Electron Transport Chain Notes

Electron Transport Chain

Learning Outcomes

• Describe the mechanism of cellular respiration (electron transport) and apply it in solving clinical disorders like pesticide, carbon monoxide, and cyanide poisoning.

• Translate the knowledge to understand the significance of uncoupling (newborn infant, non-shivering thermogenesis and brown adipose tissue).

• Relate to some of the diseases associated with cellular respiration.

Cellular Respiration: An Overview

• Living things obtain most of their energy from glucose.

• Cellular respiration is the process that releases energy by breaking down glucose and other food molecules in the presence of oxygen.

Stages of Oxidation of Food Stuffs

• First stage: Digestion in the GIT converts macromolecules into small units (carbohydrates into glucose, lipids into fatty acids, proteins into amino acids). This is called primary metabolism.

• Second stage: These products are absorbed. In the mitochondrial citric acid cycle reducing equivalents NADH or FADH2 are generated. This is called secondary or intermediary metabolism.

• Third stage: These reducing equivalents enter the electron transport chain or respiratory chain where energy is released (ATP). This is tertiary metabolism or internal or cellular respiration. The energy (ATP) is then used for body synthetic purpose.

The "Mighty" Mitochondria

• The mitochondria are the organelles where the final stages of cellular respiration occur.

• Citric acid Cycle

• Electron Transport Chain

• Cells that use a lot of energy have high numbers of mitochondria.

• Example: Muscle cells in the heart.

Cellular Respiration Flowchart

• $Glucose (C<em>6H</em>{12}O<em>6) + Oxygen (O</em>2) \rightarrow Glycolysis \rightarrow Krebs Cycle \rightarrow Electron Transport Chain \rightarrow Carbon Dioxide (CO<em>2) + Water (H</em>2O)$

What Happens if No Oxygen is Available?

• The Krebs Cycle and Electron Transport Chain can’t function.

• These are anaerobic conditions.

Strenuous Exercise

• Lactic acid is produced by muscle cells during rapid exercise when the body cannot supply enough $O_2$ to tissues.

• Without enough $O_2$, the body is NOT able to produce all of the ATP that is required.

• The buildup of lactic acid can cause painful burning in muscles.

Comparing ATP Production Mechanism

• In the presence of oxygen: the body breaks down glucose through aerobic respiration, producing 32 ATP per glucose molecule, which is more efficient.

• In the absence of oxygen: the body breaks down glucose through lactic acid fermentation (anaerobic respiration), producing 2 ATP per glucose.

Aerobic Training

• Examples: Marathons (long runs), biking, swimming.

• Can increase the size and number of mitochondria in muscle cells.

• Can increase the delivery of $O_2$ to muscles by improving the heart and lungs.

Anaerobic Training

• Examples: sprints, strides, quick bursts of energy.

• Increase the body’s tolerance to lactic acid.

• Increase the glycogen levels in the muscles.

ATP - Universal Currency of Energy in Living Cells

• The energy released from the hydrolysis of ATP is utilized for:

  • Mechanical work: Muscle contraction

  • Transport work: Sodium-potassium ATPase pump

  • Biochemical work: Initial steps of Glycolysis

  • Anabolic pathways: TAG, DNA, Protein synthesis

  • Detoxification (urea cycle), formation of active intermediates like UDP glucose

  • Heat production

Chemistry of ATP Hydrolysis

• Two high-energy bonds in ATP are represented by a squiggle bond ($\thicksim$). On hydrolysis, each releases -7.3 kcal/mole.

• More than 90% of ATP is formed through ETC (Oxidative phosphorylation).

• The rest is formed in creatine phosphate shuttle and substrate-level phosphorylation.

Biological Oxidation

• Definition: Transfer of electrons from reduced coenzymes through ETC to oxygen.

• Energy released during this process is trapped as ATP.

• This coupling of oxidation with phosphorylation is called oxidative phosphorylation.

Oxidoreductases

1. Oxidases

2. Aerobic dehydrogenases

3. Anaerobic dehydrogenases

4. Hydroperoxidases

5. Oxygenases

Enzymes, Functions, and Examples

• Oxidases: Remove hydrogen from substrates, with oxygen as the acceptor, forming water (e.g., cytochrome oxidase).

