Lecture 3B.1: Respiration: Electron Transport Chain (ETC) & ATP Production Flashcards
Energy Production and Primary Metabolism
Respiration: Electron transport chain
REDOX REACTIONS: low-potential electron donors (more __) are and the resulting electrons are driven through ___ by their affinity for a high-potential __
- electronegative
- electron transport chain
- electron acceptor
This measures the tendency of a molecule to gain or lose electrons in a redox reaction.
Redox Potential (E₀’)
Additional Info:
* A more negative E₀’ means the substance is a good electron donor (it wants to give up electrons).
* A more positive E₀’ means the substance is a good electron acceptor (it wants to gain electrons).
- This tells us whether a reaction releases energy (exergonic, spontaneous, ΔG < 0) or requires energy input (endergonic, non-spontaneous, ΔG > 0).
- It is related to redox reactions because when electrons flow from a donor to an acceptor with a higher redox potential, energy is released.
Gibbs Free Energy (ΔG)
Relationship of Gibb’s Free energy and Redox Potential in an equation
ΔG′=−nFΔE₀’
ΔG’ = Gibbs free energy change (J/mol)
n = Number of electrons transferre
F = Faraday’s constant (96,485 J/V·mol)
ΔE₀’ = Difference in redox potential between donor and acceptor (V)
What determines electron flow in redox reactions?
Electrons move from a donor with a lower redox potential (E₀’) to an acceptor with a higher redox potential.
What type of molecules serve as low-potential electron donors?
E₀’
Molecules with a more negative E₀’, meaning they readily donate electrons.
What type of molecules serve as high-potential electron acceptors?
E₀’
Molecules with a more positive E₀’, meaning they readily accept electrons.
Give examples of common electron acceptors.
In aerobic and anaerobic conditions
- O₂ (aerobic respiration)
- NO₃⁻, SO₄²⁻, Fe³⁺, CO₂ (anaerobic respiration)
Example for Redox Potential:
NADH (E₀’ ≈ -0.32 V) is a good electron __.
O₂ (E₀’ ≈ +0.82 V) is a good electron __.
Electrons naturally flow from __ → __
- donor
- acceptor
- NADH → O₂
Aerobic/Anaerobic respiration
- Aerobic respirations = __ as the terminal electron acceptor
- Anaerobic respirations = electron acceptors other than __
Oxygen
Why does anaerobic respiration yield less energy than aerobic respiration?
In terms of electron acceptors
Because alternative electron acceptors have a lower redox potential than the O₂/H₂O redox couple, resulting in less energy release.
O₂/H₂O, Em, 7 = +815 mV
Key Differences: Anaerobic Respiration vs. Fermentation
Differentiate in terms of:
* Electron Transport Chain (ETC)
* External Electron Accepto
* Proton Motive Force (PMF)
* ATP Production
* Energy Yield
Anaerobic Respiration:
* YES - Electron Transport Chain (ETC)
* YES (NO₃⁻, SO₄²⁻, etc.) - External Electron Acceptor
* YES - Proton Motive Force (PMF)
* Oxidative phosphorylation (diff e- acceptor) - ATP Production
* Higher (but less than aerobic respiration) - Energy Yield
Fermentation:
* NO - Electron Transport Chain (ETC)
* NO - External Electron Acceptor
* NO - Proton Motive Force (PMF)
* Substrate-level phosphorylation - ATP Production
* Lower - Energy Yield
What determines the amount of ATP produced in an ETC?
The difference in __ between the electron donor and the final electron acceptor. A __ difference generates __ ATP.
- redox potential (ΔE₀’)
- larger
- more
Redox Midpoint Potentials (E₀’, mV) of Key Electron Donors & Acceptors
Determine each role of the redox pair given in Electron Transport Chain (ETC):
1. H₂ / H⁺ (-420 mV)
2. Formate (HCO₂⁻) / CO₂ (-420 mV)
3. NADH / NAD⁺ (-320 mV)
4. FADH₂ / FAD (+31 mV)
5. Succinate / Fumarate (+31 mV)
6. O₂ / H₂O (+815 mV)
7. NO / N₂O (+1,300 mV)
- Strong electron donor
- Electron donor
- Major electron donor
- Intermediate electron carrier
- Intermediate in the ETC
- Final electron acceptor (Aerobic Respiration)
- Alternative acceptor (Anaerobic Respiration)
How much energy can be harvested in bacterial ETC?
Some bacterial ETCs span more than __ (e.g., from NADH to O₂), creating a large __ for __.
