1 ATP Formation by Electron-Transport Chains Mitochondrial Electron-Transport Components of the Electron-Transport Chain Oxidative Phosphorylation Recycling

1

ATP Formation by Electron-Transport Chains

ATP Formation by Electron-Transport Chains

Mitochondrial Electron-Transport

Components of the Electron-Transport Chain

Oxidative Phosphorylation

Recycling of Cytoplasmic NADH

Photosynthetic Electron-Transport

Synthesis of Carbohydrates by the Calvin Cycle

Mitochondrial Electron-Transport

Components of the Electron-Transport Chain

Oxidative Phosphorylation

Recycling of Cytoplasmic NADH

Photosynthetic Electron-Transport

Synthesis of Carbohydrates by the Calvin Cycle

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IntroductionIntroduction

Up to this point, we have dealt with

• Oxidation of substrates.

• Collection of electrons by cofactors.

Energy from the cofactors is recovered using O2 as the final electron acceptor.

This is accomplished using a series of carriers in the inner mitochondrial membrane .

3

Mitochondrial electron transportMitochondrial electron transport

Stage I and II of carbohydrate catabolism converge at the mitochondria.

Stage I

Stage II

citric acid cycle

electron-transport

oxidative phosphorylation

4

Mitochondrial electron transportMitochondrial electron transport

• Extensive inner membrane folding in the mitochondria provides a large surface area.

• There are many molecular systems on this membrane for production of ATP.

• Electron-transport chain components are arranged in packages called respiratory respiratory assembliesassemblies.

• There are also knob-like spheres called FF11 particlesparticles.

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Electron-transport andoxidative phosphorylation


Electrons obtained from nutrients and metabolic intermediates are transferred to NAD+ and FAD.

AH2 + NAD+ A + NADH + H+

BH2 + FAD B + FADH2

Since NAD+ and FAD are in limited supply, they must be recycled.

dehydrogenase

dehydrogenase

6



Recycling is accomplished by oxidation and transfer of electrons to oxygen.

NADH + H+ + 1/2 O2 NAD+ + H2O

FADH2 + 1/2 O2 FAD + H2O

ADP + Pi ATP

ADP + Pi ATP

NAD+ and FAD are then available for additionaloxidative metabolism. The energy released duringelectron transport is coupled to ATP synthesis.

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Electron-transport chainElectron-transport chain

Composed of four large protein complexes.

• Complex I - NADH-Coenzyme Q reductase

• Complex II - Succinate-Coenzyme Q reductase

• Complex III - Cytochrome c reductase

• Complex IV - Cytochrome c oxidase

Many of the components are integral membrane proteins with prosthetic groups to move electrons.

8

Electron-transport chainElectron-transport chain

Two important characteristics of the Two important characteristics of the electron-transport chainelectron-transport chain

• order of electron carriers

• quantity of energy produced

Electron carriers are arranged in order of increasing electron affinity.

This results in the spontaneous flow of electrons from carrier to carrier.

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Flow of electronsFlow of electrons

10

Energy producedEnergy produced

The amount of energy can be calculated in terms of Go’ :

NADH + H+ + 1/2 O2 NAD+ + H2O

Go’ = - 220 kJ/mol

FADH2 + 1/2 O2 FAD + H2O

Go’ = - 152 kJ/mol

Note:Note: ADP + Pi ATP Go’ = +31 kJ/mol

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Components of theelectron transport chain


Complex IComplex I

• Electrons flow from NADH to flavin mononucleotide (FMN) - similar to FAD.

• Electrons then flow to a prosthetic group on an iron-sulfur cluster - iron cycles between 3+ and 2+ states.

• Complex I terminates at ubiquinone - also called coenzyme Q or CoQ.

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Complex IComplex I

H+

13



flavoprotein

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Iron-sulfur clustersIron-sulfur clusters

Fe

S

S

Fe

Cys

Cys

Cys

Cys

Fe

S

SS

FeFe Fe

S

SS

S

S Cys

CysCys

Cys

protein

S

S

S

S

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CoQ - ubiquinoneCoQ - ubiquinoneHighlighted region serves as an anchor to

inner mitochondrial membrane.

