These are the compounds that allow mitochondria to use oxygen regardless of whether or not there is any phosphate (ADP) available. When an uncoupler is added, there is marked increase in O2 uptake.
SECTION TWO
Uncouplers of Oxidative Phosphorylation
• 2, 4 Dinitrophenol: A classic uncoupler of oxidative phos- phorylation (mechanism see below).
• Dicoumarol (Vitamin K analogue): Used as anticoagulant • Calcium: Transport of Ca++ ion into mitochondria can cause
uncoupling.
• Mitochondrial transport of Ca++ is energetically
coupled to oxidative phosphorylation. • It is coupled with uptake of Pi.
• When Ca++ is transported into mitochondria, electron
transport can proceed but energy is required to pump the Ca++ into the mitochondria. Hence, no energy is
stored as ATP.
• CCCP: Chlorocarbonyl cyanide phenylhydrazone— most active uncoupler.
• FCCP: Trifluorocarbonyl cyanide pheyl hydrazone. As compared to DNP it is hundred times more active as uncoupler.
• Valinomycin: It produced by a type of Streptomyces. Transports K+ from the cytosol into matrix and H+ from
matrix to cytosol, thereby decreasing the proton gradient. • Physiological Uncouplers:
– Excessive thyroxine hormone – EFA deficiency
– Long chain FA in Brown adipose tissue – Unconjugated hyperbilirubinaemia
Mechanism of Action of DNP as uncoupler
Dinitrophenol (DNP) a potent uncoupler is ampipathic and increase the permeability of the lipoid inner mitochondrial membrane to protons (H+), thus reducing
Fig. 10.6: Inhibitors of election transport chain and oxidative phosphorylation
the electrochemical potential and short-circuiting the ATP synthase. In this way, oxidation can proceed without phosphorylation.
Note: DNP was used for weight loss. But it was discontinued due to hyperthermia and other side effects. Inhibitors of electron transport chain and oxidative phosphorylation are shown in Figure 10.6.
CLINICAL ASPECTS
Inherited Disorders
Dysfunction of the respiratory chain can cause certain diseases which may be inherited deficiency of certain enzyme systems.
1. Infantile mitochondrial myopathy associated with renal dysfunction:
• The condition is fatal
• There is severe diminution or absence of most of the oxidoreductases of the electron transport chain.
2. MELAS: An inherited disorder associated with:
Mitochondrial myopathy, Lactic acidosis, Encephalo- pathy and Stroke
Enzyme deficiency: NADH: Ubiquinone oxido- reductase (complex 1) or deficiency of cytochrome oxidase.
SUMMARY: Various mechanisms of ATP formation are given below:
1. ATP Formation in Electron Transport Chain (ETC): The main mechanism by which ATP is formed in the body
SECTION TWO
Fig. 10.7: Reaction mechanism involved in ATP formation by quinones
I. Questions (Essay type and short notes)
1. What is ATP? Describe the formation, fate and importance with reference to electron transport chain.
2. Describe the electron transport chain reactions, and give its importance in oxidative phosphorylation
Short Notes
a. Respiratory chain,
b. Oxidative phosphorylation, c. Cytochromes,
d. Cytochrome oxidase,
e. Electron transport chain— Role and importance in biologic oxidation,
f. Cytochromes and its role in biologic oxidation. g. Redox potential, and
h. Electron transport chain
II. MCQs (Give one correct answer)
1. All of the following electron carriers are components of the mitochondrial electron transport chain except:
a. FMN b. FAD
c. NAD+ d. NADP+
e. Co-enzyme Q
2. Electron transport chain carriers are located in the mitochondria:
a. In mitochondrial matrix
b. On the inner surface of the outer mitochondrial membrane
c. On the outer surface of the outer mitochondrial membrane
d. In the intermembrane space
e. In the inner mitochondrial membrane.
is by oxidative phosphorylation in the electron transport chain (ETC).
2. Role of quinones in ATP formation: Quinol-p as intermediates in oxidative phosphorylation have been implicated for formation of ATP. Reaction mechanisms proposed by Nilkas and Lederer are as follows:
Pi reacts with the 6-chromanol derivatives of ubiquinones, or corresponding derivatives of vit K or α-tocopherol.
Steps of the Reactions
• Cyclisation of the quinones to chromanol, generating a quinone methine.
• Pi is added to the quinone-methine and the quinol-p is generated by trans-esterification.
• The quinol-p is then oxidised to a hydroxy-methyl quinone and transference of phosphate to ADP to form one ATP molecule.