- 1V
- proton motive force (PMF)
- ATP synthesis
refers to the difference in redox potential between the initial electron donor (e.g., NADH) and the final electron acceptor (O₂).
Redox span
Aerobic Bacterial Electron Transport Chain:
What is the redox span in aerobic bacterial ETCs?
eV, kcal mol⁻¹, kJ mol⁻¹
More than 1 eV, which corresponds to about 23 kcal mol⁻¹ (96 kJ mol⁻¹).
Aerobic Bacterial Electron Transport Chain:
What is the thermodynamic cost of transporting one proton?
Around 4.6 kcal mol⁻¹ (19.2 kJ mol⁻¹) when PMF is high (~200 mV).
Aerobic Bacterial Electron Transport Chain:
How many protons are pumped per electron in a ~1V redox span?
Up to five protons per electron transferred from donor (e.g., NADH) to acceptor (O₂).
Why does the ETC have a stepwise drop in potential?
- Instead of a single, large energy drop, electron transfer occurs in a stepwise manner through multiple redox centers (e.g., cytochromes, iron-sulfur clusters, quinones) in the ETC.
- This prevents energy from being lost as heat and allows respiratory enzymes to operate at high thermodynamic efficiency
Additional info:
- The stepwise electron flow helps generate and sustain the PMF, ultimately driving ATP production via oxidative phosphorylation.
How does the bacterial cell membrane contribute to respiration?
It serves as an integral part of the __, isolating and storing the __.
- proton-motive machinery
- proton motive force (PMF).
Aerobic Bacterial Electron Transport Chains:
__ help establish __ along the membrane surface and aid in proton uptake in respiratory complexes.
- Lipid headgroups
- proton conduction pathways
Aerobic Bacterial Electron Transport Chains:
Are the exact mechanisms of lipid-mediated proton conduction known?
No, the precise principles are still not fully understood.
Aerobic Bacterial Electron Transport Chains:
It is an assembly of multiple respiratory enzyme complexes that may improve efficiency and regulate activity.
respiratory supercomplex
Aerobic Bacterial Electron Transport Chains:
How do membrane lipids contribute to respiratory supercomplexes?
They help stabilize these assemblies and may regulate their function.
Aerobic Bacterial Electron Transport Chains:
What types of lipids are found in bacterial cell membranes? (3)
- Zwitterionic (an inner salt or dipolar ion, is a molecule that contains both a positively and a negatively charged functional group, resulting in an overall neutral charge.)
- anionic
- highly glycosylated lipids
Aerobic Bacterial Electron Transport Chains:
What are the major phospholipids in E. coli membranes?
Include its percentage
- Phosphatidylethanolamine (PE) → 75%
- Phosphatidylglycerol (PG) → 20%
- Cardiolipin (CL) → 5%
Aerobic Bacterial Electron Transport Chains:
What additional lipids can be present in bacterial membranes (actinobacteria)? (2)
- Phosphatidylinositol (PI)
- Phosphatidylinositol Mannosides (PIMs).
The citric acid cycle produces CO₂ and what?
Reduced redox coenzymes (NADH, FADH₂)
What is the main function of the electron transport chain?
Energy conservation
- NADH dehydrogenases bind NADH and transfer what?
- What do NADH dehydrogenases regenerate?
- 2e⁻ + 2H⁺
- NAD⁺
What is the prosthetic group in flavoproteins? (2)
class of proteins; contain flavin group;derivative of riboflavin,vit B2
flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD)
Flavoproteins accept __ but donate only __.
- 2e⁻ + 2H⁺
- electrons
What prosthetic group do cytochromes contain?
Heme
What is the redox reaction in cytochromes?
Fe²⁺ ⇌ Fe³⁺
Name three types of cytochromes. (3)
Cytochromes a, b, c
What complex do some cytochromes form?
Give three (3) examples
-
Cytochrome bc₁ complex (Complex III)
a.k.a. ubiquinol-cytochrome c reductase, facilitates electron transfer from ubiquinol (coenzyme Q) to cytochrome c, a crucial step in the ETC. -
cytochrome c oxidase (aa3)
crucial part of the electron transport chain in many bacteria and eukaryotes, acting as the terminal electron acceptor, reducing oxygen to water. -
cytochrome b6f
found in the thylakoid membrane of chloroplasts, involved in the light-dependent reactions of photosynthesis, specifically in transferring electrons from plastoquinone to plastocyanin.
What do iron-sulfur proteins contain instead of heme?
Fe-S clusters
Example of an iron-sulfur protein?
