This document discusses electron transport chain and biological oxidation. It begins by listing topics students should understand, including stages of oxidation, redox potentials, enzymes/co-enzymes in oxidation, electron transport chain organization, inhibitors, oxidative phosphorylation, chemiosmosis theory, and ATP synthesis/inhibitors. It then discusses NADH generation, ATP-ADP cycle, mitochondrial structure, and the four complexes of the electron transport chain located in the inner mitochondrial membrane, being complex I-IV. Specific inhibitors of the electron transport chain are also mentioned.

1. ELECTRON TRANSPORT CHAIN
DIPAK KUMAR SINGHA
ASST. PROFESSOR
CALCUTTA INSTITUTE OF
PHARMACEUTICAL TECHNOLOGY &
AHS
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2. BIOLOGICAL OXIDATION AND ELECTRON
TRANSPORT CHAIN
The students must be able to answer questions on the following
topics:
➢ Stages of Oxidation of Foodstuffs
➢ Redox potentials
➢ Biological oxidation
➢ Enzymes and co-enzymes involved biological oxidation
➢ High energy compounds
➢ Organization of electron transport chain
➢ Inhibitors of electron transport chain
➢ Oxidative Phosphorylation
➢ Chemiosmotic theory
➢ Proton pump
➢ ATP synthesis
➢ Inhibitors of ATP synthesis

3. BIOENERGETICS OR BIOCHEMICAL THERMODYNAMICS
Free energy
The energy actually available to do work ( utilizable)is kkown as free energy. Changes
in the free energy (ΔG) are valuable in predicting the feasibility of chemical
reactions. During a chemical reaction, heat may be released or absorbed. Enthalpy
(ΔH) is a measure of the change in heat content of the reactants, compared to
products. Entropy (ΔS) represents a change in the randomness or disorder of
reactants and products. Entropy attains a maximum as the reaction approaches
equilibrium. The react ions of biological systems involve a temporary decrease in
entropy.
The relation between the changes of free energy (ΔG), enthalpy (ΔH) and entropy
(ΔS) is
expressed as Δ G=ΔH-TΔS
T representsth e absolutet emperaturei n Kelvin (K=273+"C). The term standard free
energy represented by ΔC' (note the superscript') is often used. Lt indicates the free
energy change when the reactants or products are at a concentration of 1mol/l at pH
7.0

4. ATP ADP Cycle
• The hydrolysis of ATP is associated with the release of large amount of
energy.
• ATP + H2C ------+A DP + Pi + 7.3 Cal.

5. ATP-ADP cycle along with sources and utilization of ATP
(Note that -P does not exist in free form, but is only
transferred).

6. ORGANIZATION OF ELECTRON TRANSPORT CHAIN

7. Structure of Mitochondrion
The electron transport chain is functioning inside the mitochondria. The
mitochondrion is a subcellular organelle having the outer and inner
membranes enclosing the matrix. The inner membrane is highly selective
in its permeability, containing specific transport proteins. Certain
enzymes are specifically localized in mitochondria. The inner membrane
contains the respiratory chain and translocating systems. The knob like
protrusions represent the ATP synthase system

8. Structure of Mitochondrion

9. Inner and outer mitochondrial membrane differs greatly
in their composition. Inner membrane is 22% cardiolipin
and contains no cholesterol, whereas outer membrane is
similar to cell membrane, with less than 3% cardiolipin and
45% cholesterol.
Location of enzymes in mitochondria

10. ORGANIZATION OF ELECTRON TRANSPORT CHAIN
i. In the Electron transport chain, or respiratory chain, the
electrons are transferred from NADH to a chain of electron
carriers. The electrons flow from the more electronegative
components to the more electropositive components.
ii. All the components of electron transport chain (ETC) are
located in the inner membrane of mitochondria.
iii. There are four distinct multi-protein complexes; these are
named as complex-I, II, III and IV. These are connected by two
mobile carriers, co-enzyme Q and cytochrome c.
iv. The arrangement is schematically represented.

11. Components and sequence of reactions of
electron transport chain

12. NADH GENERATION
The NADH is generated during intermediary metabolism. A detailed list of the
reactions using NADH is given.
Malate Aspartate Shuttle
Mitochondrial membrane is impermeable to NADH. The NADH equivalents generated
in glycolysis are therefore to be transported from cytoplasm to mitochondria for
oxidation. This is achieved by malate aspartate shuttle or malate shuttle, which
operates mainly in liver, kidney and heart. The cycle is operated with the help of
enzymes malate dehydrogenase (MDH) and aspartate aminotransferase. From
one molecule of NADH in the mitochondria, 2½ ATP molecules are generated.

