Electron Transport Chain & Krebs Cycle

• The electron transport chain (ETC) is also known as the respiratory chain.

Phosphorylation Types

• At the substrate level, 2 ATP molecules are formed.
• At the oxidative level, 22 ATP molecules are formed.
• In oxidative phosphorylation, a phosphate group is added and electrons are transferred via reduced coenzymes to oxygen in the ETC.
• The net ATP produced by the Krebs cycle is 24 ATP.

Krebs Cycle

• The Krebs cycle ends with oxaloacetate.
• At the end of Krebs cycle, 2 ATP, 6 NADH, and 2 FADH2 are produced.
• NADH yields 3 ATP, and FADH2 yields 2 ATP.
• The net ATP production is 24.

Objectives

• Energy-rich molecules like glucose are metabolized through oxidation-reduction, yielding CO2 and water.
• The electron transport chain releases free energy for ATP synthesis and heat production.
• Reactions in the mitochondria are coupled to oxidative phosphorylation.

Electron Transport Chain

• The ETC is a system of electron transport that uses respiratory O2 to produce ATP (energy).
• Its purpose is ATP (energy) production.
• It resides in the inner mitochondrial membrane.
• It is key final common pathway of metabolism.
• Electrons from food metabolism are transported to O2, the terminal final acceptor of electrons.
• It utilizes the maximum amount of body's oxygen.

Metabolic Breakdown of Energy Conclusion

• Extra info: Carbohydrates transform via oxidation reactions into CO2 and water, donating electrons to NAD + FAD resulting in NADH + FADH "energy – rich reduced form.“
• These coenzymes donate a pair of electrons to specialized sets in the ETC where electrons passing down the ETC lose free energy which is used to move protons from the inner mitrochondrial membrane.
• ATP is then derived from ADP; called oxidative phosphorylation.

Mitochondria

• Mitochondria structures include the outer membrane, inner membrane, cristae, inter-membrane space, and matrix.

Mitochondria Structures

• The outer membrane contains (porin proteins), making it permeable to most ions & small molecules.
• The inter-membrane space sits between the outer and inner membranes.
• The inner membrane constitutes a specialized structure and is rich in proteins, impermeable to most small ions & molecules.
• The cristae serves to greatly increase the surface area of the inner membrane.
• The matrix comprises a gel-like solution inside the mitochondria.
• It contains TCA (tricarboxylic acid) cycle enzymes, fatty acid oxidation enzymes & MtDNA (mitochondria DNA) & mitochondrial ribosomes.

ETC Components Locations

• All elements reside in the inner mitochondrial membrane (IMM), except cytochrome C, which resides in the inter-membrane space.
• Complexes: Complex I, Complex II, Complex III, Complex IV, Complex V.
• Mobile electron carriers include CoQ and cytochrome c.

Organization of Electron Transport Chain

• Each complex receives or donates electrons to mobile carriers.
• Carriers receive electrons from donors, then donate to the next carrier in chain.
• Electrons combine with oxygen and protons to form water.
• Oxygen serves as the final acceptor as a part of the respiratory chain.

ETC Complexes

• Complex I - NADH Dehydrogenase: collects the pair of electrons from NADH and passes them to CoQ.
• Complex I Enzyme transfers 2 electrons from NADH to FMN and reduce it to FMNH2, then transfers electrons to Coenzyme Q which will get converted into CoQH2.
• Complex II -Succinate dehydrogenase: key part of *TCA cycle.
• Complex II transfers electrons to CoQ.
• It is also an enzyme that oxidizes succinate to form fumarate and the 2 electrons formed move from the coenzyme and FADH2 to the iron-sulker protein and Coenzyme Q.

Coenzyme Q (CoQ)

• Also called ubiquinone due to its ubiquitous nature in biological systems.
• It is a non-protein member of the ETC as it transports electrons.
• It is lipid-soluble and mobile.
• Facts: CoQ is a mobile electron carrier and can accept hydrogen atoms from NADH dehydrogenase (Complex I), from succinate dehydrogenase (Complex II), and from other mitochondrial dehydrogenases.
• It transfers electrons to Complex III (cytochrome bc₁) and links flavoprotein dehydrogenases to the cytochrome.