O

O

CH3H3CO

H3CO CH C

CH3

CH2)10(CH2 H

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Reduction of CoQReduction of CoQ

O

OH

R

CH3H3CO

H3CO

OH

OH

R

CH3H3CO

H3CO

O

O

R

CH3H3CO

H3CO

Oxidized formUbiquinone (CoQ)

Reduced formUbiquinol (CoQH2)

intermediate,semiquinone

e- +H+

e- +H+

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Complex IIComplex II

• Entry point for both FADH2 and Complex I.

• Succinate dehydrogenaseFrom the citric acid cycle. Directs transfer of electrons from succinate to CoQ via FADH2.

• Acyl-CoA dehydrogenaseFrom -oxidation of fatty acids. It also transfers electrons to CoQ via FADH2.

Both enzymes have iron-sulfur clusters as prosthetic groups and are integral proteins.

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All electrons from FADH2 and NADH must pass through CoQ.

Fe-S

FMN

NADH NAD+

I

II

Succinate

FAD

Fe-S

Fatty acylCoA

FAD

matrix

innermembranespace

CoQ

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Complex IIIComplex IIIElectron transfer from ubiquinol to

cytochrome c.

cytochrome c

heme prosthetic group

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Structure of cytochrome c heme group.

Fe

N

N

N N

H3C CHCH3

S

H3C

H3C

H3C CH2CH2COO-

CH2CH2COO-

CH3

Protein

Fe

N

N

N N

H3C CHCH3

S

H3C

H3C

H3C CH2CH2COO-

CH2CH2COO-

CH3

Protein

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Complex IVComplex IV

• Combination of cytochromes a and a3 - cytochrome c oxidasecytochrome c oxidase.

• Consists of 10 protein subunits, 2 types of prosthetic groups - 2 heme and 2 Cu.

• Cytochromes a and a3 are the only species capable of direct transfer of electrons to oxygen.

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matrix

ComplexI

ComplexIII

ComplexIV

CoQcyt b

cytc

cytc1

(Cu)cyta/a3

NADH O2

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Oxidative phosphorylationOxidative phosphorylation

• The electron-transport chain moves electrons from NADH and FADH2 to O2.

• The next step is the phosphorylation of ADP to produce ATP.

Catalyzed by the inner membrane enzyme ATP synthaseATP synthase.

• The steps are coupled - electrons do not flow to oxygen unless ATP is needed.

Each NADH produces 3 ATPEach FADH2 produces 2 ATP

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Coupling of electron-transportwith ATP synthesis

Coupling of electron-transportwith ATP synthesis

Chemiosmotic coupling mechanismChemiosmotic coupling mechanism

• Electron-transport causes unidirectional movement of H+ into the innermembrane space.

• The results in a H+ gradient being produced.

• The gradient then drives the synthesis of ATP.

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Coupling of electron-transportwith ATP synthase

Coupling of electron-transportwith ATP synthase

Inner mitochondrial membrane

Outer mitochondrial membrane

H+ H+

H+

H+

H+H+H+

H+H+

H+

H+

H+ H+

H+

H+

ADP + Pi ATP

Electron Transport

Chain

Electron Transport

Chain

F1-ATPsynthasecomplex

F1-ATPsynthasecomplex

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Components of ATP synthaseComponents of ATP synthase

These are knob-like projections into the matrix side of the inner membrane.

Two unitsTwo units

• F1 contains the catalytic site for ATP synthesis.

• F0 serves as a transmembrane channel for H+ flow.

F1-F0 complex serves as the molecular apparatus for coupling H+ movement to ATP synthase.

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Components of ATP synthaseComponents of ATP synthase

H+

H+H+ H+

H+

H+H+

H+H+

H+

F0

F1

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Regulation of oxidative phosphorylation

Regulation of oxidative phosphorylation

• Electrons do not flow unless ADP is present for phosphorylation

• Increased ADP levels cause an increase in catabolic reactions of various enzymes including:

glycogen phosphorylaseglycogen phosphorylase

phosphofructokinasephosphofructokinase

citrate synthasecitrate synthase

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Uncoupling of electron-transportand oxidative phosphorylation

Uncoupling of electron-transportand oxidative phosphorylation

• In some special cases, the coupling of the two processes can be disrupted.

• Large amounts of O2 are consumed but no ATP is produced.

• Used by newborn animals and hibernating mammals.

