THE ELECTRON TRANSPORT CHAIN. Oxidative phosphorylation

THE ELECTRON

TRANSPORT CHAIN

Overview of ETC

Impermiable to ions

Permiable via VDAC

Overview of Oxidative Phosphorylation

Overview of the ETC

In contrast to “electron pushing” in organic chemistry where pairs of e- were transferred the ETC generally handles one e- at a time

Details of the Complexes

Reduction Half-Reaction E_o'(V) O2 + 2H++ 2 e- H2O 0.816 Fe3+_{+ e}-_{ Fe}2+ _0.771 Photosystem P700 0.430 NO₃-_{+2 H}+_{+2 e}-_{ NO} 2-+H2O 0.421

Cytochrome f( Fe3+_{)+ e}-_{ cytochrome f(Fe}2+_{) 0.365}

Cytochrome a₃( Fe3+_{)+ e}-_{ cytochrome a}

3(Fe2+) 0.350

Cytochrome a(Fe3+_{)+ e}-_{ cytochrome a(Fe}2+_{) 0.290}

Rieske Fe-S(Fe3+_{)+ e}-_{ Rieske Fe-S(Fe}2+_{) 0.280}

Cytochrome c( Fe3+_{)+ e}-_{ cytochrome c(Fe}2+_{) 0.254}

Cytochrome c₁( Fe3+_{)+ e}-_{ cytochrome c} 1(Fe2+) 0.220 UQH + H1_{+ e}-_{ UQH} 2(UQ=coenzyme Q) 0.190 UQ + 2 H+ _{+ 2 e}-_{ UQH} 2 0.060

Cytochrome b_H(Fe3+_{) + e}-_{ cytochrome}

b_H(Fe2+₎ 0.050 Fumarate + 2 H+_{+ 2 e}-_{ succinate 0.031} UQ + H+_{+ e}-_{ UQH  0.030} Cytochrome b₅( Fe3+_{)+ e}-_{ cytochrome} b₅(Fe2+) 0.020 [FAD]+2 H+_{+2 e}-_{ [FADH} 2] _0.091* 0.003-Cytochrome b_L( Fe3+_{)+ e}-_{ cytochrome} bL(Fe2+) -0.100 Oxaloacetate + 2 H+_{+ 2 e}-_{ malate -0.166} Pyruvate + 2 H+_{+ 2 e}-_{ lactate -0.185} Acetaldehyde + 2 H+ _{+ 2 e}-_{ ethanol -0.197} FMN + 2 H+_{+ 2 e}-_{ FMNH} 2 -0.219 FAD + 2 H+_{+ 2 e}-_{ FADH} 2 -0.219 Glutathione (oxidized) + 2 H+_{+ 2 e}-_{ 2} glutathione (reduced) -0.230 Lipoic acid + 2 H+ _{+ 2 e}-_{ dihydrolipoic acid -0.290}

1 ,3-Bisphosphoglycerate + 2 H+_{+ 2 e}

- glyceraldehyde-3-phosphate+Pi -0.290

NAD+_{+ 2 H}+ _{+ 2 e}-_{ NADH + H}+ _-0.320

NADP+_{+ 2 H}+ _{+ 2 e}-_{ NADPH + H}+ _-0.320

Lipoyl dehydrogenase [FAD ] +2 H+_{+2 e}-_{ lipoyl}

dehydrogenase [FADH₂] -0.340 -Ketoglutarate + CO2 + 2 H++ 2 e-

isocitrate -0.3802

H+ _{+ 2 e}-_{ H}

2 -0.421

Ferredoxin (spinach) ( Fe3+_{) + e}-_{ ferredoxin}

(spinach) (Fe2+₎ -0.430

Succinate + CO₂+ 2 H+_{+ 2 e}-_{ -ketoglutarate +}

H₂O -0.670

Reduction Potentials at pH = 7

[FAD] = bound FAD

Complex IV

Electro-potential Gradient

Complexes I,II, III, IV Eo_{’ (v)} -0.32 +0.03 +0.04 +0.23 +0.82

Why does FADH₂ not join until after NADH + H+ _have

already gone through Complex I ?