Cytochromes and Complexes

• Cytochromes are carrier molecules in plant and animal cell mitochondria, participating in the stepwise transfer of electrons in oxidation reactions.
• Cytochromes = protein + Heme group.
• When cytochromes receive an electron, Fe3+ is converted to Fe2+ by a reduction reaction.
• When Fe2+ donates an electron to the next carrier, it reoxidizes to Fe3+.
• Cytochromes contain either a Heme group (porphyrin ring + iron in Fe3+ or Ferric state) or hemoglobin (porphyrin ring + iron in Fe2+ or Ferrous form).
• Complex III: cytochrome bc1
• Complex IV: cytochrome a + a3 (also known as cytochrome oxidase)
• Electron flow: Coenzyme Q → complex III → cytochromes C → complex IV

Inhibitors of ETC

• These inhibit the passage of electrons by binding to a component of the chain, which blocks the oxidation or reduction reaction meaning inhibit ATP synthesis.
• In such a case, all electron carriers before the block are fully reduced and all electron carriers after the block are oxidized
• Rotenone: Blocks the ETC between NADH dehydrogenase (Complex I) and CoQ.
• Antimycin A: Blocks the ETC between cytochrome bc1 and cytochrome c.
• CN- "cyanide" /CO: Blocks the ETC between cytochrome a+a3 and O2.
• Hemoglobin has a higher affinity for CO than O2, causing hypoxia when inhaled.
• It also inhibits ETC by blocking Cytochrome a+a3.

ETC and ATP Synthesis

• Electron transfer drives protons out of the matrix to the intermembrane space resulting in a lower PH outside due to increased H+.
• Complexes I, III and IV act as proton pumps.
• This creates a proton gradient across the IMM for ATP synthesis.

ATP Synthase

• Coupling of ETC (green arrow) and ATP synthesis.
• Complex II has no function in pumping H+.
• ATP synthase (Complex V) synthesizes ATP with energy from the proton gradient generated by the electron transport chain.
• ATP synthase is composed of two domains: Fo and F1.
• Fo means membrane spanning domain (it can be inhibited by oligomycin).
• F1 refers to an extra-membranous domain.

Proton Transport

• When 1 H+ ion enters the ATP synthase another H+ ion leaves the ATP synthase moving to the matrix.
• This rotation of the upper part of ATP synthase (Fo) causes conformational changes in the extra-membranous F1 domain.
• This conformational change allows it to bind ADP + Pi and releases ATP.
• When 3 H+ ions enter the matrix space, the energy enables the ATP synthase to synthesize 1 ATP from ADP+P₁.
• When both sides matrix space+ intermembrane space contain the same amount of H+ ions there will be no gradient making no energy for ATP synthesizing.

Mitochondrial Gradient

• Starting with the ETC, 2 electrons are transported from NADH to complex 1, pumping 2 H+ ions to the intermembrane space.
• Next, 2 electrons are transported by ubiquinone from complex 1 to complex 3.
• Cytochrome c takes 1 electron from complex 3 and moves it to complex 4, also coupled with H+ pumping.
• In complex 4, 4 electrons interact with an O2 molecule + 8 H+ ions, forming 2 H₂O molecules and 4 H+ ions are pumped to the intermembrane space
• Pumping of H+ ions creates gradients of electrical charge and pH (more positive charges and lower pH on the outside).

ATP Synthesis Energetics

• Energy required for phosphorylation of ADP to ATP = 7.3kcal/mol
• Energy produced from NADH to 02 pair transport = 52.58 kcal
• Excess heat energy is released or used in reactions.
• NADH ratios are 3:1 (3 ATP per reduced oxygen atom).
• FADH2 ratios are 2:1 (2 ATP per reduced oxygen atom).

Inhibitors of ATP Synthesis

• Remember that Hb has higher affinity for CO than O2, therefore it causes hypoxia when inhaled.
• It inhibits ETC in the Cyto a+a3.
• Oligomycin binds to Fo ATP Synthase domains and closes H+ channels
• Makes a channel for re-entry of protons to the matrix without any ATP production.
• Energy then released as heat!

ATP Synthase

• Coupling of electron transport (green arrow) and ATP synthesis.
• Complex II has no function in pumping H+.

The End

• The test is over