• Occurs in ‘brown fat’- dark color due to high levels of mitochondria which contain thermogeninthermogenin (uncoupling protein).

• Thermogenin allows the release of energy as heat instead of ATP.

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Energy production from glucoseEnergy production from glucose

GlycolysisGlycolysis2 ATP 2 ATP2 NADH 3 ATP/NADH 6 ATP*

Citric Acid CycleCitric Acid Cycle2 GTP 1 ATP/GTP 2 ATP6 NADH 3 ATP/NADH 18 ATP2 FADH2 2 ATP/FADH2 4 ATP

38 ATP(in heart)

* 4 ATP in muscle and brain.36 ATP / glucose

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Energy production from glucoseEnergy production from glucose

GlycolysisMitochondria

Glucose 2 PyruvateGlucose 2 Pyruvate

Oxidativephosphorylation

Oxidativephosphorylation

6 NADH+

2 FADH2

6 NADH+

2 FADH2 2 NADH2 NADH2 NADH2 NADH

2 ATP2 ATP 2 ATP2 ATP32-34 ATP32-34 ATP

2 Acetyl CoA2 Acetyl CoA

2 GTP2 GTP

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Recycling of cytoplasmic NADHRecycling of cytoplasmic NADH

Different methods are used to recycle NADH. This accounts for the different energy productions from glucose.

Glycerol-3-phosphate shuttleGlycerol-3-phosphate shuttleUsed by skeletal muscles and the brain

Malate-aspartate shuttleMalate-aspartate shuttleUsed by the heart and liver

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Glucose-3-phosphate shuttleGlucose-3-phosphate shuttle

NAD+ NADH + H+

cytoplasmicglycerol-3-phosphate

dehydrogenase

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Malate-aspartate shuttleMalate-aspartate shuttle

L-aspartate

Matrix

-ketoglutarate

L-glutamate

L-aspartate

-ketoglutarate

L-glutamate

oxaloacetate

mitochondrialaspartate

aminotransferase

L-malate L-malate

oxaloacetate

NAD+

3 ATP

NAD+NADH+ H+

NADH+ H+

Glycolysis

mitochondrialmalate

dehydrogenase

cytoplasmicaspartateaminotransferase

cytoplasmic malate dehydrogenase

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Photosynthetic electron transport


HeterotrophsHeterotrophsObtain energy by ingestion of other plants and animals.

PhototrophsPhototrophsAbsorb solar radiation and divert the energy through the electron transport chain.

They can produce their own carbohydrates from CO2 and H2O

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Two type of reactions.Two type of reactions.

Light reactions - photo phaseLight reactions - photo phaseAbsorb energy using chlorophyll and other pigments.

Dark reactions - synthesis phaseDark reactions - synthesis phaseCarbon metabolism to make carbohydrates. Light is not directly required.

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ChloroplastChloroplast

Outer membrane

Inner membrane

Granum

Stroma

Innermembranespace

Thylakoid

The apparatus for light absorption and carbonfixing in eukaryotic photosynthetic cells.

38

ChloroplastChloroplast

StromaStromaGel-like, unstructured matrix within the inner compartment. It contains the enzymes for the dark reactions.

ThylakoidsThylakoidsMembranes folded into sacs that are the sites for light receiving pigments, electron carriers and ATP synthesis. They are arranged into stacks called grana.

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Biomolecules and lightBiomolecules and light

Several types of light absorbing pigments are used.

Green plantsGreen plantsChlorophylls a and b.

BacteriaBacteriaBacteriochlorophyll.

Accessory pigmentsAccessory pigmentsCarotenes and phycobilins - absorb light outside the range of chlorophyll.

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Chlorophyll aChlorophyll a

NMg

N

N

N

O

CH3

CH2CH3

CH3CH2

CH2

H3C

H3C

H

C H2

CH

H

H3CO O

C H2

C

O

OCH3CH3CH3CH3

H3C

I II

IIIIV

NMg

N

N

N

O

CH3

CH2CH3

CH3CH2

CH2

H3C

H3C

H

C H2

CH

H

H3CO O

C H2

C

O

OCH3CH3CH3CH3

H3C

I II

IIIIV

phytol side chain

CH3

CO

in bacteriochlorophyll

COH

in chlorophyll b saturated bond inbacteriochlorophyll

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CarotenesCarotenes

HO

OH

HO

OH

-carotene

lutein

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PhycoerytherinPhycoerytherin

N H

O

H3C CH

CH3

N H

H3C

CH2

COO-

N H

O

CHH3C

N H

CH3

CH2

COO-

CH2

N H

O

H3C CH

CH3

N H

H3C

CH2

COO-

N H

O

CHH3C

N H

CH3

CH2

COO-

CH2

unsaturated bondin phycocyanin

CH2

CH3

in phycocyanin

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Photosynthetic light reactionsPhotosynthetic light reactions

Electrons flow through an electron transport chain from water to an electron acceptor.

