Electron transport and Oxidative phosphorylation

Electron transport and

Oxidative phosphorylation

The final piece of the puzzle

Take a deep breath and push on

Major Energy Pathways

Glycolysis

Oxidative phosphorylation

pyruvate

3 NADH

GlucoseGalactoseFructoseMannose

Fatty Acids

1 FADH2

Lactate

Amino Acids

O2

H2O

Anaerobic

Aerobic

Krebs Cycle

Acetyl-CoA

Electron Transport and Oxidative PhosphorylationElectron Transport and Oxidative Phosphorylation

1. The absolute heart of aerobic metabolism

2. Three Functional Phases

Electron transfer from NADH, FADH2 to O2

Energy preserved as a proton gradient

Proton gradient energy makes ATP

We are making ATP from ADP and Pi by tapping the oxidative energy generated in the transfer of electrons to O2

We are making ATP from ADP and Pi by tapping the oxidative energy generated in the transfer of electrons to O2

Anatomy of Mitochondria

Mitochondria are composed of a dual membrane system:

Outer: Porous to all molecules < 10 kDaInner: Transporter-dependent transport

Inner Membrane Transport in Mitochondria

Densely packed with specific membrane transporters andthe electron-transporting complexes

Table 1: Cytochromes in the Electron Transport System

1. Cytochrome b562 also called bH Complex III Fe2+/Fe3+

2. Cytochrome b566 also called bL Complex III Fe2+/Fe3+

3. Cytochrome b560 Complex II Fe2+/Fe3+

4. Cytochrome c1 Complex III Fe2+/Fe3+

5. Cytochrome c Mobile Fe2+/Fe3+

6. Cytochrome a Complex IV Fe2+/Fe3+ Cu+/Cu2+

7. Cytochrome a3 Complex IV Fe2+/Fe3+ Cu+/Cu2+

Electron TransportThe successive passage of electrons through a series ofmembrane complexes to Oxygen.

NADH FMN CoQ Cyt b Cyt c1 Cyt c Cyt a+a3 O2

Complex I Complex III Complex IV

Strategies p219

Transport Mechanism

NADH

NAD+

FMN

FMNH2 CoQ

CoQH2

Cyt b(Fe3+)

Cyt b(Fe2+)

Cyt c1

(Fe3+)

Cyt c1

(Fe2+)Cyt c(Fe3+)

Cyt c(Fe2+)

Cyt a+a3

(Fe3+)

Cyt a+a3

(Fe2+) O2

H2O

A bucket-brigade

Reduced Oxidized

..

..

..

..

..

..

..

..

-0.32 volts + 0.82 volts

Electron Transport Complexes

• Membranes bound heme proteins or “cytochromes”

• Iron-Sulfur proteins..high reducing potential

• Mobile electron carriers– Coenzyme Q– Cytochrome c

Electron TransportNADH

O2

Complex I

Complex III

Complex IV

-0.32

+0.82

CoQ

Cyto C

Reductive Energy

Oxidative Energy

Complex II

FADH2

H2

Fe2+

H2O

Fe

S

SFe

S

SS

S

Cys

Cys

Cys

Cys 2-

2-

Fe-S 2Fe-2S

Fe

S

ssS

Cys

Cys

Cys

Cys

Iron-Sulfur Centers

n =6-10Oxidized CoQ

CH3H3CO

H3CO ROH

OH

Semiquinone

OCH3H3CO

H3CO R

.

OH

CoQH. CoQH2Quinol Form

Form (2 electrons)(one electron)

(no electrons)

CH3H3CO

H3CO (CH2=CO

O

CH3

CH2)nHisoprenoid units

Coenzyme Q

NADH-CoQ Reductase

FMNFeS

I

CoQ

Succinate-CoQ Reductase

FADFeSCyt b560

II

CoQ-cyto c Reductase

Cyt b562

Cyt b566

FeSCyt c1

III

Cyto c

IVCu2+

Cyt aCyt a3

O2

Mobile

Succinate Fumarate

NADH

Electron TransportComplexes

Energy Time

How does the energy of oxidation translate into free energy?

Go’ = –nFEo’

F = Faraday’s constant = 96,500 J/mol x volt

n = Number of electrons

Eo’ = Standard Reduction Potential at pH 7

E = Eo’ + 0.06 log

[electron acceptor]

[electron donor]

Nernst Equation for one electron transferDetermines E under non-standard state conditions

Textbook p372

E = Eo’ + 0.06 log[electron acceptor]

[electron donor]

pH = pKa + log [proton acceptor]

[proton donor]

E (reduction potential)pH

pKa Eo’ (standard reductionpotential)

Proton Acceptor Electron acceptor (oxidant)(base)

Proton Donor (acid) Electron donor (reductant)

Acid/Base Redox

Donors(Reductants)

Acceptors(Oxidants)

e-

Eo’= Eo’ acceptor - Eo’ donorEo’= Eo’ acceptor - Eo’ donor

Eo’

NADH + H+ + 1/2 O2NAD+ + H2O

NAD+ + 2e + 2H+ NADH + H+ –0.32 volts1/2 O2 + 2e + 2H+ H2O +0.82 volts

To Arrive at equation:

NADH + H+ NAD+ +2e + 2H+ +0.32 volts

1/2 O2 + 2e + 2H+ H2O +0.82 volts

NADH + H+ + 1/2 O2 NAD+ + H2O +1.14 volts

Go’ = –nFEo’ Go’ = –220 kJ/mol

Top reduces bottom

J/CoulombCoulomb/mol

Shuttles

Problem: Cytosolic NADH cannot penetrate the mitochondria

Solution: Pass the electrons to something that canpenetrate the mitochondria membrane

Two Shuttles

Glycerol-PO4

Malate-aspartate

FADH2 2 ATP per NADHc

NADH 3 ATP per NADHc

Mammalian muscle and liver

Insect brain and flight muscle

CH2OH

C=O

CH2OP

CH2OH

HO-C-H

CH2OP

CH2OH

HO-C-H

CH2OP

CH2OH

C=O

CH2OP

Cytosol

Mitochondria

Membranetransporter

Glycerol-PO4

Shuttle

DHAPGlycerol-PO4

Flavoproteindehydrogenase

FADH2FADH2

FAD

2ATP2ATP

NAD+NADHNADH:

:

:

:

COO

HO-C-H

CH2

COO

COO

C=O

CH2

COO

COO

C=O

CH2

COO

COO

HO-C-H

CH2

COO

COO

H3N-C-H

CH2

COO

+

COO

H3N-C-H

CH2

COO

+

Glu-Kg

-Kg Glu

NADHNADH

OAA

Asp

Malate-Aspartate

L-malate

Malatedehydrogenase

Malatedehydrogenase

NAD+

NAD+NADHNADH

3ATP3ATPAminotransferase

Aminotransferase

P457P457

Mitochondria

Cytosol