• Aerobic dehydrogenases: Remove hydrogen from substrates, with oxygen as the acceptor, forming hydrogen peroxide ($H<em>2O</em>2$) (e.g., xanthine oxidase).

• Anaerobic dehydrogenases: Use NAD+, NADP+, or FAD+ as hydrogen acceptors.

  • NAD+ dehydrogenases

  • NADP+ dehydrogenases

  • FAD+ dehydrogenases

  • Cytochromes

• Hydroperoxidases: Catalyze hydrogen peroxide (e.g., catalase and peroxidases).

• Oxygenases

  • Dioxygenases: Incorporate both atoms of oxygen into the substrate: $A + O<em>2 \rightarrow AO</em>2$

  • Monooxygenases: Incorporate one atom of oxygen into the substrate, with the other reduced to water: $A - H + O<em>2 + ZH</em>2 \rightarrow A - OH + H_2O + Z$

NAD+ Linked Dehydrogenases

• Examples:

  1. Glyceraldehyde-3-phosphate dehydrogenase

  2. Isocitrate dehydrogenase

  3. Glutamate dehydrogenase

  4. Pyruvate dehydrogenase

  5. Alpha-ketoglutarate dehydrogenase

• NADP+ Linked dehydrogenases take part in reductive biosynthesis.

FAD Linked Dehydrogenases

• $AH<em>2 + FAD \rightarrow FADH</em>2$

• FAD is derived from riboflavin (Vitamin B complex).

• Both hydrogen atoms are attached to the flavin ring.

• Examples:

  1. Succinate dehydrogenase

  2. Fatty acyl CoA dehydrogenase

Substrate Level Phosphorylation

• Energy from a high-energy compound is directly transferred to ADP or GDP to form ATP or GTP without the electron transport chain.

• Examples:

  1. Bisphosphoglycerate kinase (glycolysis)

  2. Pyruvate kinase (glycolysis)

  3. Succinate thiokinase (TCA cycle)

Case Study: Smoke Inhalation

• A 35-year-old male rescued from a fire presents with disorientation, distress, soot in sputum, and breathing difficulty.

• Vitals: BP 90/60, HR 120, RR 30, $O_2$ sat 95%.

• Exam: Burned nasal hair, soot around mouth, burns to face, arms, and back.

• Immediate concerns: Consider cyanide (CN) exposure, often overlooked in smoke inhalation cases which can be produced from the combustion of paper, silk, wool, plastic, and cotton.

Components of ETC

• 5 Complexes:

  1. Enzyme complex I (NADH dehydrogenase)

  2. Enzyme complex II (Succinate dehydrogenase)

  3. Enzyme complex III (Cytochrome reductase)

  4. Enzyme complex IV (Cytochrome oxidase)

  5. ATP Synthase (V)

Two Mobile Carriers

• Coenzyme Q, Cytochrome C.

• Coenzyme Q connects complex I and II.

• Cytochrome C connects complex III and IV.

• Electrons flow from more electronegative to electropositive components.

Coenzyme Q

• It is a lipophilic electron carrier.

• It can accept hydrogen atoms both from complex-1 and 2 FMNH2 and FADH2.

Cytochromes

• Cytochrome C is a mobile component of ETC.

• Conjugated proteins containing heme group having porphyrin ring and iron atom.

• Iron in cytochromes is alternatively oxidized ($Fe^{+3}$) and reduced ($Fe^{+2}$) in contrast to iron of hemoglobin and myoglobin which remains in ($Fe^{+2}$) state.

• Electrons are transported from coenzyme Q to cytochromes in the order of b, c1, c, a, and a3 during electron transport.

ATP Synthase

• It has two functional subunits: Fo (oha) portion embedded in the IMM, and F1 portion protrudes into the mitochondrial matrix.

Chemiosmotic Theory

• Proposed by Peter Mitchell to explain oxidative phosphorylation.

• The transport of electrons from inside to outside of IMM is accompanied by the generation of a proton gradient across the membrane.

• Protons ($H^+$) accumulate outside the membrane, creating an electrochemical potential difference.

Chemiosmotic Theory (cont.)

• The proton pumps (complexes I, III, IV) expel $H^+$ from inside to outside of the membrane.