13. Glycerol-3-phosphate Shuttle
In skeletal muscle and brain, the reducing equivalents from cytoplasmic NADH are
transported to mitochondria as FADH2 through glycerol-3-phosphate shuttle (Fig.
20.6). Hence only 1½ ATPs are generated when this system is operating.

14. ETC Complex-I
i. It is also called NADH-CoQ reductase or NADH dehydrogenase complex. It
is tightly bound to the inner membrane of mitochondria.
ii. It contains a flavoprotein (Fp), consisting of FMN as prosthetic group and an
iron-sulfur protein (Fe-S). NADH is the donor of electrons, FMN accepts
them and gets reduced to FMNH2 (Fig. 20.8). Two electrons and one
hydrogen ion are transferred from NADH to the flavin prosthetic group of
the enzyme. NADH + H+ + FMN → FMNH2 + NAD+
iii. The electrons from FMNH2 are then transferred to Fe-S. The electrons are
then transferred to co-enzyme Q (ubiquinone) (CoQ).
iv. Overall function of this complex is to collect the pair of electrons from
NADH and pass them to CoQ. The reactions are shown in Figure 20.8.
v. There is a large negative free energy change; the energy released is 12
kcal/mol. This is utilized to drive 4 protons out of the mitochondria.

15. Complex II or Succinate-Q-Reductase
The reaction in Complex-II is represented in Fig. 2.9. The electrons from FADH2 enter
the ETC at the level of coenzyme Q. This step does not liberate enough energy to
act as a proton pump. In other words, substrates oxidized by FAD-linked enzymes
bypass complex-I. The three major enzyme systems that transfer their electrons
directly to ubiquinone from the FAD prosthetic group are:
i. Succinate dehydrogenase,
ii. Fatty acyl CoA dehydrogenase
iii. Mitochondrial glycerol phosphate dehydrogenase(Fig. 2.6).

16. Co-enzyme Q
i. The ubiquinone (Q) is reduced successively to semiquinone (QH) and finally to
quinol (QH2).
ii. It accepts a pair of electrons from NADH or FADH2 through complex-I or complex-II
respectively (Figs2.7 and 2.13).

17. Components and sequence of reactions of
electron transport chain

18. iii. Co-enzyme Q is a quinone derivative having a long isoprenoid tail. The
chain length of the tail is different
in various species, mammals have 10 isoprene units (Fig. 2.10). Two
molecules of cytochrome c are reduced.
iv. The Q cycle thus facilitates the switching from the two electron
carrier ubiquinol to the single electron carrier cytochrome c.

19. Complex III or Cytochrome Reductase
i. This is a cluster of iron-sulfur proteins, cytochrome b and cytochrome c1, both
contain heme prosthetic group. The sequence of reaction inside the Complex III is
shown in Figure 20.11.
ii. During this process of transfer of electron, the iron in heme group shuttles between
Fe3+ and Fe2+ forms.
iii. The free energy change is—10 kcal/mol; and 4 protons are pumped out.
Cytochrome c
• It is a peripheral membrane protein containing one heme prosthetic group. The
term cytochrome is derived from Greek, meaning cellular colors. It is one of the
highly conserved proteins among different species. Axel Theorell (Nobel
• prize, 1955) isolated it. Cytochrome c collects electrons from Complex III and
delivers them to Complex IV.

20. Complex IV or Cytochrome Oxidase
i. It contains different proteins, including cytochrome a and cytochrome a3. The
Complex IV is tightly bound to the mitochondrial membrane.
ii. The reaction is depicted in Figure 20.12. Four electrons are accepted from
cytochrome c, and passed on to molecular oxygen.
4 H+ + O2 + 4 Cyt. c-Fe++ → 2 H2O + 4 Cyt. c+++
iii. 2 protons are pumped out to the inter-membrane space.
iv. Cytochrome oxidase has 4 redox centers, namely, a, a3, CuA and CuB. The electron
transfer in this complex is as shown
Cyto c →CuA →Cyto a →Cyto a3 →CuB
Cytochrome oxidase contains two heme groups and two copper ions. The two heme
groups are denoted as cytochrome-a and cytochrome a-3. The functional unit
of the enzyme is a single protein, and is referred to as cytochrome a--a3.
The sequential arrangement of members of electron transport chain is shown in Box
20.2 and Fig. 20.13.

21. INHIBITORS OF ELECTRON TRASPORT
CHAIN

22. ANY
QUESTION?