Part 3: Electron Shuttles, ETC & Chemiosmosis

Cellular Respiration

Recall: Location

Glycolysis occurs in the cytoplasm

Electron transport chain (ETC) starts in the mitochondrial matrix

The NADH produced during glycolysis must make its way into the mitochondria

Electron Shuttle

Electron ShuttleNADH produced in glycolysis must be

transported from the cytoplasm into the mitochondria to enter the ETC.

Mitchondria is not permeable to NADH produced in the cytoplasm

Shuttling of NADH is indirect

Two shuttle mechanisms:glycerol phosphate shuttlemalate-aspartate shuttle

Glycerol Phosphate Shuttle In brain tissue and skeletal muscle tissue Results in FADH2 in the matrix 2e- from NADH get shuttled into the mitochondria

and become part of FAD to become FADH2

CYTOPLASM INTERMEMBRANE SPACE

MATRIX

NADH

NAD+

DHAP

Glycerol 3-phosphate

FADH2

FAD

ETC

Glycerol 3-phosphate

DHAP

Malate-Aspartate Shuttle In liver, kidney, heart Results in NADH in the matrix 2e- from NADH get shuttled into the mitochondria

and become part of NAD+ to become NADHINTERMEMBRANE

SPACECYTOPLASM MATRIX

ETC

Aspartate

NADH

NAD+

Oxaloacetate

Malate

NADH

NAD+

Oxaloacetate

Malate

Aspartate

Two Methods of ATP Synthesis

Two Methods of ATP SynthesisSubstrate-level phosphorylation direct ATP formation through phosphate

transfer from substrate to ADP Occurs in glycolysis & Kreb cycle

Oxidative phosphorylation indirect ATP formation through redox

reactions involving O2 as a final electron acceptor

Driven by the electron transport chain

Substrate-level Phosphorylationenzyme transfers a phosphate group

from a substrate molecule to ADP rather than adding an inorganic

phosphate to ADP as in oxidative phosphorylation

Figure 9.7

Enzyme Enzyme

ATP

ADP

Product

SubstrateP

+

Oxidative Phosphorylation Involves 2 processes that occur on the

inner mitochondrial membrane (IMM) Electron Transport Chain:

Energy in electrons of NADH and FADH2 used to drive H+ against its concentration gradient

Electrons fall to oxygen (final electron acceptor)

Chemiosmosis: Using facilitated diffusion of H+ to drive the

synthesis of ATP

Electron Transport Chain OverviewThe ETC removes energy stored in

the NADH and FADH2 molecules to: Move protons against its concentration

gradient creating a proton gradient across the inner mitochondrial membrane (IMM)

Final electron acceptor is oxygen which converts H2O

All reactions are redox reactions

Electron Transport Chain (ETC)

Made of electron carrier molecules embedded in the inner membrane of mitochondria

Most carriers are protein molecules except for ubiquinone (Q)

ETC Thermodynamics Each electron transfer

step is energetically favourable

Each carrier in the chain has a higher electronegativity than the carrier before it

Electrons from NADH and FADH2 lose energy (pulled downhill) by each electron carrier

ETC & Oxygen Final electron

acceptor is the very electronegative oxygen

Oxygen drives the redox reactions

Lack of oxygen prevents this whole process from occurring

H2O

O2

NADH

FADH2

FMN

Fe•S Fe•S

Fe•S

O

FAD

Cyt b

Cyt c1

Cyt c

Cyt a

Cyt a3

2 H + + 12

I

II

III

IV

Multiproteincomplexes

0

10

20

30

40

50

Free

ene

rgy

(G) r

elat

ive

to O

2 (k

cl/m

ol)

ETC & Chemiosmosis Overview

Oxidativephosphorylation.electron transportand chemiosmosis

Glycolysis

ATP ATP ATP

InnerMitochondrialmembrane

H+

H+H+

H+

H+

ATPP i

Protein complexof electron carners

Cyt c

I

II

III

IV

(Carrying electronsfrom, food)

