09(1)BIO462e-Trnsprt Oxid Phos

7/31/2019 09(1)BIO462e-Trnsprt Oxid Phos

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Electron Trans ortElectron Trans ortand Oxidativeand Oxidative

PhosphorylationPhosphorylation


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Electron Transport ChainElectron Transport Chain

Carried out by four closely related multisubunit

membrane-bound complexes and two electroncarriers, coenzyme Q and cytochrome c

in a series of oxidation-reduction reactions, electrons

from FADH2 and NADH are transferred from onecomplex to the next until they reach O2 O2 is reduced to H2O

as a result of electron transport, protons are pumpedacross the inner membrane to the intermembranespace

2 H2 O+ 4 H+ + 4 e -O2


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Proton gradient across inner mitochondrial membrane


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the proton gradient establishes a voltage

gradient the proton and voltage gradients together

provide the mechanism to couple electron

transport with phosphorylation of ADP

+ADP Pi ATP + H2O


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Electron Transport Chain


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Complex I

NADH-CoQ oxidoreductase

electrons are passed from NADH to FMN

NH 3 C

O 2 H + + 2 e -

NH 3 C N O

C H 2

C H 2 O PO 32 -

Flavin mononucleotide(FMN)

N

NH 3 C

H3

CN

N H

O

OC H 2

C H 2 O PO 32 -

Dihydroflavin mononucleotide(FMNH 2 )

H

H

( C H O H ) 3

( C H O H ) 3


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Complex I

electrons are then passed to the iron-sulfur

clusters finally, electrons are passed to coenzyme Q

(also called ubiquinone)

C H 3 O

C H 3 O

( C H 2 C H = C C H 2 ) n

C H 3O

O

C H 3

H

Coenzyme Q(oxidized form)

C H 3 O

C H 3 O

( C H 2 C H = C C H 2 ) n

C H 3

O H

O H

C H 3

H

Coenzyme QH 2(reduced form)

reduction

oxidation2 H + + 2 e -+


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Complex I

the overall equation for the reaction of complex I is

E-FMNH2 + 2 Fe -S( ox) E-FMN 2 Fe -S( re d)+ 2 H+

+

NADH + H+ + E-FMN N AD + + E-FMNH2

this transfer of electrons is strongly exergonic andis sufficient to drive the phosphorylation of ADP

e - re+

o 2+

e - ox+

o 2

NADH + H+ + CoQ N AD+

+ CoQH2 G' = -81 kJmol-1

G' = +30.5 kJ

-1

ADP+

Pi + H2OATP


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Complex II

Succinate-coenzyme Q oxidoreductase

C O O -

C H 2

C H 2+ E - F A D

C

CH C O O -

+ E - F A D H 2

the overall reaction is exergonic, but not enough todrive ATP production

no H+ is pumped out of the matrix during this step

C O O-

Succinate

- O O C

Fumarate

E-F A D H 2 + C o Q E- F A D C o Q H 2+

G' = -13.5 kJmol-1

Succinate + C o Q Fumarate C o Q H 2+


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Redox Forms of CoQ

C H 3 O

C H 3 O

R

C H 3

O

O

C H 3 O

C H 3 O

R

C H 3

O

O -

e -

e -

-

C H 3 O

C H 3 O

R

C H 3

O H

O H

Coenzyme QH 2

2 H + + e -

2 H + + e -


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Complex III

CoQH2-cytochrome c oxidoreductase

+ +

+CoQH2 2Cyt c[Fe( III) ]

+ 2 F II ' = - -1

this decrease in free energy is sufficient todrive the phosphorylation of ADP to ATP (GG00

= + 30.5 kJmol= + 30.5 kJmol--11

)


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Complex IV

Cytochrome oxidase:

+ 2 H +

+ H O2 C t c F e I I I

2 C y t c [ F e( I I ) ]

G' = -110 k mol -1

+ 12 O 2

complex IV contains cytochrome a,cytochrome a3, and Cu(II), which are alsoinvolved in the electron transport

complex IV is the link with molecular oxygen

+NADH + H+

NAD+

+ H2O

1

2 O2


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The heme group of the

cytochromes


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Electron Transport Chain


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Coupling Ox and Phos

the energy-releasing oxidations give rise to

proton pumping and a pH gradient across theinner mitochondrial membrane

in addition, differences in the concentration of

voltage gradient

a coupling process converts theelectrochemical potential to the chemical

energy of ATP the coupling factor is ATP synthase, a

complex protein oligomer, separate from theelectron transport complexes


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ATP Synthase


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Coupling Ox and Phos

P/O ratio:P/O ratio: the number of moles of Pi consumed

in phosphorylation to the number of moles ofoxygen atoms consumed in oxidation

Phos hor lation P

P/O = 2.5 or 3.0 when NADH is oxidized

P/O = 1.5 or 2.0 when FADH2 is oxidized

A D P + P i A T P + H 2 O

1 / 2 O 2 + 2 H+

+ 2 e- H 2 O

Oxidation (O)


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Mechanism of Ox/Phos

The mechanism by which the proton gradient

leads to the production of ATP depends on ionchannels through the inner mitochondrialmembrane

protons flow back into the matrix through channels the flow of protons is accompanied by formation of

ATP

the details of how phosphorylation takes place as a

result of the linkage to the proton gradient are notexplicitly specified by this mechanism


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The overall effect of electron transport reaction series is to moveprotons out of the matrix into the intermembrane space, creating

a difference in pH across the membrane.


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Formation of ATP accompanies the flow of

protons back into the mitochondrial matrix.


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End of lecture

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09(1)BIO462e-Trnsprt Oxid Phos