Tema 4. Electron Transport

Cap. 4 pages 120 -145

Aox

Ared

INOUT

-n∆Eh = y∆p

# protons extruded -e

The generation of energy for growth-related physiological processes in respiringprokaryotes is by coupling the flow of electrons in membranes to the creation

of an electrochemical proton gradient.

Bred

Box

-n∆Eh = y∆p

# e transfered

yH+yH+

Coupling site

-e

Who is responsible for the movement of the electrons through the membrane?

The electron carriers are either proteins or lipids!

1) Flavoproteins which carry a hydrogen and an electron

2) Quinones are lipids which carry a hydrogen and an electron

3) Iron-sulfur proteins which carry only electrons

4) Cytochromes which carry only electrons

The electrons are not carried in the protein, but in a non-protein portion called

Prosthetic group

Prosthetic groups

1) Flavoproteins is a flavin, either flavin adenine dinucleotide (FAD) orflavin mononucleotide (FMN)

2) Iron-sulfur proteins is a cluster of iron-sulfur (FeS)

3) Cytochromes is heme.

Flavoproteins (Fp)

Flavin comes from the Latin flavius, which means yellow.

The flavins (FAD, FMN) are synthesizedby cells from the vitamin riboflavin (B2).

FMN

FMN

Quinones

Their hydrophobic lipid nature allow them to transfer electrons to carries that are notmobile.

UQ coenzyme Q

n = 6 to 10 The isoprenoid side chains contribute the hydrophobicity.

menaquinone

Menaquinones are derivatives of vitamin Kand differ from UQ in being a naphthoquinone

They also have a lower Ehand are used predominatelyduring anaerobic respiration

Iron-sulfur proteinsThey contain non-heme iron andUsually acid-labile sulfur.

These proteins cover a wide rangeof potentials -400mV to +350 mV.

Example:ferredoxin

Cytochromes

The iron is the electron carrier and is oxidizedto ferric or reduced to ferrous ion during e- transport.

Different types of heme:a, b, c, d, and o.

D and o have only been found inprokaryotic cytochrome oxidases

They can usually be identified byspectrophotometry.

Cytochrome b

In Mitochondria

Organization of the electron carriers

Coupling sitesProton extrusion

NADH fp FeS UQ b c1c aa3

O2

E´o-320 mV

E´o+ 815 mV

Complex I Complex III Complex IV

succinate fp

FeS

Complex II

NADH dehydrogenase

Succinate dehydrogenase

Ubiquinol-cytochrome c oxidoreductase

Cytochrome aa3 oxidase

In bacteria

AH2 dehydrogenase quinone

bc1c aa3 O2

b

o

Aerobic respiration

AH2 dehydrogenase quinone Yreductase

Anaerobic respiration

This creates flexibility, oxidases with differentaffinity to oxygen, they could differ in ∆p.

Adaptability to environmental conditions: E. colican use a reductase or an oxidase depending onoxygen availability.

How to identify a coupling site?

NADH fp FeS

Complex I

UQ b c1

Complex III

c aa3

Complex IV

O2

succinate fp

FeS

Complex II

E´o-320 mV

E´o+ 815 mV

The # of ATP’s made for every 2e- transfer to oxygen is called the P/O ratio. It is equal to the # of ATP formed per oxygen consumed.

You could use P/2e- when the e- acceptor is not O2

NADH O2 P/O ratio = 3

succinate O2 P/O ratio = 2

succinateComplex II

Each coupling site is characterizedby a drop in midpoint potential of about 200 mV.

The Q loop

InOut

fp

FeS

NADH + H+

NAD+2H+Site 1

Note that the electron carriersalternate between those that carry both H2 and e- and thosethat only carry e-.

2 X [e-]

QQH2 2H+

Cytochrome

oxidase

2H+

½ O2 + 2H+

2H+ 2H+H2O

Site 2

Site 3

that only carry e-.

2 X [e-]

Q cycle ( explains observations of e- transfer in mitochondria,chloroplasts, and many bacteria)

Out

UQH

UQH2 UQUQ-O2

1e-

bL

2 H+

P site

1e-

In

UQH2

UQ- UQ

bH

UQH2

2 H+

N site

1e-

P site can by inhibited by myxothiazol and stigmatellin

N site is inhibited by antimycin.

E. coli

NADH dehydrogenase NDH1UQ

MQ

DMQ

NO3

70%

30%

fumarate

74%

16%

10%

O2

60%

37%

3%

NADH dehydrogenase NDH2

DMQ 70% 16%37%

b562 o b558 d

O2

High affinity to O2

NDH1 and bo 8H+ NDH2 and bd 2H+

NADH dehydrogenase NDH1 UQ bc1 c aa3O2

cyt, bb3

dehydrogenasemethanolNitrate reductase NO3

Paracoccus denitrificansNon-fermenting G-, facultative anaerobe

Nitrite reductase NO2

Nitric oxide reductase

Nitrous oxide reductase

NO

N2O

½ N2

Wolinella succinogenesG-, anaerobe isolated from the rumen.

cty bFeSNi

H2

2 H+fumarate

+ 2H+

INOut

MK

cty bFeSMO

MO

cty b FeS

FAD

HCO2

H+ + CO2

2H+

succinate