Give one (1) example
Ferredoxin
Additional info: iron-sulfur proteins that act as electron carriers in various metabolic processes, including photosynthesis, nitrogen fixation, and other redox reactions, found in bacteria, plants, and animals.
a class of cyclic organic compounds containing two carbonyl groups (C=O) within a six-membered unsaturated ring, acting as biological pigments and playing roles in electron transport, photosynthesis, and other processes.
quinone
Are quinones proteins?
No
What do quinones transfer?
Electrons (2e⁻)
Link Fe-S proteins to cytochromes
Quinone
Two common types of quinones? (2)
-
Ubiquinone
a lipid-soluble molecule involved in mitochondrial energy production -
Menaquinone
also known as vitamin K2, is an essential nutrient and electron carrier, particularly important in bacterial electron transport chains
Electron Transport Chain Order
- What is the first carrier in the electron transport chain?
- What follows (1) in the ETC?
- What follows (2)?
- What follows (3)?
- What are the final carriers in the ETC?
- NADH dehydrogenase
- Flavoproteins
- iron-sulfur proteins
- Quinones
- Cytochromes
What reaction establishes this energized state?
Electron transport chain (ETC) reactions
What is separated across the membrane in ETC?
Protons (H⁺) and electrons (e⁻)
How are ETC components arranged?
In terms of reduction potential
Reduction potential - a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respectively.
By increasing reduction potential
Explanation:
* Electron Flow: Electrons move from molecules with a lower reduction potential (higher tendency to donate electrons) to those with a higher reduction potential (higher tendency to accept electrons).
* Energy Release: This ordered arrangement ensures that energy is released in controlled steps, which is harnessed to pump protons across the inner mitochondrial membrane.
* Proton Gradient Formation: The movement of electrons powers the active transport of protons (H⁺), creating an electrochemical gradient used for ATP synthesis.
What is the main force driving ATP synthesis?
Proton Motive Force (PMF)
Explanation:
* Proton (H⁺) Gradient: It refers to the electrochemical gradient of protons across a membrane (e.g., mitochondrial, bacterial, or thylakoid membrane).
* Motive (Driving Force): The gradient stores potential energy, driving protons back across the membrane.
* Force for Cellular Processes: PMF powers ATP synthesis, flagellar movement in bacteria, and active transport.
* Combination of Gradients: It consists of a chemical gradient (proton concentration difference) and an electrical gradient (membrane potential).
What causes charge separation in PMF?
H⁺ extrusion and OH⁻ accumulation
Explanation:
* H⁺ Extrusion: Protons (H⁺) are actively pumped out of the cell or organelle, creating a positive charge outside.
* OH⁻ Accumulation: As H⁺ leaves, hydroxide ions (OH⁻) remain inside, making the interior more negative.
* This separation generates both a chemical gradient (proton concentration difference) and an electrical gradient (membrane potential), which together drive processes like ATP synthesis and transport.
What cellular processes use PMF? (3)
- ATP biosynthesis
- nutrient transport
- flagellar movement
What alternates in the ETC?
Electron-only and electron-plus-proton carriers
Explanation:
* Electron-only carriers (e.g., cytochromes, iron-sulfur (Fe-S) proteins) transfer only electrons.
* Electron-plus-proton carriers (e.g., flavoproteins, quinones) transfer both electrons and protons, contributing to proton extrusion and the formation of the proton motive force (PMF).
Generation of PMF: Complexes I & II
What is Complex I also called?
NADH: Quinone Oxidoreductase
Generation of PMF: Complexes I & II
What does FMNH₂ donate in Complex I?
2e⁻ to Fe/S proteins
Generation of PMF: Complexes I & II
How many protons does Complex I extrude?
4H⁺
Generation of PMF: Complexes I & II
What is the function of ubiquinone (Q)?
Takes up 2H⁺ from cytoplasm
Generation of PMF: Complexes I & II
What is Complex II also called?
Succinate Dehydrogenase Complex
Generation of PMF: Complexes I & II
What electron donor does Complex II use?
FADH₂
Generation of PMF: Complexes I & II
How does Complex II compare to Complex I in proton pumping?
Pumps 4H⁺ fewer
Complexes III & IV
What is Complex III called?
Cytochrome bc₁ complex
Complexes III & IV
What molecule transfers electrons to cytochrome c?
QH₂ (Ubiquinol)
Complex III & IV
What cycle does Complex III engage in?
Q cycle
occurring in Complex III (cytochrome c reductase), where the electron carrier Coenzyme Q (CoQ) undergoes sequential oxidation and reduction, facilitating proton translocation and contributing to the proton gradient used for ATP synthesis
Complex III & IV
How many protons does Complex III pump per 2e⁻?