NADH+

FADH2

NAD+

FAD+ 2 H+ + 1/2 O2

H2O

ADP +

Electron transport chainElectron transport and pumping of protons (H+),

which create an H+ gradient across the membrane

ChemiosmosisATP synthesis powered by the flowOf H+ back across the membrane

ATPsynthase

Q

Oxidative phosphorylation

Intermembranespace

Innermitochondrialmembrane

Mitochondrialmatrix

Figure 9.15

ETC Components: Complex I

2 e- from NADH are transferred to Complex I

Protons are pumped across the inner mitochondrial membrane (IMM) by Complex I

ETC Components: Q

e- are transferred from Complex I to ubiquinone (Q)

Q is lipid soluble can move within

the phospholipid bilayer

mobile component within the IMM

ETC Components: Complex III

e- are transferred from Q to Complex III

Protons are pumped across the IMM by Complex III

ETC Components: Cyt C

e- are transferred from Complex III to cytochrome c (cyt c)

cyt c is a mobile component on the surface of IMM (peripheral), in the intermembrane space

ETC Components: Complex IV

e- are transferred from cyt c to Complex IV

Protons are pumped across the IMM by Complex IV

ETC Components: O2

O2 is the final electron acceptor of the ETC

enough e- pass through the ETC to produce full H2O molecules

FADH2 Pathway

Figure 9.15

Oxidativephosphorylation.electron transportand chemiosmosis

Glycolysis

ATP ATP ATP

InnerMitochondrialmembrane

H+

H+H+

H+

H+

ATPP i

Cyt c

I

II

III

IV

FADH2

FAD+ 2 H+ + 1/2 O2

H2O

ADP +

ATPsynthase

Q

Intermembranespace

Innermitochondrialmembrane

Mitochondrialmatrix

X

ETC Components: Complex II

2e- are transferred from FADH2 to Complex II

no protons are pumped across the IMM

e- are transferred from Complex II to Q and proceed through the rest of ETC

Complex II

FADH2 FAD+

Q

ETC Components: Complex II

Oxidativephosphorylation.electron transportand chemiosmosis

Glycolysis

ATP ATP ATP

InnerMitochondrialmembrane

H+

H+H+

H+

H+

ATPP i

Protein complexof electron carners

Cyt c

I

II

III

IV

(Carrying electronsfrom, food)

NADH+

FADH2

NAD+

FAD+ 2 H+ + 1/2 O2

H2O

ADP +

Electron transport chainElectron transport and pumping of protons (H+),

which create an H+ gradient across the membrane

ChemiosmosisATP synthesis powered by the flowOf H+ back across the membrane

ATPsynthase

Q

Oxidative phosphorylation

Intermembranespace

Innermitochondrialmembrane

Mitochondrialmatrix

Figure 9.15

Electrochemical Proton Gradient

Low H+

High H+

ETC Summary

NADH e- transferred to O2; three proton pumps activated

FADH2 e- transferred to O2; two proton pumps activated

electrochemical proton gradient formed across IMM

H2O

O2

NADH

FADH2

FMN

Fe•S Fe•S

Fe•S

O

FAD

Cyt b

Cyt c1

Cyt c

Cyt a

Cyt a3

2 H + + 12

I

II

III

IV

Multiproteincomplexes

0

10

20

30

40

50

Free

ene

rgy

(G) r

elat

ive

to O

2 (k

cl/m

ol)

Figure 9.13

Electron Transport Chain NADH is oxidized and FMN (complex I)

is reduced↓

FMN is oxidized as it passes electrons to an iron sulfur protein, FeS (complex I).

↓ FeS is oxidized as it pass electrons to

ubiquinone Q. FADH2 enters at complex II to Q, bypassing complex I.

↓ Q passes electrons on to a succession

of electron carriers (in complex III and IV), most of which are cytochromes.

↓ Cyt a3 , the last cytochrome passes

electrons to oxygen.

