The Electron Transport Chain

Substrate Level Phosphorylation

Direct transfer of phosphate from a high energy substrate to ADP

• does not require oxygen

Examples of substrate level phosphorylation in metabolism

• Glycolysis: 1,3-BPG → 3-PG (1ATP)

• Glycolysis: PEP → Pyruvate (1ATP)

• TCA Cycle: Succinyl-CoA → Succinate (1 GTP)

Oxidative Phosphorylation

ATP synthesis by electron transport and the proton gradient

• occurs in the inner mitochondrial matrix

What is the process of oxidative phosphorylation?

1. Electrons from NADH/FADH flow through ETC complexes₂

2. Energy released pumps H across membrane⁺

3. H gradient (proton-motive force) drives ATP synthase⁺

4. ATP synthesized as H flows back into matrix

• Yield: 2.5 ATP per NADH , 1.5 ATP per FADH₂

Mitochondrial Structure

inner and outer membranes, cristae (folds), matrix (low [H+]), and intermembrane space (high [H+])

• Inner mitochondrial membrane and cristae: location of ETC complexes and ATP synthase

Parts of the Mitochondria

Outer Membrane:

• permeable to small molecules

• contains porins

Intermembrane space

• High [H+] → acidic

• similar in composition to cytosol

• contains cytochrome c

Matrix

• Low [H+] → alkaline

• contains TCA cycle enzymes

• contains PDH complex

• contains mtDNA and ribosomes

Inner Membrane

• Impermeable to most molecules

• contains ETC complexes

• Contains ATP synthase

• Highly folded into Cristae (high surface area)

Key Concept: The inner membrane's impermeability is ESSENTIAL for maintaining the proton gradient!

ETC Four Complexes

• Complex I: NADH-ubiquinone oxidoreductase (NADH dehydrogenase)

  • Transfers electrons from NADH → CoQ (ubiquinone)

  • Pumps 4 H+

• Complex II: Succinate dehydrogenase

  • Transfers electrons from FADH → CoQ

  • No H+ pumping

• Complex III: Ubiquinone-cytochrome c oxidoreductase

  • Transfers electrons from CoQ → Cytochrome c

  • pumps 4H+

• Complex IV: Cytochrome c oxidase

  • Transfers electrons from Cytochrome c → O₂ (reduces O₂ to H₂O)

  • Pumps 2 H+

Mobile Carriers of the ETC

• Ubiquinone (CoQ) - lipid-soluble, in membrane

• Cytochrome c - water-soluble, in IMS

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Chemiosmosis

The Key to ATP Synthesis

• The movement of ions across a selectively permeable membrane, down their electrochemical gradient, coupled to ATP synthesis

The Chemiosmotic Hypothesis and Proton Motive Force

1. Electron transport pumps H+ from matrixs to IMS

2. This creates a PMF

• Chemical gradient (ΔpH): More H outside

• Electrical gradient (Δψ): Positive outside, negative inside

3. H+ flows back through ATP synthase

4. This flow drives ATP synthesis

PMF: Δp = Δψ - (2.3RT/F) ΔpH ≈ 200 mV

What are uncouplers and what are their characteristics

Uncouplers:

• molecules that dissipate the H+ gradient

• Electron transport continues but no ATP is made

• Energy is released as heat

Characteristics:

• Weakly Acidic (can accept/donate H+)

• Hydrophobic (can cross the membrane)

• Ex: 2,4-dinitrophenol, dicoumarol, and FCCP

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ETC Complex I

Function: transfers electrons form NADH to CoQ

Structure: Largest complex(44 subunits ~980kDa), L shaped

Prosthetic groups: 1 FMN(Flavin Mononucleotide), 6-8 Fe-S clusters

Reaction: NADH + H⁺ + Q → NAD⁺ + QH₂

PROTON PUMPING: 4 H per 2 electrons (per NADH)

ETC Complex I Path and Mechanism

Electron path: NADH -> FMN -> Fe-S clusters -> CoQ (ubiquinone)

