Biological oxidation lecture 2

BIOLOGICAL OXIDATION

Electron Transport Chain

Dr. Dalia Shaalan

Lecturer of Medical Biochemistry

Faculty of Medicine – Mansoura University


In non-biologic systems,

• Energy is produced in the form of heat by direct

reaction between hydrogen and oxygen,

• Then heat can be transformed into mechanical

or electric energy.

• This process is explosive, inefficient and

uncontrolled.


In biologic systems,

• Cells use electron transport chain to transfer

electrons stepwise from substrates to oxygen.

• Thus producing energy gradually.

• This process is stepwise, efficient and

controlled.


• Definition:

It is a chain of catalysts of increasing redox

potential that collects reducing equivalents

(hydrogen atoms and electrons) from substrates

transferring it stepwise to be oxidized in a final

reaction with oxygen to form water and energy.

• It is also known as redox chain or respiratory

chain.


Components of the electron transport chain

1. Hydrogen and electron carriers.

2. Four membrane-bound enzyme complexes.

All are imbedded in the inner mitochondrial

membrane.


NAD→ FMN→ Co Q→ Cyt b→ c1 → c → a → a3Hydrogen and electron carriers

FAD

Electron Transport ChainHydrogen and electron carriers

1- NAD+

• A coenzyme acts as a hydride ion carrier (H- =

Hydrogen atom with 2 electrons).

• It receives 2H from substrates as isocitrate,

malate, β-hydroxy acyl CoA and β-hydroxy

butyrate.

• Its reduced form (NADH+H+) passes both

hydrogens to flavoprotein containing FMN


Hydrogen and electron carriers

2-Flavoproteins

• FAD and FMN are tightly bound to flavoproteins to

prevent their reduced form reacting with oxygen directly.

• Flavoprotein Fp1 containing FMN receives (2 H) atoms

from reduced NAD+ passing them to coenzyme Q.

• Flavoprotein Fp2 containing FAD receives (2 H) atoms

from substrates as succinate, acyl CoA and choline

passing them to coenzyme Q.


3- Ubiquinone (Coenzyme Q)

• It is a small molecule, freely mobile in the inner

mitochondrial membrane.

• collects reducing equivalents from more fixed

components (reduced FMN or FAD).

N.B: Ubiquinones are compounds containing quinine ring

(vary according to number of side chain isoprene units).

The most common ubiquinone is coenzyme Q that has

structural similarity to vitamin K.




• Receives 2 hydrogen atoms to form ubiquinol (reduced

coenzyme Q) or 1 hydrogen atom forming semiquinone.

• Reduced coenzyme Q passes the 2 e- to 2 cytochrome b

and releases 2H+ into the mitochondrial matrix.

• It is a bridge between flavoproteins, which can carry 2

hydrogen atoms, and cytochrome b, which can carry one

electron only.




Oxidation of ubiquinol involves 2 successive actions:

• Ubiquinol (coenzyme Q) dehydrogenase:

transfers electrons to (cyt b, FeS protein, cyt c1 which

are its coenzymes) then to cytochrome c .

• Cytochrome oxidase:

transfers electrons from cyt c to (cyt a, cyt a3 which are

its coenzymes) then to oxygen.


4- Cytochromes

• Electron carriers transfer only electrons from CoQ to O2.

• The letters; a, b and c according to their order of discovery.

• All ETC cytochromes are haemoproteins (differ in redox

potential).

• The haem in cytochromes differs from that of haemoglobin

as the Fe atom oscillates between oxidation and reduction

during the physiological action of cytochromes, while the Fe

of haemoglobin remains in the reduced form during its

physiological action.



4- Cytochromes

• Cytochrome c is a water soluble, peripheral membrane

protein. It is relatively mobile.

• Cytochrome a3 contains copper in addition to the

haeme group.

• N.B. the mobile components of the electron transport

chain include Co-Q and cyt c. They collect reducing

equivalents from the other fixed components.


5- Iron sulfur protein (FeS or none- haeme iron)

• associated with flavoproteins and cytochrome b.

• consists of cluster of cysteine residues which bind Fe by

covalent bonds (with S of cysteine).

• The S and Fe are thought to take part in the oxidation-

reduction mechanism between flavoprotein and

coenzyme Q as Fe atom in these complexes oscillates

between oxidation and reduction that allow them to

either give up or accept electrons.

Enzyme Complexes of the Electron Transport

Chain organized in the inner mitochondrial

membrane


Enzyme Complexes of the Electron Transport Chain

• Complex I: NADH dehydrogenase (NADH-ubiquinone

oxidoreductase)

• It is a flavoprotein that contains FMN as well as FeS protein

as coenzymes.

• It transfers hydrogen atoms from NADH+H+ to ubiquinone.



• Complex II: Succinate dehydrogenase (succinate-

ubiquinone oxidoreductase)

• It is a flavoprotein that contains FAD as well as FeS

protein as coenzymes.

• It transfers hydrogen atoms from succinate to

ubiquinone.



Complex III: Ubiquinol dehydrogenase (ubiquinol-

cytochrome c oxidoreductase).

• It transfers electrons from ubiquinol to cytochrome c

using cyt b and cyt c1 as coenzymes.

Complex IV: Cytochrome oxidase (cytochrome-oxygen

oxidoreductase)

• It transfers electrons from cytochrome c to oxygen.

• It needs cyt a and cyt a3 as coenzymes.



N.B: In addition to these four enzyme complexes, there is

fifth complex (complex V) which is the ATP synthase

that responsible for biosynthesis of ATP from ADP and

inorganic phosphate.


AH2 NAD FMNH2 Oxidized reduced Oxidized reduced

A NADH+H+ FMN reduced Oxidized reduced Oxidized

Coenzyme Q

Coenzyme Q

Cyt b (2 Fe +2) Cyt c1 (2 Fe +3) Cyt c (2 Fe +2)

Cyt b (2 Fe +3) Cyt c1 (2 Fe +2) Cyt c (2 Fe +3)

reduced Oxidized reduced

Oxidized reduced Oxidized

½ O2

Sequence of events in the electron transport chain

Cyt a3 (2 Fe +2)

Cyt a (2 Fe +2) Cyt a3 (2 Fe +3)Cyt c (2 Fe +3)

Cyt c (2 Fe +2) Cyt a (2 Fe +3)

O= H2O

2H+

NAD linked

dehydrogenases


Complex IIIComplex I

Complex IV

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Biological oxidation lecture 2