NADP+ is the acceptor in green plants.

2 H2O + 2 NADP+

2 H+ + O2 + 2 NADPHlight

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PhotosystemsPhotosystems

Two typesTwo typesEach contain one primary acceptor molecule - usually chlorophyll

A set of accessory molecules help funnel additional light.

Cat

Chl aReactionCenter

Chl Chl

CatCat

Chl Chl

Chl

Chl

Chl

Chl Chl

Chl

Chl

Chl

Chl

Cat

Cat

CatChl

h

45

PhotosystemsPhotosystems

Photosystem I - P700Photosystem I - P700• Chlorophyll a and accessory pigments• Absorb in 600-700 nm range

Photosystem II - P680Photosystem II - P680• Chlorophyll a, b and accessory pigments• Absorb light with a maximum at 680 nm

All photosynthetic cells have P700. Both are present in O2 evolving organisms - higher plants, algae and cyanobacteria.

46

Linkage of photosystems I and IILinkage of photosystems I and II

In green plants, the two systems are linked.In green plants, the two systems are linked.

• Light is absorbed by Photosystem I.

• Energy is transmitted to the P700 center and an electron is excited.

• Electron is passed via an electron transport chain.

• The ‘electron hole’ is filled by another electron transport chain driven by Photosystem II.

47

Photosystem IPhotosystem I

P700

P700*A0

A1

Fe-SComplex Ferredoxin

Ferre doxin-NADPreductase

+NADP+

NADPH + H+ proton gradient

+

Photosystem I

lightRed

uct

ion p

ote

nti

al, V

48

Photosystem IIPhotosystem II

2 H O2

O + 4 H +proto n gradient

+2

P680

Water-splittingcomplex

P680*

QA

QB

Photosystem II

light

0.0

+0.5

+1.0

-0.5

-1.0R

ed

uct

ion p

ote

nti

al, V

49

Linkage of photosystems I and IILinkage of photosystems I and II

light

50

Photosystems I and IIPhotosystems I and II

Net reactionNet reaction

2 H2O + 2 NADP+

O2 + 2 NADPH + 2 H+

Eight photons are required to transfer four electrons.

8 h

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PhotophosphorylationPhotophosphorylation

• Converting light into chemical bonds- very similar to oxidative phosphorylation.

• Photoinduced electron transfer from water to NADP+ pumps H+ through thylkaloid membrane - from stromal side to inner compartment.

• Protein complexes CF0 and CF1 are the ATP synthases of chloroplasts.

52


H+ H+

H+H+ADP

ATP

Stroma

High Mg Low H+2+

Low Mg High H+2+Lumen

thylakoidmembrane

proton pump withinthe light-inducedelectron transportsystem.

53


Process is non-cyclicProcess is non-cyclic

• Starts with H2O and ends with NADPH and O2.

• Products will accumulate as long as there is light.

A cyclic process exists for photosystem I.A cyclic process exists for photosystem I.

• No H2O is consumed and no NADPH or O2 is produced.

• ADP is phosphorylated.

54

Cyclic photophosphorylationCyclic photophosphorylation

Cytochromebf complex

proto n gradient

P700

P700*A0

A1Fe-S

Complex

Ferredoxin

light

Plastocyanin

55

Synthesis of carbohydratesSynthesis of carbohydrates

The Calvin CycleThe Calvin Cycle• The ‘dark’ reactions - fixation of carbon

from CO2.

• Four stages - fix one carbon at a time.• Six cycles per glucose.

Overall reaction for one glucoseOverall reaction for one glucose

6 CO2 + 12 NADPH + 12H+ + 18 ATP + 12 H2O

C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi

56

Calvin cycleCalvin cycle

Stage 1Stage 1

• Addition of CO2 to an acceptor molecule.

• Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubiscorubisco) catalyzes the addition of CO2

• The ribulose-1,5-bisphosphate that is produced will immediately cleave into two molecules of 3-phosphoglycerate.

57

Calvin cycleCalvin cycleStage 1Stage 1

ribulose-1,5-bisphosphate

3-phosphoglycerate

-keto acid intermediate

O

CH2

C

C

C

CH2

O

P

O

O-

O-

O

H

H

OH

OH

P

O

O-

O-

O

CH2

C

C

C

CH2

O

P

O

O-

O-

C

H

O

OH

P

O

O-

O-

O

O-

O

C O-

CH OH

C H2

O P

O

O-

O-

+ CO2

H+

2

H2O

H+

58


Stage 2Stage 2• Phosphorylation of the C1 carboxyl group,

producing 1,3-bisphosphoglycerate.• Stromal 1,3-bisphosphoglycerate (1,3-PBG) is

reduced to glyceraldehyde-3-phosphate.

COO-

CH OH

CH2OPO32-

COPO32-

CH OH

CH2OPO32-

O HC

C

O

CH2OPO32-

H OH3-phosphoglycerate

kinaseglyceraldehyde

3-phosphatedehydrogenase

ATP ADP NADPH

+ H+NADP+

+ Pi

59


Stage 3Stage 3• Carbohydrates are formed from glyceraldehyde-

3-phosphate. The same gluconeogenesis pathways used earlier are used.

glyceraldehyde-3-phosphate dihydroxyacetone phosphate

DHAP + glyceraldehyde-3-phosphate fructose-1,6-bisphosphate

fructose-1,6-bisphosphate + H2O fructose-6-phosphate + Pi

fructose-6-phosphate glucose-6-phosphate

glucose-6-phosphate glucose-1-phosphate

isomerase

aldolase

phosphatase

isomerase

phosphoglucomutase

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Stage 4Stage 4• Only one of each six cycles results in

carbohydrate production.• The other passes through the cycle are used to

regenerate the ribulose-1,5-bisphosphate.• The first step is the conversion of glyceraldehyde-

3-phosphate to dihydroxyacetone phosphate.

C

C

C

O

OHH

H H

P-O O-

O

C

C

C

O

OH

H H

P-O O-

O

OH

H

H

H O

isomerase

C

C

C

O

OHH

H H

P-O O-

O

C

C

C

O

OH

H H

P-O O-

O

OH

H

H

H O

isomerase

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Ribulose-5-phosphate

Ribulose-1,5-bisphosphate

Glycerate-1,3-bisphosphate

Glyceraldehyde-3phosphate (G3P)

Dihydroxyacetonephosphate (DHAP)

Sucrose, starch,cellulose, etc.

Fructose-1,6-bisphosphate

Fructose-6-phosphate

Glucose-6-phosphateGlucose

X5P

X5P

R5P

G3PDHAP

G3P

DHAP

G3P

FbisPF6P

E4PS7P

ATP

ADP

CO2

H O2ATP

ADP

NADPH

NAD+

Pi

PiPi

Calvin

cycle

Calvin

cycle

3-phospho-glycerate

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PhotorespirationPhotorespiration

Rubisco can act as an oxygenase by substituting O2 for CO2.

CH2OPO32-

C O

CHOH

CHOH

CH2OPO32-

+ O2

CH2OPO32-

C-O O

C

CHOH

-O O

CH2OPO32-

+rubisco

CH2OPO32-

C O

CHOH

CHOH

CH2OPO32-

+ O2

CH2OPO32-

C-O O

C

CHOH

-O O

CH2OPO32-

+rubisco

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PhotorespirationPhotorespiration

• This appears to be a counter productive path - oxygen is consumed.

• Some plants have adapted this process as an optional pathway for carbon fixation.(sugar cane, corn, sorghum, ...)

• This can be described by the Hatch-Slack pathway - C4 pathway

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Hatch-Slack pathwayHatch-Slack pathway

Mesophyllcell

Bundlesheath cell

Documents

1 ATP Formation by Electron-Transport Chains Mitochondrial Electron-Transport Components of the Electron-Transport Chain Oxidative Phosphorylation Recycling