Mechanism:

1. NADH binds and transfers 2 electrons to FMN

FMN -> FMNH₂

2. Electrons pass through Fe-S clusters one at a time

Fe³⁺ Fe²⁺ (one-electron transfers)

3. Electrons reduce ubiquinone (Q) to ubiquinol (QH₂)

Q → QH⁺ (semiquinone) → QH₂

• Conformational changes drive H⁺ translocation

• 4 H pumped per NADH oxidized

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ETC Complex I: Fe-S clusters

IRON-SULFUR (Fe-S) CLUSTERS: One-Electron Carriers

FUNCTION: Transfer electrons ONE at a time

Fe³ + e^- ⇌ Fe²⁺

• Key Point: Fe-S clusters bridge the gap between 2-electron carriers (NADH, FADH₂) and 1-electron carriers (cytochromes)

ETC Complex I Inhibitors – Rotenone

• Natural isoflavonoid from plant roots

• Potent Complex I inhibitor

• Blocks electron transfer from Fe-S to CoQ

Traditional Use:

• Indigenous peoples used rotenone-containing plants to catch fish

• Fish rise to surface due to impaired cellular respiration

• Relatively safe for humans (poor absorption)

ETC Complex II

The ONLY enzyme in BOTH TCA cycle AND ETC!

REACTION:

Succinate + Q Fumarate + QH→ ₂

STRUCTURE:

• 4 subunits (smallest ETC complex)

• Prosthetic groups: FAD, Fe-S clusters

ETC Complex II Electron Path

Succinate FAD Fe-S CoQ (QH₂)

Critical Difference: NO PROTON PUMPING

• Less energy released (ΔG less negative)

• FADH has higher reduction potential than NADH₂

• This is why FADH yields LESS ATP than NADH!

ETC Complex III

Function: Transfers electrons from CoQ to cytochrome c

Structure:

• Dimeric complex (functions as a dimer)

• Contains: 3 heme groups + 1 Fe-S cluster (Rieske)

ETC Complex III Prosthetic Groups

Prosthetic Groups:

• Heme bL and bH (cytochrome b)

• Heme c₁ (cytochrome c₁)

• [2Fe-2S] Rieske iron-sulfur protein

ETC Complex III Overall Reaction and Proton Pumping

OVERALL REACTION:

QH₂ + 2 Cyt c (ox) + 2 H⁺ (matrix) → Q + 2 Cyt c (red) + 4 H⁺ (IMS)

PROTON PUMPING:

4 H per 2 electrons

e^_ PATH:

QH₂ → Q → Fe-S → Cyt c

The Q Cycle Complex III

• QH₂ carries 2 electrons, but cytochrome c accepts only 1

• The Q cycle efficiently handles this mismatch

Q Cycle First Half

1. QH₂ binds at Qp site (P = positive/IMS side)

2. One electron → Rieske Fe-S → Cyt c₁ → Cyt c

3. Other electron → Heme bL → Heme bH → Q at Qn site

4. Q becomes semiquinone (Q•^-)

5. 2 H⁺ released to IMS

Q Cycle Second Half

1. Another QH₂ binds at Qp site

2. Same process: one e^- to Cyt c, one e^- to Qn site

3. Semiquinone at Qn + e^- + 2H⁺ (matrix) → QH₂

4. 2 or more H⁺ released to IMS

NET RESULT: 4 H⁺ pumped per 2 electrons to Cyt c

Complex IV Function and Reaction

FUNCTION: Transfers electrons from Cyt c to O₂ (final acceptor)

OVERALL REACTION:

4 Cyt c (red) + 8 H (matrix) + O₂ → 4 Cyt c (ox) + 2H₂O + 4 H⁺(IMS)

Complex IV Proton Movement

4 H used to reduce O₂ to H₂O ("chemical" protons)

• 4 H pumped to IMS ("pumped" protons)

• Total: 8 H consumed from matrix per O₂

Complex IV