Oxidative Phosphorylation Overview

Oxidative Phosphorylation Learning Objectives

• Explain the significance of oxidative phosphorylation
• Understand electron flow and proton pumping in the electron transport chain (ETC) complexes
• Recognize initial and final electron donors/acceptors in each ETC complex
• Explain ATP formation by ATP synthase
• Recognize two different shuttle systems for cytosolic NADH to enter the ETC

Oxidative Phosphorylation Details

• Glycolysis and the TCA cycle produce very little ATP directly (substrate-level phosphorylation)
• Glycolysis: 2 ATPs; TCA cycle: 2 ATPs per glucose
• Remaining energy stored in reduced cofactors (10 NADH and 2 FADH2 per glucose)
• Overall reaction: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (ΔG°' = -2870 kcal/mol)
• Oxidation of reduced co-factors releases energy:
  • NADH + H⁺ + 1/2 O₂ → NAD⁺ + H₂O (ΔG°' = -220 kcal/mol)
  • FADH₂ + 1/2 O₂ → FAD + H₂O (ΔG°' = -182 kcal/mol)
• Oxidation of one glucose mole stores 2564 kJ of energy (about 90% of total under standard conditions)

NAD+/NADH and NADP+/NADPH

• Diffusible 2-electron carriers
• Accept or donate one hydride ion (H⁻) = (1 proton + 2 electrons)

FAD/FADH₂ and FMN/FMNH₂

• Bound to enzymes as prosthetic groups
• Accept or donate one H⁺ + one electron at a time

Oxidation of Fatty Acids and Amino Acids

• Oxidation of fatty acids and some amino acids produce acetyl-CoA and NADH/FADH₂

The Mitochondrion

• Aerobic oxidation of biomolecules occurs within the mitochondrion
• Structure includes outer membrane, inner membrane, cristae, matrix, and intermembrane space

Electron Transport Chain (ETC) in Mitochondria

• Electron flow is favorable from donors with lower reduction potential (more negative E values) to acceptors with higher reduction potential.
• ETC consists of four integral membrane protein complexes (I-IV)

Summary of Electron Flow in Inner Membrane

• Diagram showing the flow of electrons from NADH to oxygen through complexes I, II, III, and IV. The table shows the standard reduction potentials for each complex's substrates.

Standard Reduction Potentials

• Table of standard reduction potentials for various ETC components and complexes

NADH Dehydrogenase (Complex I)

• Cofactors: FMN, iron-sulfur clusters
• Transfers electrons from NADH to ubiquinone (Q)
• Ubiquinol (QH₂) carries electrons through the inner membrane to Complex III
• Overall reaction: NADH + H⁺ + Q → NAD⁺ + QH₂
• Proton pumping occurs, moving protons from the matrix to the intermembrane space

FMN

• Protein-bound co-factor
• Accepts or donates one H⁺ + one electron at a time
• Exists in oxidized (FMN), radical (FMNH•), reduced (FMNH₂) forms

Iron-Sulfur (Fe-S) Clusters

• Single electron carriers
• Each iron (Fe) is coordinated by four sulfur atoms (S)
• Four cysteine residues from a protein contribute to the cluster
• Can have a single Fe coordinated by four cysteine residues

Q (or CoQ)/QH₂

• Diffusible through inner membrane
• Accepts or donates one electron and one proton at a time

Succinate Dehydrogenase (Complex II)

• Only membrane-bound enzyme in the TCA cycle
• Contains internal electron transfer cofactors (FAD and Fe-S centers)
• QH₂ carries electrons through inner membrane to Complex III
• No proton pumping occurs

Cytochrome bc₁ Complex (Complex III)

• Electron carriers: Fe-S centers, cytochrome b and cytochrome c₁
• Transfers electrons from ubiquinol (QH₂) to cytochrome c
• Complex III pumps four protons per two electrons transferred

Cytochromes

• Proteins containing a heme group (which coordinates an Fe(II/III) atom)
  • Heme a: within complex IV
  • Heme b: within complex III
  • Heme c: within complexes III and IV

The Q Cycle in Complex III

• Diagram and description of the Q cycle, including rounds 1 and 2.
• The cycle transports electrons and pumps protons

Cytochrome Oxidase (Complex IV)

• Electron carriers include two copper ions and two heme A groups (cytochrome a proteins).
• Transfers electrons from cytochrome c to oxygen.
• Four cytochromes c (reduced) + O₂ 4 cytochrome c (oxidized) + 2 H₂O
• Incompletely reduced intermediates (e.g. peroxide, hydroxyl radicals) remain bound until complete reduction to water

Electron Transport and Proton Pumps

• Overall schematic showing electron flow through complexes I, II, III, and IV, alongside proton pumping.

Proton Gradient

• Protein gradient across the inner membrane
• Energy released when protons flow down gradient drives ATP synthesis

ATP Synthesis by ATP Synthase

• ATP synthase (FoF₁ complex) is in inner membrane.
• Protons flow through Fo unit down the gradient, driving ATP synthesis by the F₁ unit (ATPase) from ADP and Pi
• For every 2 electrons donated by NADH, 2.5 ATPs are synthesized. For every 2 electrons donated by FADH₂, 1.5 ATPs are synthesized.

Shuttle Systems for NADH Generated in Cytoplasm

• Inner mitochondrial membrane is impermeable to NADH.
  • Glycerol 3-phosphate shuttle: prevalent in skeletal muscle and brain
  • Malate-aspartate shuttle: prevalent in liver, kidney, and heart
• These shuttles ferry reducing equivalents (NADH equivalents) across the membrane.

Net Profit of Aerobic Metabolism

• Summary of ATP yield from complete glucose oxidation (30-32 ATPs) - accounting for different NADH shuttle systems.

This quiz explores the significance and mechanisms of oxidative phosphorylation. It covers the electron transport chain, ATP formation by ATP synthase, and the role of various electron donors and acceptors. Dive into the details of energy production from glucose and learn about the shuttle systems for NADH.