5. Electron Transport Chain

8/2/2019 5. Electron Transport Chain

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Electron Transfer

Chain


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Total of all chemical changes that occur in body. Includes:

Anabolism:

Energy-requiring process where small molecules joined to

form larger molecules

E.g. Glucose + Glucose

Catabolism:

Energy-releasing process where large molecules broken down

to smaller Energy in carbohydrates, lipids, proteins is used to

produce ATP through oxidation-reduction reactions

METABOLISM


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Metabolic Pathways

The enzymatic reactions of metabolism form a network of

interconnected chemical reactions, or pathways.

The molecules of the pathway are called intermediates

because the products of one reaction become the substrates

of the next.

Enzymes control the flow of energy through a pathway.


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An electron transfer reaction:

Aox

+ Bred

Ared

+ Box

Aox

is the oxidized form of A (the oxidant)

Bred

is the reduced form of B (the reductant).

Fe+++ + e- Fe++


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1. NAD+/NADH and FAD/FADH2

2. FMN (Flavin MonoNucleotide)

3. Coenzyme Q

functions as a mobile e- carrier within the mitochondrial inner

membrane

Q + 2 e- + 2 H+ QH2

trans-membrane H+ transport

4. Cytochromes

proteins with heme prosthetic groups

Some cytochromes are part of large integral membranecomplexes,

each consisting of several polypeptides & including multiple electron

carriers

Electron Carriers


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5. Iron-sulfur centers (Fe-S)

prosthetic groups containing 2, 3 , 4 or 8 iron atoms complexed to

elemental & cysteine S.

4 -Fe centers have a tetrahedral structure, with Fe & S atoms

alternating as vertices of a cube.

Fe

Fe

S

S

S

Fe

Fe

S

S

S

SS

Cys

Cys

Cys

Cys

S

Fe

S

Fe

S

S

S

S

Cys

CysCys

Cys

Iron-Sulfur Centers


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Proteins in specific order

Transfers 2 electrons in specific order

Proteins localized in complexes

i. Embedded in membrane

ii. Ease of electron transfer

Electrons ultimately reduce oxygen to water

2 H+ + 2 e- + O2 H2O

ELECTRON TRANSPORT COMPLEXES


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Complex Name

No. of

Proteins

Prosthetic Groups

Complex I NADH

Dehydrogenase

46 FMN,

9 Fe-S cntrs.

Complex II Succinate-CoQ

Reductase

5 FAD, cyt b560,

3 Fe-S cntrs.

Complex III CoQ-cyt c

Reductase

11 cyt bH, cyt bL, cyt

c1, Fe-S

Complex IV Cytochrome

Oxidase

13 cyt a, cyt a3, CuA,

CuB

Composition of Respiratory Chain Complexes


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It is the largest of all complexes

Consisting of 46 protein chains in two domains that form an L-shape.

Complex I transports two electrons from NADH in the mitochondrial

matrix to Q within the membrane, via FMN and a series of seven

iron-sulphur clusters.

In addition to the redox function, Complex I transports four H+ across

the membrane.

Complex I - NADH-ubiquinone oxidoreductase


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Complex II oxidizes succinate to fumarate

Reduces Q to QH2 within the membrane

The transfer proceeds initially via an FAD cofactor and then through

a series of three iron-sulphur clusters.

Finally, electrons pass through the heme iron of cytochrome b560 and

then outside the protein to ubiquinone, which binds near the heme

group.

Complex II - Succinate Dehydrogenase


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Dimeric complex

Each half of dimeric Complex III has a binding site for the lipid-mobile

carrier ubiquinol/ubiquinone (QH2/Q).

Quinol oxidation takes place near the intermembrane space, while

quinone reduction takes place near the matrix side of the membrane

Because the redox of Q/QH2 also involves the transfer of protons

The overall reaction can be represented as:

2 QH2 + Q + 2H+("in") + 2 cyt cIII ==> 2 Q + 4H+("out") + QH2 + 2 cyt c

II

Complex III - Cytochrome bc1 Complex


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Cytochrome c Oxidase (CcO, Complex IV) is a large, membrane-

bound dimeric enzyme, with each half of the dimer consisting of 13

protein chains.

The complex acts as the terminus of mitochondrial electron

transport in all aerobic life, by using four electrons to reduce

dioxygen:

O2 + 4H+ + 4e==> 2 H2O

This reaction is coupled with the transfer of four protons across the

mitochondrial membrane, driving the synthesis of ATP.

Complex IV - Cytochrome c Oxidase


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5. Electron Transport Chain