Loss in Free Energy from High Energy

Electrons to Water

Energy per proton pumped

• NAD+ + 2H+ + 2e- → NADH + H+ -0.32 v

• (1/2) O₂ + 2H+ + 2e- → H₂O +0.82 v

• So NADH + H+ + (1/2) O₂ → NAD+ + H₂O +1.14 v

• ∆Go’ = -220.1 kJ/(mole NADH) or about 7.5 ATP (makes

2.5 ATP)

• Have 10 protons pumped or -220.1/10 = -21.8 kJ/mole e

Free Radicals

•

Up to now, we’ve had

heteroytic bond

cleavage which

resulted in transferring

electrons in pairs

•

Can also have

hemolytic bond

cleavage—these

Free Radicals

• For example ozone depletion

Free Radicals and ETC

•

All complexes in ETC transfer electrons one

at a time

.

•

Therefore need stable free radicals to “feed”

electrons to the ETC one electron at a time. An

example is Flavin Mononucleotide (FMN)

•

Indeed many of the flavin molecules like flavin

FMN and CoQ can accept or discharge

either One or Pair of Electrons

Overview of Complex I

-Electron carrier in wall of inner Membrane is QH₂ (UQH₂)

FMN accepts 2e- _{at a time}

All NADHs Aren’t

Transfer Along Chain

Complex I

Complex I – 2

nd

_-6

th

_Reactions

• FMNH₂ transfers e- (one at a time) to iron complexes in

protein, oxidizing iron from Fe+2 _{to Fe}+3

2Fe-2S 4Fe-4S 2[Fe3+_{] + 2e}- _2[Fe2+ _{] +0.1 to 0.4 v} +0.5 -96.5 Eo_{’ ∆G}o_{’ kJ} FMNH₂ → FMN + 2 H+ _{+ 2 e}- _{+0.219 v}

Complex I – 3

rd

_Reaction

• Ubiquinone (Ubiquitous Quinone, Q)

• Fat soluble charge carrier – membrane resident

Quinone Q + 2 H+ _{+ 2 e}- _{ QH} 2 0.060 2[Fe3+ _{]+ 2e}- _{-0.1 to -0.4} 2[Fe2+_] one =ketone ol = alcohol

Complex I

• Overall reaction

• NADH + Q +6H+_{matrix side} → NAD+ + QH₂ + 4H+_{cytoplasm side}

• NADH + H+ → NAD+ + 2e- + 2H+ 0.328

• CoQ + 2e- +2H+→ CoQH₂ 0.06

• 0.36 v

• G = -69.5 kJ/mole NADH

• Current thinking is that complex I protein binds H+ on the

matrix side and undergoes conformation changes to release them in the inner membrane space

Complex II (Succinate Dehydrogenase)

Complex II

• Introduced first in Citric Acid cycle

• No protons transported by complex II

• But energetic electrons transferred onto complex III and

IV to pump protons

Complex - II

• We came across this enzyme, succinate dehydrogenase

before in the Citric Acid cycle, where as you recall, it was embedded in the mitochondrial membrane

Complex - II

• The succinate dehydrogenase compound is part of

Complex II called the succinate-Q-reductase complex

• FADH₂’s electrons are transferred to Fe-S centers and

then onto Q to make QH₂ and oxidized back to FAD

• This step comes in after the NADH oxidation step since

it is lower in free energy

• Overall reaction (potentials from Table 19.1

• Succinate → Fumarate + 2e- + 2H+ -0.031

• CoQ + 2e- + 2H+ → CoQH₂ 0.06

• +0.029

Complex III

Complex III

•

In complex III, we first meet cytochromes

•

Group of red and brown heme proteins

•

Spectra undergo color changes when

undergoing oxidation and reduction

•

First observed by an Englishman (Keilin) using

a microscope on the flight muscles of insects

when the immobilized insect tried to free itself

•

Classified as a, b, or c depending on spectral

Complex III

•

Also known as

cytochrome c

oxidreductase

•

Where do we get the

names cytochrome:

from early work where

these were identified

by their absorbance

color

Red –oxidized Green - reduced

Complex III

•

Oxidizes QH

coming

from complexes I and II to

Q; transfer to cyt c

•

Pumps 2 protons

•

Rieske Fe-S differs from

2Fe-2S (seen before)

since one Fe bound to

two histidine groups

•

The Heme groups are

Heme Group

A cytochrome is an electron-transferring protein that contains the Heme group

All complexes process electrons one at a

time

• Up to now, had FMN or CoQ interface between two electron

carriers and discharging one electron at a time.