• The final step in the reaction cycle is the reduction of the hydroxy methyl quinone to the original quinone structure (Fig. 10.7).
3. Substrate Level Phosphorylation: ATP can be produced at substrate level without mediation of electron transport chain.
Examples
a. In glycolytic cycle
1.
2.
b. In TCA cycle: In conversion of succinyl CoA to succinic acid.
4. In Löhman reaction: ATP can be formed from creatine ~ P in muscles. The high energy phosphate is trans- ferred to ADP and ATP is formed, called as Löhmann reaction.
• Myokinase reaction: In muscle, two ADP molecules can react to produce one molecule of ATP and AMP, the reaction being catalysed by the enzyme myokinase (adenylate kinase).
SECTION TWO
3. Electrons from pyruvic acid enter the mintochondrial electron transport chain at:
a. Nadh – Q reductase b. Coenzyme Q
c. QH2– cytochrome c-reductase
d. Cytochrome c oxidase
e. Between CoQ and cytochrome b 4. Match the following:
a. Coenzyme Q b. Flavoprotein
c. Cytochrome oxidase d. Cytochrome b
1. Catalyses electron transport between NADH and coenzyme Q _________.
2. Catalyses electron transport between coenzyme Q and cytochrome c _________.
3. Terminates the electron transport chain _________. 4. Catalyses electron transport between Fp and
cytochrome b ___________.
5. The phosphate: Oxygen (P:O) ratio is defined as:
a. The moles of phosphate consumed divided by moles of oxygen consumed,
b. The moles of ATP formed divided by the milligrams of protein,
c. The moles of CO2 produced divided by moles of O2
consumed,
d. The moles of ATP synthesised divided by the atom equivalents of O2 consumed,
e. The moles of O2 consumed in presence of ADP divided
by the moles of O2 consumed in absence of ADP
6. Which of the following oxidation-reduction systems has highest redox-potential? a. Fe3+ cytochrome b/Fe2+ b. Fe3+ cytochrome a/Fe2+ c. Fumarate/succinate d. Ubiquinone ox/red e. NAD+/NADH
7. Cyanide is poisonous as it stops respiration because of its: a. Inhibition of TCA cycle
b. Combination with RB cell membrane c. Inhibition of myoglobin
d. Formation of complex with Hb. e. Inhibition of cytochrome oxidase
8. In oxidative phosphorylation the oxidation of one molecule NADPH produces how many ATPs?
a. Zero b. 2
c. 3 d. 4
e. 5
9. In oxidative phosphorylation, one molecule of reduced flavoprotein produces how many ATPs:
a. Zero b. 2
c. 3 d. 4
e. 5
10. In the presence of rotenone:
a. NADH is oxidised by electron transport, b. FAD H2 is oxidised by electron transport
c. Cytochrome a is reduced by electron transport d. Cytochrome c is reduced
e. Cytochrome a3 is reduced
11. Which of the following statements describing cytochrome oxidase is true?
a. It is inhibited by copper
b. It is also known as cytochrome b
c. It transfers electrons from CoQ to cytochrome b d. It transfers four electrons and four protons to form
H2O molecule
e. It is a single cytochrome
12. Dinitrophenol (DNP) causes which of the following in biologic oxidation:
a. Increased bydrolysis of ATP b. Increased synthesis of ATP c. Prevents electrons transfer d. Lowers the body heat production e. All of the above
13. The chemical energy required for the synthetic processes is provided by:
a. Phosphorylation of ATP b. Dephosphorylation of ATP c. Phosphorylation of ADP d. Dephosphorylation of ADP e. All of the above processes
14. Which of the following vitamins of not a component of electron transport chain?
a. Nicotinamide b. Ubiquinone
c. Biotin d. Riboflavin
e. None of the above 15. Match the following:
A. Inhibitors of site I B. Inhibitors of site II C. Inhibitors of site III
1. Antimycin A __________. 2. Piericidin A ___________.
3. H2S ___________.
4. Amobarbital __________. 5. BAL ____________.
16. Aerobic dehydrogenases have the prosthetic group:
a. NAD+ b. NADP+
c. ATP d. FAD+
e. ADP
17. The oxidation and phosphorylation in intact mitochondria is blocked by: a. Puromycin b. Oligomycin c. Gentamicin d. Streptomycin e. Septran Answers to MCQs 1. d 2. e 3. a 4. 1-B, 2-D, 3-C, 4-A 5. d 6. b 7. e. 8. a. 9. b 10. b 11. d 12. a 13. b 14. c 15. 1-B, 2-A, 3-C, 4-A, 5-C 16. d 17. b.
Major Concepts