Ch 18 Oxidative Phosphorylation

Front 1

Alternative names for NADH-Q oxidoreductase, Q-cytochrome c oxidase, cytochrome c oxidase, and succinate-Q reductase

Back 1

complex I, complex III, complex IV, complex II (does not pump protons)

Front 2

What is the supramolecular complex?

Back 2

respirasome
-large protein complexes

Front 3

What is the purpose of the supramolecular complex?

Back 3

To facilitate rapid transfer of substrate
To prevent release of reaction intermediates

Front 4

Name the 4 large protein complexes in ETC

Back 4

NADH-Q oxidoreductase
succinate-Q reductase
Q-cytochrome c oxidase
cytochrome c oxidase

Front 5

Name the 2 electron carriers in ETC

Back 5

coenzyme Q / ubiquinone
cytochrome c

Front 6

coenzyme Q

Back 6

Q/ubiquinone
-hydrophobic quinone
-diffuses rapidly within inner mito membrane
-carries e- from complex I to III

Front 7

Which carrier shuttles e- from complex I to III?

Back 7

coenzyme Q

Front 8

What is coenzyme Q?

Back 8

quinone derivative with long tail of 5-C isoprene units
-most common is 10 isoprene (Q10)

Front 9

How many oxidation states does quinone have?

Back 9

3

Front 10

Describe coupling with Q

Back 10

e- transfer coupled to proton binding and release

Front 11

What is the Q pool?

Back 11

ubiquinone is soluble in membrane, so a pool of Q and QH2 is thought to exist within the inner mito membrane

Front 12

What is cytochrome c?

Back 12

A small soluble protein that shuttles e- from complex II to VI

Front 13

Which carrier shuttles e- from complex II to VI?

Back 13

cytochrome c

Front 14

complex I

Back 14

NADH-Q oxidoreductase
NADH dehydrogenase

Front 15

complex II

Back 15

succinate dehydrogenase

Front 16

complex III

Back 16

cytochrome c reductase

Front 17

complex IV

Back 17

cytochrome c oxidase

Front 18

Which protein does not pump protons?

Back 18

succinate dehydrogenase

Front 19

How is coupling of proton binding and release to e- transfer accomplished with Q?

Back 19

QH2 (fully reduced) holds onto protons more tightly

Front 20

Where are the genes that encode for 3 proteins in respirasome?

Back 20

mitochondria and nucleus

Front 21

Shape of NADH-Q oxidoreductase

Back 21

L
arm projects to matrix

Front 22

Reaction catalyzed by NADH-Q oxidoreductase

Back 22

NADH + Q + 5H(matrix) --> NAD+ + QH2 + 4H(cyto)

Front 23

1st prosthetic group of complex I

Back 23

flavin mononucleotide
FMN -> FMNH2

Front 24

e- acceptor of FMN

Back 24

isoalloxazine ring
identical with that of FAD

Front 25

2nd prosthetic group of complex I

Back 25

series of iron-sulfur clusters (non-heme proteins)
complex I contains 2Fe-2S and 4Fe-4S

Front 26

two states of Fe-S

Back 26

Fe2+(reduced) and Fe3+(oxidized)

Front 27

Which prosthetic groups bind/release protons in redox?

Back 27

quinones and flavins (not Fe-S clusters)

Front 28

Where do the redox reactions take place in complex I?

Back 28

extramembranous part

Front 29

When 2e- flow from NADH to coenzyme Q...

Back 29

4H+ pumped out of matrix into intermembrane space

Front 30

Q accepts 2 - from - and 2 - from - to be reduced to QH2. QH2 leaves - to -.

Back 30

Q accepts 2 e- from complex I and 2 H+ from matrix to be reduced to QH2. QH2 leaves enzyme to hydrophobic interior of membrane.

Front 31

What are the sources of NADH for ETC?

Back 31

-citric acid cycle
-fatty acid degradation
-cytoplasmic

Front 32

Where is FADH2 generated?

Back 32

CAC in oxidation of succinate to fumarate by succinate deHase which is part of complex II

Front 33

flow of e- in complex I

Back 33

(NAD+ -> NADH) -> (FMN -> FMNH2) -> n(Fe3+S -> Fe2+S)

Front 34

flow of e- in complex II (Q-cytochrome c oxidoreductase)

Back 34

(FADH2 -> FAD) -> n(Fe3+S -> Fe2+S) -> (Q -> QH2)

Front 35

Which oxidation produces less ATP

Back 35

oxidation of FADH2 b/c it doesn't pump protons

Front 36

What other enzymes are electron carriers for FADH2 to O2?

