Oxidative Phosphorylation Flashcards

``Oxidative Phosphorylation

Learning Outcomes

• Describe the chemical nature and arrangement of electron carriers in the mitochondrial inner membrane.

• Explain how electron carriers are organized in complexes that conserve energy from NADH and FADH2 reoxidation by pumping protons to create a proton and electrochemical gradient.

• Describe how the proton gradient's energy synthesizes ATP from ADP via the F0, F1-ATP synthase.

• Indicate how electron transport rate is controlled by ADP and ATP levels (respiratory control or tightly-coupled oxidative phosphorylation).

• Explain the effects of inhibitors and uncouplers on the electron transport chain.

Electron Transport Chain Overview

• The electron transport chain (ETC) is also known as OXPHOS (oxidative phosphorylation).

Initial Steps Leading to Electron Transport Chain

• Several pathways converge to produce reduced electron carriers:

  • Glucose → pyruvate → acetyl-CoA → enters TCA cycle → TCA cycle produces NADH + H+ and FADH2

Hydrogen Atom Transfer

• Transfer of a hydrogen atom involves transferring a proton (H+) and an electron (e-).

• When two H atoms are transferred to NAD^+, one proton is released to the solution.

  • NAD^+ + 2H \rightarrow NADH + H^+ (one H^+ released to solution)

• NADH + H+ and FADH2 are considered electron carriers.

Reduction and Oxidation

• Reduction involves gain of electrons; oxidation involves loss of electrons.

• NAD^+ is reduced to NADH + H^+.

Structure of Mitochondria

• Mitochondria contain their own DNA and ribosomes.

Electron Transport Chain (ETC) Function

• Electrons are transferred from NADH + H^+ and FADH2 along protein complexes, reducing O2 to form H_2O.

  • \frac{1}{2} O2 + 2H^+ \rightarrow H2O

Components of the Electron Transport Chain

• Proteins/protein complexes are embedded in the inner mitochondrial membrane.

  • Contain prosthetic groups: FMN, FAD, haem, Fe-S clusters.

• Ubiquinone (Q) is a non-protein-bound mobile element.

• Prosthetic groups are non-amino-acid structures tightly bound to proteins.

• Complex V synthesizes ATP.

Flavins (FMN and FAD)

• Act as hydrogen and electron acceptors.

• FMN = Flavin mononucleotide

• FAD = Flavin adenine dinucleotide

Cytochromes

• Proteins containing haem.

• Carry electrons on the Fe^{2+} ion.

• Fe^{2+} \rightleftharpoons Fe^{3+} + e^-

Iron-Sulphur Proteins (Fe-S Proteins)

• Contain Fe-S clusters anchored via cysteine residues.

• Carry electrons on the Fe^{2+} ions.

• Fe^{2+} \rightleftharpoons Fe^{3+} + e^-

Ubiquinone (Q)

• Also known as Coenzyme Q (CoQ).

• Mobile carrier, not bound to a protein.

• Accepts hydrogen atoms (and thus carries electrons).

• Transfers electrons from Complex I and II to Complex III.

Redox Potential (E'0)

• Measure of electron affinity.

• More negative E'0, more likely to pass on electrons (reduce another molecule).

• More positive E'0, more readily a molecule will accept electrons (be reduced).

Electron Flow in ETC

• Electrons pass to acceptors of progressively higher redox potential (E'0).

• Example sequence: Complex I → Q → Complex III → Complex IV → O_2

• Alternative inputs: Complex II → Q

Stepwise Energy Release

• Stepwise breakdown releases energy in useable small 'packages'.

• Direct burning of sugar releases all energy as heat.

• Stepwise oxidation stores some free energy in activated carrier molecules.

Energy Release and Redox Potential

• Electrons are passed stepwise along the electron transport chain, releasing energy.

• The redox potential increases along the chain from NADH to oxygen, facilitating electron transfer.

Proton Pumping

• Energy from electron transport pumps protons (H^+) from the matrix to the intermembrane space.

• This creates an electrochemical gradient (higher H^+ concentration in intermembrane space).

Complex II

• Complex II does not pump protons.

• The difference in redox potential between Complex II and Q is insufficient for proton transfer.

Mechanisms for Proton Transfer

• Each complex uses a different mechanism for transferring protons.

Electrochemical Gradient

• Proton transfer creates a pH gradient and a voltage gradient (membrane potential).

ATP Synthase

• Proton-motive force harnessed to synthesize ATP.

• ADP + P_i \rightarrow ATP

ATP Synthase Mechanism

• Protons flow through ATP synthase, causing rotation.

• F0 subunit: embedded in the inner membrane; rotates as protons flow through.

• F1 subunit: site of ATP synthesis.

• Stator: prevents rotation of F1 subunit.

• ATP synthase is also known as Complex V.

ATP Synthesis from ADP and Pi

• Stalk transmits energy from F0 to F1, causing conformational changes.

• Three sites in F1 progress sequentially through O, L, and T states, synthesizing ATP.

Chemiosmotic Coupling

• Coupling of ATP synthesis to electron transport is called oxidative phosphorylation (OXPHOS).

• Approximately 3H+ are required for each ATP synthesized.

Oxidative Phosphorylation Defined

• Oxidative: components of the ETC are oxidized as they pass electrons.

• Phosphorylation: ADP is phosphorylated using Pi to create ATP.

ATP Yields

• 1 NADH reoxidized yields ~2.5 ATP.

• 1 FADH2 reoxidized yields ~1.5 ATP.

• Respiratory control: Increased [ADP] increases the rate of O2 uptake; increased [ATP] decreases O2 uptake.

Inhibitors of Oxidative Phosphorylation

• Examples: CN^-, CO, rotenone.

• Inhibitors reduce electron transfer, reducing ATP synthesis.

• Can lead to damaging radical production (e.g., superoxide) if only one electron is passed to oxygen.

Uncouplers of Oxidative Phosphorylation

• Examples: dinitrophenol (DNP), thermogenin in brown adipose tissue.

• Uncoupling agents create channels in the inner mitochondrial membrane, making it permeable to protons.

• Proton gradient is dissipated without ATP synthesis; energy released as heat.

DNP

• DNP has been marketed as a diet pill but is dangerous and can cause death.

ATP Production Balance Sheet

• Glycolysis: 2 ATP

• TCA cycle: 2 ATP (via GTP)

• 2 NADH from pyruvate dehydrogenase: 5 ATP

• 6 NADH from TCA cycle: 15 ATP

• 2 FADH2 from TCA cycle: 3 ATP

• 2 NADH from glycolysis: 3 or 5 ATP (depends on shuttle)

• Total: 30 or 32 ATP per glucose molecule