Unit 05 Part 2

Unit 05: Energy

• Course Reference: Molecular Cell Biology (MBoC, 7th Ed), Chapter 14, pages 811-868 (not covering chloroplasts).

  • Author(s) include Alberts B, Johnson A, Lewis J, et al.

  • Published by Garland Science, 2022.

Part 2 Outline of Topics

• Key Topics:

  • Redox Potential: Measure of electron affinities.

  • Electron Transfer:

    • Through electron carriers with transition metals.

    • Through three large H+ pumping complexes.

  • Electron Transfer to O2 at Cytochrome C Oxidase (COX).

  • Respiratory Chain Supercomplexes.

  • Movement of Protons and Energy Capture.

  • ATP Synthase: Described as a nanomachine generating ATP.

  • Transport Mechanisms across the Inner Membrane (IMM).

  • Respiratory Control and its implications.

  • Chemiosmotic Mechanisms in Bacteria.

Redox Potential

• Definition: A measure of electron affinity, associated with oxidation-reduction reactions.

  • NADH/NAD+:

    • Strong electron donor vs. weak electron acceptor.

  • Voltage difference used to calculate redox potential.

    • Example: Electrons move from NADH/NAD+ (low redox potential) to O2/H2O (high redox potential), with a difference of 1140 mV (~26 kcal/mol).

  • Important Concept: Redox potential represented as E’0 (measures ΔG).

  • Note: Understanding of measurement/calculation of redox potentials is not required.

Electron Transfer Through Electron Carriers

• Transition Metals: Fundamental components in respiratory chain complexes, typically transferring 1 electron at a time.

  • Types of Electron Carriers:

    • Cytochromes: Contain heme (porphyrin) and apocytochrome; similar structure to hemoglobin and chlorophyll.

      • Detected by absorbance.

    • Fe/S Clusters: Identified using electron paramagnetic resonance (EPR).

    • Mobile Carriers:

      • Cytochrome c: Heme protein, soluble in aqueous solutions, transfers 1 electron.

      • Ubiquinone (Coenzyme Q): Lipid soluble, can transfer 1-2 electrons.

  • Characteristics: Transition metals exhibit several oxidation states; allow for easy electron transfer.

  • Knowledge of structures of electron carriers and complexes not necessary but their properties should be understood.

Electron Transfer Through H+ Pumping Complexes

• Three Major H+ Pumps:

  • Complex I: Contains ~46 subunits, utilizes flavin and >7 Fe/S, transfers electrons to CoQ.

  • Complex III: Composed of 11 subunits, includes 3 hemes and Fe/S, accepts electrons from CoQ and transfers to cyt c.

  • Complex IV (Cytochrome C Oxidase): Contains 13 subunits and binds molecular O2, crucial for oxygen utilization.

  • Note: Memorization of complex subunit numbers is not required.

Energy Conversion in H+ Pumping

• Energy Conversion Machine: Each complex functions as an energy conversion machine.

  • Demonstrable in vitro with isolated complexes in liposomes using artificial electron acceptors/donors.

  • Mechanisms for H+ pumping vary among complexes but are based on electron and proton transfer principles.

Electron Transfer to O2 at Cytochrome C Oxidase (COX)

• Function of Complex IV:

  • Acts as the terminal electron acceptor, binds molecular O2, essential for aerobic respiration.

  • Responsible for ~90% of O2 uptake in cells.

  • High electron affinity of O2 leads to a significant energy release when reduced to water.

  • Mechanism involves caution: O2 can form superoxide if not fully reduced; COX delays O2 binding until all 4 electrons are accepted.

  • Inhibitors: Cyanide and azide bind to COX at the binuclear O2 binding site.

Structural Components of COX

• Cardiolipin: Tight association with COX, aiding in its functional structure.

  • Cardiolipin is a double phospholipid (with 4 fatty acid tails) known to stabilize the protein complex.

Respiratory Chain Supercomplexes

• CryoEM Studies: Identified supercomplexes that enhance electron transfer efficiency within the crista membrane.

  • Functionality depends on cardiolipin as a hydrophobic medium, important for COX structural integrity.

Movement of Protons and Energy Capture

• Proton Mobility: Protons are highly mobile in water, navigating through hydrogen-bonded networks.

  • H+ Pumping Proteins: Operate as 'proton wires' with polar/ionic side chains allowing rapid movement of H+ (40 times faster than in water).

  • Conformational changes in proteins enable this movement, which is driven by electron transport mechanisms.

ATP Synthase Mechanism

• Mechanism: ATP synthase generates ATP via a favorable concentration ratio of ATP to ADP.

  • Highly negative ΔG from ATP's hydrolysis requires a high ATP concentration to maintain a favorable energy state.

  • Functions in reverse by hydrolyzing ATP to pump H+ back across the membrane, dependent on electrochemical gradient and local conditions.

  • Efficiency of ATP production via oxidative phosphorylation (OXPHOS) far exceeds that of glycolysis.

Cristae Functionality

• Cristae Structure: EM visualization shows ATP synthases arranged like lollipops, aiding in mitochondrial membrane curvature and H+ trapping.

Transport Across the Inner Membrane (IMM)

• Transport Mechanisms: Coupling of favorable H+ flow to necessary metabolite transport (e.g., pyruvate, phosphate, ADP/ATP) via specific transporters.

  • Adenine Nucleotide Translocator (ANT) facilitates ATP out and ADP in.

  • Discovery of mitochondrial PYR transport machinery was recent (2012).

Respiratory Control and Uncoupling Agents

• Uncoupling Agents: Such as DNP disconnect electron transport from ATP production by dissipating the proton-motive force (PMF).

  • This leads to increased O2 uptake, illustrating respiratory control dynamics.

  • Brown fat uses uncoupling proteins (UCP) to manage energy expenditure more as heat rather than ATP.

Chemiosmotic Mechanisms in Bacteria

• Prokaryotic Energy Generation: Bacteria can generate energy similarly, using gradients for propulsion.

  • They can utilize various substrates (sulfur, carbon, nitrogen) with electron carriers in their plasma membranes.

Historical Context of Electron Transfer Chains

• Key milestones in understanding electron transfer chain:

  • 1880s: Cytochromes identified.

  • 1920s/30s: Role in oxygen utilization & ATP discovery.

  • 1961: Chemiosmotic hypothesis introduction.

  • 1980s: Mitochondrial DNA sequencing and structure research, culminating in Nobel Prizes for related discoveries.

Unit 05 – Part 2: Overview

• Discussion Points:

  • Metals involved in respiratory chain protein complexes.

  • Mobile electron carriers within mitochondrial respiratory chains.

  • Unique features of cytochrome oxidase.

  • The concept of respiratory chain supercomplexes and their significance.

  • Basic principles of ATP synthase functionality and its evolutionary distinctions.