4H⁺
Complex III & IV
What is Complex IV called?
Cytochrome oxidase
Complex III & IV
What does Complex IV reduce?
O₂ to H₂O
Complex III & IV
What is the role of Complex IV in PMF?
Pumps additional protons
What are the two components of ATPase? (2)
- F₁ (cytoplasmic)
- F₀ (membrane)
ATP Synthesis via ATP Synthase (ATPase)
- What does F₀ do?
- What does F₁ do?
- Translocates protons
- Catalyzes ATP synthesis
What type of phosphorylation does the ETC drive?
Oxidative phosphorylation
What type of ATP synthesis is independent of ETC?
Substrate-level phosphorylation
Can ATPase run in reverse?
Yes
- How many H⁺ are required per ATP molecule?
- How many ATP are produced per 2e⁻?
- 3-4 H⁺
- ~3 ATP
What type of respiration uses alternative electron acceptors instead of O₂?
Anaerobic respiration
What are two examples of electron acceptors in anaerobic respiration? (2)
- Nitrate (NO₃⁻)
- sulfate (SO₄²⁻)
What is the main electron donor in methanogenesis?
H₂
What metabolic process uses inorganic compounds as electron donors?
Chemolithotrophy
What are two examples of electron donors in chemolithotrophy? (2)
- hydrogen sulfide (H₂S)
- ammonium ion (NH₄⁺)
What is the electron donor in oxygenic photosynthesis?
H₂O
What is the carbon source for chemoautotrophs?
CO₂
What alternative to PMF do some bacteria use for energy conservation?
Sodium motive force
What enzyme generates a sodium-motive force from NADH?
Na+-NQR (sodium-translocating NADH:ubiquinone oxidoreductase)
Na⁺-NQR (sodium-translocating NADH:ubiquinone oxidoreductase) is an enzyme that generates a sodium-motive force (Na⁺ gradient) by transferring electrons from NADH to ubiquinone while simultaneously pumping Na⁺ ions across the membrane. This enzyme is found in some bacteria and helps drive cellular processes like ATP synthesis and solute transport by using the generated Na⁺ gradient.
What enzyme contributes to sodium translocation in some bacteria?
Oxaloacetate decarboxylase
Oxaloacetate decarboxylase is an enzyme that contributes to sodium translocation in some bacteria by catalyzing the decarboxylation of oxaloacetate to pyruvate. This reaction is coupled with the active transport of Na⁺ ions across the membrane, generating a sodium-motive force. This Na⁺ gradient can then be used for ATP synthesis, solute transport, or flagellar movement in bacteria.
What transporter switches between proton and sodium gradients?
Na+/H+ antiporter
What amino acid residues participate in proton conduction?
Give any of the six (6)
- Tyrosine (Tyr)
- Glutamine (Gln)
- Aspartic acid (Asp)
- Histidine (His)
- Serine (Ser)
- Threonine (Thr)
What metal centers are key to electron transport?
Give four (4)
- Fe-S clusters
- flavins
- quinones
- haem groups
What distance typically separates electron donors and acceptors?
5-14 Å (Angstroms)
Measures ATP yield per electron transfer
phosphate/oxygen (P/O) ratio
What two-electron carriers donate electrons to the ETC? (2)
- NADH
- FADH₂
What enzyme transfers electrons from hydrogen?
Hydrogenase
What enzyme transfers electrons from succinate?
Complex II (Succinate dehydrogenase)
What enzyme oxidizes methanol for electron transfer?
Methanol dehydrogenase
What enzyme oxidizes methylamine for electron transfer?
Methylamine dehydrogenase
What enzyme oxidizes formate for electron transfer?
Formate dehydrogenase
What is the reduced form of quinone?
Quinol (QH₂)
What determines bacterial quinone composition?
O₂ availability
Under anaerobic or low-oxygen conditions, bacteria shift to using menaquinone (MK) or demethylmenaquinone (DMK), which are better suited for alternative electron acceptors like nitrate, fumarate, or sulfate.
Electron Transport in Paracoccus denitrificans
What molecule reduces ubiquinone in P. denitrificans? (2)
- NADH
- Succinate
In Paracoccus denitrificans, ubiquinone is reduced by:
- NADH, through NADH:ubiquinone oxidoreductase (Complex I)
- Succinate, through succinate dehydrogenase (Complex II)
Both pathways transfer electrons to ubiquinone (UQ), which then passes them through the electron transport chain for energy production, adapting to aerobic and anaerobic conditions.