Back 36

glycerol phosphate deHase, fatty acyl CoA deHase
these don't pump protons (like Fe-S in complex II)

Front 37

What is the fate of QH2 (ubiquinol) generated from complex I and II?

Back 37

passed to complex III to cytochrome c

Front 38

What is the role of Q-cytochrome oxidoreductase?

Back 38

Catalyze transfer of e- from QH2 to oxidized cytochrome c (water soluble), and pump protons out of the matrix

Front 39

How many H+ does complex III pump out?

Back 39

2
less than complex I between of smaller thermodynamic force

Front 40

eq of QH2 and Cyt c e- transfer

Back 40

QH2 + 2Cytc(ox) + 2H+(matrix) -> Q + 2Cytc(red) + 4H+(cyto)

Front 41

Definition of cytochrome

Back 41

e- transferring protein that contains heme prosthetic group

Front 42

How many total hemes are in Q-cytochrome oxidoreductase (cytochrome bc1)? what type are they?

Back 42

3 hemes total
-2 within cytochrome b (bL and bH)
-1 within cytochrome c1

iron-protoporphyrin IX type hemes

Front 43

How many subunits are in Q-cytochrome oxidoreductase?

Back 43

2
b and c1

Front 44

What type of heme is present in both cytochrome bc1 and myo/hemoglobin?

Back 44

iron-protoporphyrin IX

Front 45

Describe the Rieske center

Back 45

2Fe-2S center in cytochrome bc1 (Q-cytochrome c oxidoreductase)
-1 Fe coordinated with his, not cys

Front 46

Strategy of Fe-S coordination

Back 46

stabilizes Fe center in reduced form, raising reduction potential so it can readily accept electrons

Front 47

Q cycle

Back 47

mechanism of coupling of e- transfer (Q -> cytochrome c) to transmembrane proton transport
1. 2 QH2 bind to complex consecutively, each giving up 2e- and 2H+ (protons released to cyto)

Front 48

Can cytochrome c accept both 2e- from QH2?

Back 48

no, 1 at a time - Q cycle

Front 49

Q cycle sum

Back 49

4H+ released to cyto, 2H+ removed from matrix

2QH2 + Q + 2Cytc(ox) + 2H+(matrix) -> 2Q +QH2 + 2Cytc(red) + 4H+(cyto)

Front 50

How to solve problem of efficiently funneling e- from 2e- carrier to 1e- carrier?

Back 50

Q cycle

Front 51

Component of cytochrome that does Q cycle

Back 51

cytochrome b

Front 52

Role of cytochrome c oxidase (complex IV)

Back 52

catalyzes transfer of e- from reduced form of cytochrome c to molecular oxygen

Front 53

eq of cytochrome c oxidase

Back 53

4Cytc(red) + 8H+(matrix) -> 4Cytc(ox) +2H2O + 4H+(cyto)

Front 54

Prosthetic groups of cytochrome c oxidase

Back 54

2 heme A groups, 3 Cu ions arranged as 2 Cu centers (A, B)
CuA center: (CuA/CuA) linked by 2 cys residues
CuB center: 3 his residues

Front 55

^Go' of cytochrom c oxidase reaction

Back 55

-231.8 kJ/mol (highly favorable)
energy captured as a proton gradient

Front 56

e- flow in cytochrome c oxidase

Back 56

cytochrome c -> CuA/CuA -> heme a -> heme a3 -> CuB -> O2
-heme a3 and CuB directly adjacent -> formed active center

Front 57

Which prosthetic groups form active center at which O2 reduced to H2O?

Back 57

heme a3 and CuB

Front 58

How many cytochrome c bind to cytochrome c oxidase to reduce one O2?

Back 58

4 bind consecutively

Front 59

Steps of cytochrome c e- transfer to O2

Back 59

1. 2 molecules of cytochrome c sequentially transfer e- to reduce CuB and heme a3 --> ready to bind O2
2. reduced CuB and Fe (heme a3) bind O2, which forms peroxide bridge (O22-)
3. addition of 2 more e- and 2 more H+ from 2 cytochrome c, which travel to active center and cleave peroxide bridge
4. addition of 2 more H+ leads to release of H2O from heme and Cu. cytochrome c oxidase regenerated
*4H+ come from matrix -> contribute to H+ gradient

Front 60

How much energy does each proton contribute to gradient?

