—a stationary component

anchored in the membrane—and a ring of 10 to 15 c -subunits (see Table 20.3)

constitutes a major component of the rotor of the motor Protons flowing through the

a–c complex cause the c-ring to rotate in the membrane Each c subunit is a folded pair

of-helices joined by a short loop, whereas the a-subunit is presumed to be a cluster

of-helices The b-subunit, together with the d- and h-subunits and the oligomycin

sen-sitivity-conferring protein (OSCP), form a long, slender stalk that connects F0in the membrane with F1, which extends out into the matrix The b, d, and h subunits form

long-helical segments that comprise the stalk, and OSCP adds a helical bundle cap

that sits at the bottom of an -subunit of F1(Figure 20.21) The stalk is a stable link be-tween F0and F1, essentially joining the two, both structurally and functionally

The Catalytic Sites of ATP Synthase Adopt Three Different Conformations

The F1structure appears at first to be a symmetric hexamer of - and -subunits.

However, it is asymmetric in several ways The - and -subunits, arranged in an

al-ternating pattern in the hexamer, are similar but not identical The hexamer

con-Stator

Rotor shaft Rotor

FIGURE 20.21The ATP synthase, a rotating molecular

motor The c-,

portion (the rotor) of the motor Flow of protons from

the a-subunit through the c -subunit turns the rotor and

drives the cycle of conformational changes in  and 

that synthesize ATP (pdb id  1C17; 1E79; 2A7U; 2CLY;

and 2BO5).

Protein Subunit Mass

*The number of c subunits varies among organisms: yeast mitochondria, 10; Ilyobacter tartaricus, 11; Escherichia coli, 12; spinach chloroplasts, 14; Spirulina platensis, 15.

†The subunit nomenclature can be confusing E coli ATP synthase lacks a -subunit in its rotor; its -subunit is analogous

TABLE 20.3 Yeast F 1 F 0 –ATP Synthase Subunit Organization