• So how can we accommodate a two-electron carrier like QH₂

without a flavin intermediary to transduce from 2 e- _{to 1 e}- _{at a}

time?

• The complex III operates in 2 stages as shown on the next

slide – called the ‘Q-cycle’

• Complex III produces a single electron carrier: cytochrome c

• The overall equation is:

• 2QH₂ +2Cyt c_ox + 2H+_matrix → 2Q + 2Cyt c_red + 4H+_inner

Note; 2 H+ _{pumped from matrix, 2H}+ _{comes from loss of 2H+ from QH} 2

Complex III

UQ –

universal

quinone

given as Q in

our book

Proton/Electron Balance for Complex III

Fed Exiting

H+ _e- _H+ _e

-2 QH₂ 4 4

Q

Protons from matrix 2

2 Cytochrome c 2

QH₂ 2 2

Protons 4

Totals 6 4 6 4

First half of cycle

• QH₂ enters Q_p site and releases 2H+ & e- (see paths below)

giving QH·- _{The e}- _{goes to Cyt c which departs}

• QH·- releases 1 e- giving Q and e- goes to Cyt b_L→b_H

• This electron then adds to Q at Q_n site giving Q- which

remains bound

• We need to further reduce Q- so need second cycle

Second Half of Cycle

• As in 1st cycle QH₂ enters at Q_p and releases 2H+ & e- and e

-goes to Cyt c

• In this 2nd half of cycle, the second e- released is transferred

(via b_L & b_H) to Q- _{and with 2H}+ _{from matrix reduces it to QH}

2

• QH₂ is released and joins Q pool

-Complex III

• Overall reaction:

• 2H+ are “pumped” from QH₂ (higher energy H+)

• 2H+ are “pumped” from the matrix (lower energy H+)

• It permits a 2e- carrier interact with b_L, b_H, Rieske

complex, and cytochrome c₁ all of which are 1e- _carriers

• Cytochrome c₁ is mobile (water soluble) 1e- carrier

• Releases about -36. 6 kJ/(mole NADH)of free energy

QH₂ + 2H+

Cytochrome c

• Mobile electron carrier (like Q) with a heme group

• Carries 1e- from Complex III onto Complex IV

Complex IV

Cu+1 _{and Cu}+2 _{used here}

Cu_A

Cu_B

Complex IV

•

1 e- carriers

4

Complex IV

• Overall equation

• 4Cyt c_red + 8 H+_matrix + O₂ → 4Cyt c_ox + 2H₂O + 4H+_inner

• ΔGo’ = -231.8 kJ

• Need to capture as much as possible of this loss in

free energy in protons pumped to cytoplasm side of membrane

Complex IV

-Mechanism

Red- reduced

Possible Mechanism

Complex IV overall reaction

• Overall equation for oxygen reduction

• 4 Cyt c_red + 4H+_matrix + O₂ → 4 Cyt c_ox + 2H₂0

• ΔGo’ = -87.2 kJ/mole

• But we have a total of ΔGo’ = -112.0 kJ in this

reaction

• Use the remainder of the energy to pump four

more H+ _{to cytoplasmic side of membrane}

• Thus overall equation is

Summary of Redox Among Complexes

Overall Picture

Water soluble Fat soluble

Oxygen Radicals

• The transfer of 4 e- and 4 H+ to oxygen result in the

following electrochemical reaction

• O₂ + 4e- + 4H+ → 2H₂0 0.82 v

• However the partial reduction of O₂ leads to very reactive

radical species: O₂- _{and O}

2

2-• The enzyme does not release these radicals

• However, they are inevitably formed so

• Superoxide dismutase: 2O₂- + 2H+ → O₂ + H₂O₂ • Catalase: 2H₂O₂ → O₂ + 2H₂O

Diseases Originating in ETC

Atherogenesis

Emphysema; bronchitis Parkinson disease

Duchenne muscular dystrophy Cervical cancer

Alcoholic liver disease Diabetes

Acute renal failure Down syndrome

Retrolental fibroplasia

Cerebrovascular disorders Ischemia; reperfusion injury