Oxidative Phosphorylation

Low H+

High H+

Proton Motive Force: Chemiosmosis The electrochemical gradient produced by the

ETC: High proton concentration in the intermembrane space

Chemiosmosis: facilitated diffusion of proton down the concentration gradient

Oxidative phosphorylation: Oxidation (ETC) coupled with phosphorylation (ATP

synthesis) Occurs by coupling passive movement of protons to

produce ATP through the enzyme complex ATP synthase

ATP is produced as protons flow through ATP synthase

ATP Synthase: Complex V

Fo = rotor Transmembrane Proton channel

F1 = knob, rod Peripheral catalytic sites

that phosphorylate ADP to ATP

Figure 9.14

INTERMEMBRANE SPACE

H+

H+

H+

H+

H+

H+ H+

H+

P i

+ADP

ATP

A rotor within the membrane spins clockwise whenH+ flows past it down the H+

gradient.

A stator anchoredin the membraneholds the knobstationary.

A rod (for “stalk”)extending into the knob alsospins, activatingcatalytic sites inthe knob.

Three catalytic sites in the stationary knobjoin inorganic Phosphate to ADPto make ATP. MITOCHONDRIAL MATRIX

ATP Synthase: Complex V

Rotational Catalysis Protons flowing through the F0

component cause the subunits in the rod to rotate. electrochemical gradient translates into

mechanical energy When an irregularly shaped

subunit rotates, it contacts a different subunit.

As a result, the subunits change conformation, which changes their affinity for ATP.

subunit

Catalytic subunit 3 conformation:

loosely binds ADP and P tightly binds ADP and Pi to form ATP releases ATP

When the interacts closely with the subunit, ATP is synthesized

Each of the 3 subunits in the F1 component take turn catalyzing the synthesis of ATP

Animation ETC

http://www.youtube.com/watch?v=lRlTBRPv6xM&feature=related

Proton Gradient & ATP synthasehttp://www.youtube.com/watch?v=3y1dO4nNaKY&feature=related

ATP synthasehttp://www.youtube.com/watch?v=PjdPTY1wHdQ&feature=related

Cellular Respiration Lecture:

http://www.youtube.com/watch?v=qviLDKDJNKM&feature=related

ATP Production

In general:1 NADH 3 ATP molecules1 FADH2 2 ATP moleculesThe ETC is coupled with ATP

synthesis. The latter is dependent on the former.

ATP Production Summary

Cellular Respiration Step

Energy Molecules Produced

ATP Totals

Glycolysis 2 ATP2 NADH

2 ATP4-6 ATP

Oxidative Decarboxylation

2 NADH 6 ATP

Krebs Cycle 6 NADH2 FADH2

2 ATP

18 ATP4 ATP2 ATP

TOTAL 36-38 ATP

ATP Production Summary

Cellular Respiration Step

Energy Molecules Produced

ATP Totals

Glycolysis 2 ATP2 NADH

2 ATP4-6 ATP

Oxidative Decarboxylation

2 NADH 6 ATP

Krebs Cycle 6 NADH2 FADH2

2 ATP

18 ATP4 ATP2 ATP

TOTAL 36-38 ATP

Electron shuttlesspan membrane

CYTOSOL 2 NADH

2 FADH2

2 NADH 6 NADH 2 FADH22 NADH

Glycolysis

Glucose2

Pyruvate

2AcetylCoA

Citricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

MITOCHONDRION

by substrate-levelphosphorylation

by substrate-levelphosphorylation

by oxidative phosphorylation, dependingon which shuttle transports electronsfrom NADH in cytosol

Maximum per glucose:About

36 or 38 ATP

+ 2 ATP + 2 ATP + about 32 or 34 ATP

or

Figure 9.16

Cellular Respiration Summary

Amino acids

Sugars Glycerol Fattyacids

GlycolysisGlucose

Glyceraldehyde-3- P

Pyruvate

Acetyl CoA

NH3

Citricacidcycle

Oxidativephosphorylation

FatsProteins Carbohydrates

Figure 9.19

Catabolism of various molecules from food

Glucose

Glycolysis

Fructose-6-phosphate

Phosphofructokinase

Fructose-1,6-bisphosphateInhibits Inhibits

Pyruvate

ATPAcetyl CoA

Citricacidcycle

Citrate

Oxidativephosphorylation

Stimulates

AMP

+– –

Figure 9.20

Regulation of Cellular Respiration