• This causes high $H^+$ concentration outside.

• This causes $H^+$ to enter into mitochondria through the channels (Fo-F1 complex), this proton influx binds to oxygen of Pi+ADP to form ATP.

Current Concept of ATP Synthesis

• When 1 NADH transfers its electrons to oxygen, 10 protons are pumped out. This accounts for approximately 3 ATP synthesis.

• Oxidation of 1 FADH2 is accompanied by the pumping of 6 protons accounting for 2 molecules of ATP.

• Around 3 protons are required per ATP synthesized.

ATP Production (Recent Findings)

• Peter Hinkle proved that actual energy production is less because there is always leakage of protons.

• According to recent findings:

  • 1 NADH generates - 2.5 ATP

  • 1 FADH2 generates - 1.5 ATP

Oxidative Phosphorylation

• The process of synthesizing ATP from ADP and Pi coupled with the electron transport chain is known as oxidative phosphorylation.

• Complex V of the IMM is the site of oxidative phosphorylation.

Inhibitors of Oxidative Phosphorylation

1. Oligomycin: an antibiotic, prevents the cell from using the established $H^+$-gradient to make ATP.

2. Atractyloside: Plant toxin, inhibits translocase.

Inhibitors of ETC

• The inhibitors bind to one of the components of ETC and block the transport of electrons.

• This causes the accumulation of reduced components before the blockade step and oxidized components after that step.

• The synthesis of ATP is dependent on ETC.

• Hence, all the site-specific inhibitors of ETC also inhibit ATP formation.

Site-Specific Inhibitors

• At complex IV (cytochrome oxidase):

  1. Carbon monoxide: odorless, toxic gas frequently released during incomplete combustion reactions (e.g., in car engines or during coal gasification).

  2. Cyanide: the most potent inhibitor of ETC, an extremely toxic compound; low doses are lethal to humans.

  3. Hydrogen sulfide.

Uncouplers

• These increase the permeability of IMM to protons ($H^+$).

• Thus, an uncoupler allows ETC but blocks the establishment of a proton gradient across the IMM.

• The result is that ATP synthesis does not occur.

• The energy linked with the transport of electrons is dissipated as heat.

• Compounds that can uncouple or delink the electron transport chain from oxidative phosphorylation are known as uncouplers.

Chemical Uncouplers

1. 2,4-dinitrophenol (extensively studied).

2. Dinitrocresol.

3. Pentachlorophenol.

4. Trifluorocarbonyl cyanide phenyl hydrazone (FCCP).

5. Aspirin (high doses).

Physiological Uncouplers

1. Thyroid hormones.

2. Long-chain fatty acids.

3. Unconjugated Bilirubin.

• These act as uncouplers only at high concentrations.

Significance of Uncoupling

• Brown adipose tissue, present in the upper back and neck portions and around the kidney, is rich in mitochondria and carries oxidation uncoupled from phosphorylation.

• This causes the liberation of heat when fat is oxidized in this tissue.

Significance of Uncoupling - Examples

1. Newborn infants (Non-shivering Thermogenesis).

2. Hibernating animals.

Brown Adipose Tissue

• Brown adipose tissue is located in the neck area and is more physiologically active in women than in men.

  • In certain individuals due to presence of this brown adipose tissue it is believed to protect them from becoming obese.

  • The excess calories consumed by these people are burnt and liberated as heat, instead of being stored as fat.

  • However, research, that was published in the New England Journal of Medicine, is showing that brown adipose tissue helps adults burn more calories than white adipose tissue.

Diseases Associated with Oxidative Phosphorylation

• LHON (Leber's Hereditary Optic Neuropathy): A mitochondrially inherited degeneration of retinal ganglion cells and their axons leads to an acute or subacute loss of central vision; this affects predominantly young adult males.

• MELAS (Mitochondrial Encephalopathy, Lactic Acidosis, Stroke-like episodes): A condition that affects many of the body's systems, particularly the brain and nervous system (encephalo-) and muscles (myopathy).

References

• Textbook of Biochemistry - Lippincott's 5th Edition (Recommended book)

• Textbook of Biochemistry - By Vasudevan (Reference Book) 8th edition

• www.google.com/google images