Back 60

21.8 kJ/mol

Front 61

Where does free energy from reduction of water go to?

Back 61

-pump 4H+ into matrix
-pump 4 additional H+ into matrix

Front 62

eq of Cyt c

Back 62

4Cytc(red) + 4H+(matrix) + O2 -> 4Cytc(ox) + 2H2O
overall:
4Cytc(red) + 8H+(matrix) + O2 -> 4Cytc(ox) + 2H2O 4H+(cyto)

Front 63

What are the effects that contribute to mechanism of how protons transport through cytochrome c oxidase

Back 63

1. charge neutrality maintained in interior of proteins
-addition of e- to site favors binding of H+ to nearby site
2. conformation changes take place (heme a3-CuB center)
-in one conformation, protons may enter exclusively from matrix side, or exit exclusively to cytoplasm in another

Front 64

transfer of single e- to O2 yields:

Back 64

superoxide anion

Front 65

transfer of 2e- to O2 yields:

Back 65

peroxide

Front 66

Why does O2 get reduced?

Back 66

-ideal terminal e- acceptor
-high affinity for e- provides large thermdynamic driving force

Front 67

Strategy to reduce O2 safely

Back 67

catalyst does not release partly reduced intermediate
-cytochrome c oxidase holds O2 tighly between Fe and Cu

Front 68

Name some reactive oxygen species

Back 68

superoxide, hydrogen peroxide, OHdot

Front 69

Defense strategies against oxidative damage by ROS

Back 69

-superoxide dismutase in mito(Mn) and cyto(Cu):
O2-dot + 2H+ <-> O2 + H2O2
-catalase: heme
2H2O2 <-> O2 + H2O
-glutathione peroxidase
-antioxidant vitamins E and C
-aerobic exercise

Front 70

What can ROSs be helpful for?

Back 70

signal transduction pathways
-growth factors increase ROS levels
-ROS regulate channels and transcription factors

Front 71

What is the average separation between e- carrying groups?

Back 71

15 angstroms

Front 72

Why are proteins more efficient in e- transfer?

Back 72

In vacuum, e- transfer rate decreases by a factor of 10 for every 0.8 A.
Through proteins, the rate decreases by 10 for every 1.7 A, making it efficient.

Front 73

Which protein in ETC has been conserved?

Back 73

cytochrome c

Front 74

What proves that cytochrome c is preserved?

Back 74

Cytochrome c of any eukaryotic species reacts in vitro with cytochrome c oxidase of any other species
-wheat germ cyto c and human cyto c oxidase

21/104 residues have been invariant for over 1.5 billion years

Front 75

eq for flow of e- from NADH to O2

Back 75

NADH + .5O2 + H+ <-> H2O + NAD+
^Go' = -220.1 kJ/mol
exergonic

Front 76

eq for ATP synthesis coupled with proton flow

Back 76

ADP + Pi + H+ <-> ATP + H2O
^Go' = +30.5 kJ/mol
endergonic

Front 77

Name of enzyme that synthesizes ATP

Back 77

ATP synthase or Complex V
(discovered as mitochondrial ATPase or F1F0ATPase)

Front 78

Chemiosmotic hypothesis

Back 78

e- transport and ATP synthesis are coupled by proton gradient across inner mito membrane
-flow of protons back into matrix drives ATP synthase

Front 79

What is the proton-motive force?

Back 79

energy-rich unequal distribution of protons
-chemical (pH) and charge (psi) gradients both power synthesis of ATP
-pH outside is 1.4 units higher
-membrane potential is 0.14 V (outside +)

Front 80

Describe the structure of ATP

Back 80

'ball and stick'
stick - F0 subunit in the inner mito membrane
ball - F1 subunit in the mito matrix (85 D diameter)

Front 81

Which subunit of ATP synthase contains catalytic activity?

Back 81

F1 subunit

Front 82

Structure of the F1 subunit

Back 82

5 types of polypeptide chains (a3, B3, y, del, e)
-a and B arranged in hexameric ring (a3B3), members of P-loop NTPase family. both bind nucleotides but only B units participate directly in catalysis
-y subunit is a long helical coiled coil in the center of hexamer, breaks symmetry of hexamer

Front 83

How are the 3 beta subunits in F1 distinguished?

