electron transport chain and mechanism

1. DEVIPRIYA P V
M PHARM

2.  Electron transport chain (ETC) and its mechanism.
 Oxidative phosphorylation & its mechanism
 Substrate level phosphorylation
 Inhibitors ETC and oxidative phosphorylation
/Uncouplers

3.  Oxidation is the loss of electrons and reduction is the gain of
electrons.
 oxidation-reduction is applicable to biological systems also.
 The transfer of electrons from the reduced coenzymes through
the respiratory chain to oxygen is known as biological
oxidation.
 The enzymes involved in biological oxidation are:
1. Oxidases
2. Dehydrogenases
3. Hydroperoxidases
4. Oxygenases.

4.  Energy-rich molecules, such as glucose, are metabolized by a
series of oxidation reactions and yield CO2 and water.
 The metabolic intermediates of these reactions donate
electrons to specific coenzymes, nicotinamide adenine
dinucleotide (NAD+) and flavin adenine dinucleotide (FAD), to
form the energy-rich reduced forms, NADH and FADH2.
 These reduced coenzymes donate a pair of electrons to a
specialized set of electron carriers, collectively called the
electron transport chain
 The ETC is located in the inner mitochondrial membrane.
 The inner mitochondrial membrane have 5 enzyme complexes-
complex I, II, II , IV and V.
 The complexes I- IV are carriers of electrons while complex V
is responsible for ATP synthesis.

5.  There are five distinct carriers that participate in the ETC.
(1)Nicotinamide nucleotides:
 NAD+ is reduced to NADH + H+ by dehydrogenases with the removal
of 2 hydrogen atoms from the substrates (glyceraldehyde-
3phosphate, pyruvate,isocitrate,α-ketoglutarate and malate).
(2) Flavoproteins:
 NADH dehydrogenase reductase is a flavoprotein with FMN as the
prosthetic group.
 The coenzyme FMN accepts 2 electrons and a proton to form
FMNH2.
 Succinate dehydrogenase is also a flavoprotein with FAD as the
coenzyme. This accept 2 hydrogen atoms from succinate
(3) Iron- sulfur proteins:
 The iron-sulfur (FeS) proteins exist in the oxidized (Fe3+) or
reduced (Fe2+) state and they participates in the transfer of
electron

6. (4) Coenzyme Q (ubiquinone):
 It can accept electrons from FMNH2 produced in the ETC by
NADH dehydrogenase or FADH2 produced outside ETC
(5) Cytochromes:
 cytochromes are conjugated proteins containing heme group
 The electrons are transported from coenzyme Q to cytochromes
(in the order) b, c1, c, a and a3.

7. Overview of biological oxidation
(ETC- Electron transport chain).

8.  The transport of electrons through the ETC is linked with the
release of free energy.
 The process of synthesizing ATP from ADP and Pi coupled with
the ETC is known as oxidative phosphorylation.
 The complex V of the inner mitochondrial membrane is the
site of oxidative phosphorylation.
 There are three reactions in the ETC that are exergonic to
result in the synthesis of 3 ATP molecules.
1. Oxidation of FMNH2 by coenzyme Q.
2. Oxidation of cytochrome b by cytochrome c1 .
3. Cytochrome oxidase reaction.

9.  MECHANISM OF OXIDATIVE PHOSPHORYLATION.
Chemical coupling hypothesis :
 According to chemical coupling hypothesis, during the course of
electron transfer in respiratory chain, a series of phosphorylated
high-energy intermediates are first produced which are utilized
for the synthesis of ATP.
Chemiosmotic hypothesis :
 Proton gradient : The inner mitochondrial membrane is
impermeable to protons (H+) and hydroxyl ions (OH-).
 The transport of electrons through ETC is coupled with the
translocation of protons (H+) across the inner mitochondrial
membrane (coupling membrane) from the matrix to the
intermembrane space.
 The pumping of protons results in a proton gradient which results
in the synthesis of ATP from ADP and Pi.
 ATP synthase, present in the complex V, utilizes the proton
gradient for the synthesis of ATP.

10. Rotary model for ATP generation:
 The changes in the mitochondrial membrane proteins leads
to the synthesis of ATP.
Structure of mitochondrial ATP
synthase complex
Inner mitochondrial
membrane

11.  ATP may be directly synthesized during substrate oxidation
in the metabolism.
 The high-energy compounds such as phosphoenolpyruvate
and 1,3-bisphosphoglycerate (intermediates of glycolysis)
and succinyl CoA (of citric acid cycle) can transfer high-
energy phosphate to ultimately produce ATP.

12. (1)NADH and coenzyme Q :
 Fish poison rotenone, barbituate drug Amytal and antibiotic
piercidin A inhibit this site.
(2)Between cytochrome b and c1 :
 Antimycin A -an antibiotic, British antilewisite (BAL)-an
antidote.
(3)Inhibitors of cytochrome oxidase:
 Carbon monoxide, cyanide, hydrogen sulphide and azide
effectively inhibit cytochrome oxidase.
 Carbon monoxide reacts with reduced form of the cytochrome
while cyanide and azide react with oxidized form.
 Cyanide is a potent inhibitor of ETC. It binds to Fe3+ of
cytochrome oxidase blocking mitochondrial respiration leading
to cell death.

14.  Uncouplers are compounds that can uncouple (or delink) the
electron transport from oxidative phosphorylation
 They increase the permeability of inner mitochondrial
membrane to protons (H+), so that ATP synthesis does not
occur.
 The uncouplers allow oxidation of substrates without ATP
formation.
 Eg: 2,4-dinitrophenol (DNP), dinitrocresol, pentachlorophenol,
trifluorocarbonylcyanide phenylhydrazone( FCCP).
 Physiological substances like thermogenin, thyroxine and long
chain free fatty acids
 The antibiotic Oligomycin prevents the mitochondrial oxidation
as well as phosphorylation.
 Plant toxin Atractyloside is an uncoupler