Electron Transport in Paracoccus denitrificans
What complex transfers electrons from the quinone pool to cytochrome c?
Complex III (Cytochrome bc1)
- Ubiquinol (QH₂) donates electrons to the cytochrome bc₁ complex.
- Electrons follow the Q-cycle, where one electron moves to cytochrome c via the Rieske iron-sulfur protein and cytochrome c₁, while the other cycles back to regenerate ubiquinone.
- This process also pumps protons across the membrane, contributing to the proton motive force for ATP synthesis.
Electron Transport in Paracoccus denitrificans
What is the function of Complex IV in P. denitrificans? (2)
- Reduces O₂ to H₂O
- pumps protons
- Reduces O₂ to H₂O → It receives electrons from cytochrome c and transfers them to molecular oxygen (O₂), reducing it to water (H₂O).
- Pumps protons across the membrane → This contributes to the proton motive force, which drives ATP synthesis.
Electron Transport in Paracoccus denitrificans
What enzyme reduces NO to N₂O in P. denitrificans?
Nitric oxide reductase (NOR)
- NOR catalyzes the reaction:
2 NO + 2 e⁻ + 2 H⁺ → N₂O + H₂O - This step is crucial in the denitrification pathway, allowing P. denitrificans to survive under low-oxygen conditions by using NO as an alternative electron acceptor.
Electron Transport in Escherichia coli
Which complex is missing in E. coli compared to mitochondria?
Complex III
Why?
* Instead of Complex III, E. coli uses alternative electron transport pathways.
* Electrons from the quinone pool are transferred directly to terminal oxidases (e.g., cytochrome bo₃ or bd oxidases) without a cytochrome bc₁ complex.
In the electron transport chain, quinones act as a “__” or reservoir of electrons, receiving electrons from various dehydrogenases (enzymes that remove electrons) and then donating them to other complexes further down the chain.
- pool
Quinone Pool
Electron Transport in Escherichia coli
What quinone pool does E. coli use?
Q8 quinone pool
Enzyme Families Involved in NADH to Quinone Electron Transfer
Which NADH dehydrogenase in bacteria pumps protons? How many protons does this pump per electron?
- NDH-1
- 2H+/e−
NADH-quinone oxidoreductase (complex I/NDH-1)
Enzyme Families Involved in NADH to Quinone Electron Transfer
Which NADH dehydrogenase does not pump protons?
NDH-2
type II NADH:quinone oxidoreductase
Enzyme Families Involved in NADH to Quinone Electron Transfer
Which enzyme translocates sodium instead of protons?
Na+-NQR
sodium-translocating NADH:ubiquinone oxidoreductase
Enzyme Families Involved in NADH to Quinone Electron Transfer
Which NADH dehydrogenases are expressed in E. coli? (2)
- NDH-1
- NDH-2
Redox Loop of Complex III
What is another name for Complex III in prokaryotes?
Cytochrome bc1
Redox Loop of Complex III
What is the function of Complex III?
Electron transfer between quinol and cytochrome c
What is the main function of bacterial ETC? (2)
- Extract high-energy electrons from chemical substrates
- generate a proton-motive force (PMF) or sodium-motive force (SMF)
What is the typical voltage range of the PMF?
100-180 mV
What else does the PMF drive?
__ of solutes against __ (via __)
- Active transport
- concentration gradients
- secondary active transporters
What two components make up the PMF? (2)
- Electrical gradient
- pH gradient (chemiosmotic force)
How does bacterial ETC differ from mitochondrial ETC?
It is branched rather than linear
Why is a branched ETC beneficial?
Allows flexibility in electron transfer routes depending on growth conditions
What is the main driver of PMF in neutral pH bacteria like Escherichia coli?
Electrical membrane potential
In E. coli (which lives at neutral pH), the proton motive force (PMF) is mainly driven by electrical membrane potential (ΔΨ) rather than a proton gradient (ΔpH).
Since the inside and outside of the cell have similar pH, there isn’t a strong proton concentration difference. Instead, E. coli maintains a charge difference across the membrane, where the inside is more negative and the outside more positive.
This electrical potential (ΔΨ) pulls protons back into the cell, fueling ATP synthesis, transport, and motility.
What dominates the PMF in pathogens like Helicobacter pylori and Salmonella enterica?
A pH gradient (acidic exterior, alkaline cytoplasm)
Are bacterial ETC components conserved across domains?
Yes, but their exact mechanisms are still not fully understood