Back 83

by the direction of the y subunit

Front 84

Which subunit contains the proton channel of ATP synthase?

Back 84

F0
ring with 10-14 c subunits embedded in membrane
a single a (NOT alpha) subunit binds to outside of ring

Front 85

How are F1 and F0 subunits connected?

Back 85

by central y-e stalk, and by an exterior colummn (a, b, del subunits)

Front 86

Formation of ATP from ADP and orthophosphate

Back 86

ADP3- + HPO42- + H+ <-> ATP4- + H2O

Front 87

What kind of intermediate is formed in ATP synthesis?

Back 87

pentacovalent intermediate on the terminal phosphate

Front 88

What is the very main role of the proton gradient in ATP synthesis?

Back 88

to release ATP from the synthase
(isotopic exchange experiments, 18O labeled Pi)

Front 89

Which part of the ATP synthase is the roto?

Back 89

c ring and ye stalk

Front 90

Which part of the ATP synthase is the stator (stationary unit)

Back 90

hexamer

Front 91

binding change mechanism of ATP synthesis

Back 91

1. ADP and Pi binding (L conformation)
2. ATP synthesis (T conformation)
3. ATP release (O conformation)

Front 92

What are the conformations and sequence of the hexamer?

Back 92

L (loose) -> T (tight) -> O (open) -> L -> T -> O ->.....

Front 93

What drives the interconversion of the 3 forms of the ATP binding sites?

Back 93

rotation of the y subunit

Front 94

What direction does the y subunit rorate in?

Back 94

counterclockwise

Front 95

How does proton flow through F0 drive the rotation of the y subunit?

Back 95

2 half channels in the a subunit does not span the membrane - each half channel directly interacts with one c subunit
-the c subunit with a proton attached it its asp residue will rotate until it is in a proton-poor environment

Front 96

What powers the rotation of the c ring?

Back 96

movement of protons through the half-channels from the high proton concentration of the cytoplasm to the low proton concentration of the matrix
-ability of neutralized asp residue to occupy hydrophobic membrane

Front 97

How does c ring rotation lead to synthesis of ATP?

Back 97

the c ring is tightly bound to y and e subunits - as the c ring turns, the y and e subunits are turned inside the hexamer unit of F1 and cause binding changes
-2 b chains and del subunits on the exterior prevent the hexamer from rotating

Front 98

About how many protons are required to generate 1 ATP?

Back 98

10 / 3 ~ 3.33 protons

e- from NADH pump enough protons for 2.5 ATP
e- from FADH2 yield 1.5 ATP

Front 99

How is ATP synthase analogous to G proteins?

Back 99

they are both P-loop NTPases
G proteins don't bind unless they interact with other proteins
-analogous to the binding-change mechanism
-the P-loop regions of B subunits will bind either ADP or ATP (or release) depending of y subunit direction

Front 100

Is the inner mito membrane permeable to NADH and NAD+?

Back 100

no!

Front 101

How is the respiratory chain important for glycolysis?

Back 101

it regenerates NAD+

Front 102

How is cytoplasmic NADH reoxidized to NAD+ under aerobic conditions?

Back 102

e- from NADH are passed across mito membrane
-glycerol 3-phosphate shuttle (muscle)
-malate-aspartate shuttle (heart, liver)

Front 103

glycerol 3-phosphate shuttle

Back 103

1. Transfer e- from NADH to dihydroxyacetone phosphate to form glycerol 3-phosphate (catalyzed by glycerol 3-phosphate dehydrogenase in cytoplasm)
2. Glycerol 3-phosphate is reoxidized to dihydroxyacetone phosphate on the outer surface of the inner mito membrane by a membrane-bound isozyme of glycerol 3-phosphate deHase
3. An e-pair from glycerol 3-P is transferred to FAD in the enzyme to form FADH2, also regenerating dihydroxyacetone phosphate
4. The reduced flavin transfers e- to Q, which enters respiratory chain as QH2
**1.5, not 2.5 ATP formed because FAD, not NAD+ is e- acceptor
***use of FAD enables e- from cytoplasmic NADH to be transported into mito against an [NADH] gradient
****price = 1 ATP per 2e-

Front 104

Where is the glycerol 3-P shuttle prominent?

Back 104

in muscle
-enables it to sustain a very high rate of oxidative phosphorylation

Front 105

If insects lack lactate dehydrogenase, how do they regenerate cytoplasmic NAD+?

