Download Biochemistry (2) 4 Oxidation phosphorylation

OXIDATIVE PHOSPHORYLATION
Electron transport chain (ETC) reaction
Definition
Components of the ETC
Mechanism of oxidation phosphorylation
(formation of ATP)
Chemiosmotic hypothesis
 The flow sheet of respiration,
showing the origin of the pairs of
hydrogen atom removed by
dehydrogenases with the transfer
of their electrons (2е-) to the
electron transport chain which
carries them to oxygen .
Reduction of each atom of oxygen
requires 2е- +2H+.Energy set free
during transport of a pair of
electron from NADH to oxygen is
harnessed to cause the coupled
synthesis of three of ATP from
ADP and phosphate in the
process of oxidation
phosphorylation. The electron
transport chain is shown here in
the abbreviation form.
Definition
 The oxidation of a substrate with accompanying phosphorylation of
ADP to ATP is known as oxidation phosphorylation.
 This process takes place in mitochondria as the enzymes concerned
with Citric acid cycle are located in the space between the cristae
and the enzyme concerned electron transport are arranged in
appropriate patter in cristae themselves.
 Oxidation phosphorylation is fundamental to all aspect of cellular
life in aerobic organisms since it is their main source of useful
energy.
 Oxidation phosphorylation involves the reduction of O2to H2O with
electrons donated by NADH and FADH2 .
Components of the ETC
 Pyridine nucleotides
Dehydrogenases enzymes
cannot catalyze the removal of
hydrogen from substrate unless
low molecular weight cofactors
are also present to act as
hydrogen acceptors. There are
frequently two pyridine
nucleotides :
1- Nicotinamide adenine
dinucleotide (NAD)
2- Nicotinamide adenine
dinucleotide phosphate
(NADP)
Components of the ETC

Flavoproteins
Reversible hydrogen acceptors
are flavoprotein, which consist of
protein moiety and a prosthetic group
containing riboflavin.
There are two flavin prosthetic group
1- Flavin mononucleotides
(FMN).
2- Flavin dinucleotides
(FDN).
Components of the ETC
 Coenzyme Q
An important part of the
chain of compounds involved in
cellular respiration. Associated
with the lipoprotein part of
mitochondria.
Coenzyme Q is found widely in
living materials and has therefore
given the name of ubiquinone
(latin ubique, every where).
Components of the ETC
 Cytochromes
Cytochrome means cellular pigment.
Cytochromes are redox catalysts of
major important in cellular
respiration.
They act by reversibly donating and
accepting ELECTRONS (not hydrogen).
Electrons are transported by the haem
iron which undergoes a change of
valency in the process.
There are many cytochromes designated
a, b , c.
The electron transport chain
Mechanism of oxidation
phosphorylation (formation of ATP)
There are 3 hypothesis regarding the
mechanism of oxidation phosphorylation:
1- Chemical coupling hypothesis.
2- Conformational coupling hypothesis.
3- Chemiosmotic hypothesis.
1- Chemical coupling hypothesis.
• It proposes that the electron transport is
coupled ATP synthesis by sequence of
consecutive reaction in which a high
energy intermediate is formed by
electron transport and subsequently is
cleaved and donates its energy to
generate ATP.
2- Conformational coupling hypothesis.
• It proposes that the transport of electron along the
respiratory chain causes a conformational change
in protein component of inner membrane to yield a
high-energy form. The conformational change so
produced is transmitted to F0F1ATPase molecule,
causing it to become energized. Relaxation of the
energized F0F1ATPase to its normal conformation
has proposed to provide the energy for the
synthesis of ATP and its release from the enzyme
F0F1ATPase .
3- Chemiosmotic hypothesis.
• It postulates that electron transport, pumps H+ from the matrix across
the inner mitochondria membrane to the outer aqueous phase thus
generating a H+gradient across the inner membrane. The osmotic
energy inherent in its gradient was postulated to drive the energy
requiring synthesis ATP.
• This mechanism mainly involves the following 3 important stages:
1- Ionization of hydrogen,
2- Electron transport chain.
3- Formation of water.
 Hydrogen atom ionized to hydrogen
ion H+ .
 Electrons removed from hydrogen
atom enter an electron transport
chain.
This ETC include flavoprotein, iron
sulfide protein, ubiquinone and the
cytochromes b,c1, c ,a and A3.
 Each electron gets shuttled from one
of these acceptors to the next until it
finally reaches cytochrome A3 which
called cytochrome oxidase because it
is capable, by giving 2 electron, of
causing elemental oxygen to
combine with hydrogen ions to form
water.
 The next step in the oxidation
phosphorylation is to convert ADP to
ATP by a large protein in inner
membrane called ATPase
(F0F1ATPase)