Back 105

They use the glycerol 3-P shuttle

Front 106

malate-asparate shuttle

Back 106

Mediated by 2 membrane carriers and 4 enzymes
1. e-'s are transferred from cytoplasmic NADH to oxaloacetate, forming malate, which passes inner mito membrane in exchange for alpha-kg
2. Malate is reoxidized by NAD+ to form oxaloacetate and NADH in the matrix by malate deHase from the citric acid cycle
3. Oxaloacetate undergoes transamination reaction to form aspartate which can be transported to the cyto in exchange for glutamate
4. Glutamate donates an amino group to oxaloacetate to form aspartate and a-kg
5. In the cytoplasm, asparate is deaminated to form oxaloacetate

Front 107

How are ATP and ADP transported across the inner mitochondrial membrane?

Back 107

via ATP-ADP translocase
*the flows of ATP and ADP are coupled
ADP3- transported into matrix, ATP4- transported to cytoplasm

Front 108

What type of transport protein is ATP translocase? How abundant is it?

Back 108

-antiporter
-constitutes about 15% of proteins in the inner mito membrane

Front 109

Describe the ATP-ADP exchange

Back 109

ATP and ADP bind to the translocase (not Mg2+)
-in an actively respiring mito with positive membrane potential, ATP4- transport out of mito matrix and ADP3- transport into the matrix are favored
-this is energetically expensive: 1/4 energy yield from e- transfer is consumed to REGENERATE the membrane potential tapped by the exchange process
-inhibition of this process leads to inhibition of respiration

Front 110

Describe the common tripartite structure of mitochondrial transporters

Back 110

-tripartite structure: 3 tandem repeats of 100 aa module, each with 2 transmembrane segments
-the transmembrane helices form a tepeelike structure with the nucleotide binding site in the center

Front 111

What other molecules do the various mitochondrial transporters transport?

Back 111

(matrix/cytoplasm)
ADP/ATP - translocase
phosphate/malate - dicarboxylate carrier
malate/citrate + H+ - tricarboxylate carrier
pyruvate/OH- - pyruate carrier
phosphate/OH- - phosphate carrier

Front 112

Which carrier works in concert with ATP-ADP translocase?

Back 112

The phosphate carrier
-mediates exchange of H2PO4- for OH-
-this leads to the exchange of cytoplasmic ADP and Pi for matrix ATP at the cost of influx of H+ (as said before, OH- outflux)

Front 113

What comprises the ATP synthasome?

Back 113

ATP-ADP translocase and the phosphate carrier

Front 114

Other homologous carriers in the inner mito membrane

Back 114

-dicarboxylate carrier enables malate, succinate, fumarate export in exchange for Pi
-tricarboxylate carrier exchanges citrate and H+ for malate

Front 115

How many molecules does the complete oxidation of glucose yield?

Back 115

30 molecules of ATP

Front 116

Can you know the number of ATP (or GTP) formed in glycolysis and the citric acid cycle for certain? How about ETC?

Back 116

Yes, because it is determined by the stoichiometries of chemical reactions.
Not in ETC because ATP yield because the stoichiometries of proton pumping, ATP synthesis, and metabolite-transport processes are not integers or fixed values

Front 117

What is the estimate for number of H+ pumped out of the matrix per e- pair by NADH-Q oxidoreductase, Q-cytochrome c oxidoreductase, and cytochrome c oxidase?

Back 117

4, 2, 4

Front 118

How many protons are consumed in transporting ATP out of the matrix?

Back 118

1 proton influx
from coupling with the phosphate carrier which outfluxes 1 OH-

Front 119

How many molecules of cytoplasmic ATP generated from the flow of e- pair from NADH to O2?

Back 119

2.5 molecules of ATP

Front 120

How many molecules of cytoplasmic ATP generated from the flow of e- pair from complex II (succinate oxidation or cytoplasmic NADH) to O2?

Back 120

1.5 molecules of ATP

Front 121

Describe respiratory control or acceptor control

Back 121

Regulation of the rate of oxidative phosphorylation by ADP level
-rate of O2 consumption by mito increases markedly when ADP is added, returns to normal when ADP converted to ATP

Front 122

How is the citric acid cycle affected by the level of ADP?

Back 122

At low [ADP], NADH and FADH2 are not consumed by the ETC. The citric acid slows because there is less NAD+ and FAD to feed the cycle. As [ADP] rises and ETC speeds up, NADH and FADH2 are oxidized and the citric acid cycle speeds up.

Front 123

Describe the regulatory significance of energy change in ETC

Back 123

e- do not flow from fuel molecules to O2 unless ATP needs to be synthesized

Front 124

How can you generate heat from oxidative phosphorylation?

Back 124

Uncouple ETC from ATP synthesis.
-in humans, coupling is in brown adipose tissue for nonshivering thermogenesis

Front 125

What is brown fat?

Back 125

-specialized tissue for nonshivering thermogenesis
-very rich in mitochondria
-brown from greenish cytochromes in many mitochondria and red hemoglobin in extensive blood supply that carries heat

Front 126

Uncoupling protein (UCP-1)

Back 126

also known as thermogenin (resembles translocase)
-Forms a pathway for flow of protons from cytoplasm to matrix
-short-circuits the mito proton battery
-activated when core body temp falls -> released hormones release fatty acids that activate thermogenin
-others include UCP-2 and UCP-3 in other tissues

Front 127

Where can oxidative phosphorylation be inhibited?

Back 127

1. inhibition of ETC
2. Inhibition of ATP synthase
3. Uncoupling from ATP synthesis
4. Inhibition of ATP export

Front 128

rotenon and amytal

Back 128

block e- transfer in complex I and prevent use of NADH as a substrate
However, e- can flow from oxidized succinate because they enter through QH2, not through complex I

Front 129

antimycin A

Back 129

interferes with e- flow from cytochrome bH in complex III

Front 130

cyanide (CN-) and azide (N3-) and carbon monoxide (CO)

Back 130

interferes with complex IV
-cyanide and azide react with ferric heme a3
-CO inhibits ferrous heme a3

Front 131

oligomycin and dicyclohexylcarbodiimide (DCC)

Back 131

prevent the influx of protons through ATP synthase
-oligomycin an antifungal agent

Front 132

2,4-dinitrophenol (DNP) and some acidic aromatic compounds

Back 132

uncouples the tight coupling of ETC and phosphorylation
-carry H+ across inner mito membrane along [gradient]
-ATP not synthesized because the proton-motive force is dissipated
-O2 and NADH consumed
-energy not captured as ATP, but released as heat

Front 133

atractyloside and bongkrekic acid

Back 133

-atractyloside binds to translocase when it faces the cytoplasm
-bongkrekic acid binds toe translocase when it faces the mito matrix
-oxidative phosphorylation stops soon after - translocase is essntial for maintaining ADP to accept proton-motive force energy

Front 134

Name the uses for the energy produced by the proton gradient

Back 134

active transport
electron potential
heat production
NADPH synthesis
ATP
flagellar motion

Front 135

apoptosis

Back 135

programmed cell death regulated by mitochondria

Front 136

mitochondrial outer membrane permeabilization (MOMP)

Back 136

Process by which the outer membrane of damaged mitochondria becomes highly permeable
-initiated by Bcl family proteins
1. cytochrome c exits the mitochondira and interacts with apoptotic pertidase-activating factor 1 (APAF-1), which leads to formation of the apoptosome
2. apoptosome activates proteolytic enzyme called caspase 9 that activates a cascade of other caspases

also destroys the inhibitor of caspase-activated DNAase or CAD

Front 137

What are the e- carriers in the respirasome?

Back 137

quinones, flavins, iron-sulfur complexes, heme groups of cytochromes, copper ions

Front 138

Sequence of ETC

Back 138

1. Electrons from NADH are transferred to the FMN prosthetic group of NADH-Q oxidoreductase (Complex I), the first of four complexes. This oxidoreductase also contains Fe-S centers.
2. The electrons emerge in QH2, the reduced form of ubiquinone (Q). The citric acid cycle enzyme succinate
dehydrogenase is a component of the succinate-Q reductase
complex (Complex II), which donates electrons from FADH2 to Q to form QH2.
3. This highly mobile hydrophobic carrier transfers its electrons to Q-cytochrome c oxidoreductase (Complex III), a complex that contains cytochromes b and c1 and an Fe-S center. This complex reduces cytochrome c, a water-soluble peripheral membrane protein.
4. Cytochrome c, like Q, is a mobile carrier of electrons, which it then transfers to cytochrome c oxidase (Complex IV). This complex contains cytochromes a and a3 and three copper ions.
5. A heme iron ion and a copper ion in this oxidase transfer electrons to O2, the ultimate acceptor, to form H2O.