Chapter 20 - Electron Transport Chain

Where does the electron transport chain take place and what is it?

Four large protein complexes embedded in the inner mitochondrial membrane; three of these complexes pump protons out of the matrix into the intermembrane space, which generates a proton gradient used to power ATP synthesis

Where does the citric acid cycle occur?

In the mitochondrial matrix

Does ATP synthase aid in electron flow?

NO, it uses the proton-motive force generated by electron flow to synthesize ATP

Is the outer mitochondrial membrane permeable to small molecules and ions?

Yes, it is permeable to these because of channel proteins called mitochondrial porins → most abundant protein in outer mitochondrial membrane

Is the inner mitochondrial membrane permeable to small molecules and ions?

No, the inner membrane is impermeable to nearly all ions and polar molecules

What are the relative positions and roles of complexes of the ETC?

• Complexes I and III are the major proton pumpers → each pump 4 H+

• Complex II does not pump protons

• Complex IV is the final electron acceptor, uses O₂ → pumps 2 H+

What is reduction/redox potential?

A measure of a molecule’s tendency to donate or accept electrons

• Negative reduction potential → readily donates electrons, strong reducing agent, wants to get oxidized; ex. NADH

• Positive reduction potential → readily accepts electrons, strong oxidizing agent, wants to get reduced; ex. O₂

• High reduction potentials will release more free energy when reduced, allowing more protons to be pumped

How do electrons flow?

From low reduction potential (more negative, NADH) to high reduction potential (more positive, O₂)

What are the 5 landmark reduction potentials in the ETC?

Top (most energy, strongest reductant)

• NADH → -320 mV

• FADH2 → -80 mV

• Ubiquinone (CoQ) → 0 mV

• Cytochrome C → +235 mV

• O₂ → +816 mV

Bottom (least energy, strongest oxidant)

Where does NADH enter the ETC and how much ATP does it yield?

NADH enters at Complex I via flavin mononucleotide (FMN) and yields ~2.5 ATP

• FMN accepts 2 e- and 1 H+ from NADH, becoming FMNH₂

Where does FADH2 enter the ETC and how much ATP does it yield?

FADH2 enters at Complex II and yield ~1.5 ATP

What are the electrons carriers in the ETC?

• FMN → where NADH passes e- to in Complex I

• Fe-S cluster proteins → present in Complexes I, II, and III

• Fe-Heme cytochromes → present in Complex III, cytochrome c, and Complex IV

• Coenzyme Q → lipophilic, mobile shuttle b/w Complex I/II and III

What is the role of coenzyme Q (CoQ)?

CoQ carries electrons between Complex I/II and Complex III

• lipophilic, can move through IM membrane

• Three oxidation states: full oxidized ubiquinone (Q), semiquinone radical (QH), fully reduced ubiquinol (QH₂)

• bridge between fixed complexes in ETC

• carries 2 e-

What is fraxatin and what can mutations in the fraxatin gene cause

Fraxatin is a protein required for synthesis of Fe-S proteins, required for Complexes I, II and III, so without it the ETC and ATP synthesis fails causing neurodegeneration and cardiomyopathy

What three complexes in the ETC pump protons?

Complexes I, III, and IV

Where is [H+] high and where is [H+] low?

[H+] high in IM space, [H+] low in matrix

How do electrons move from NADH to O₂?

Through Complex I (NADH-Q oxioreductase), then Complex III (Q-cytochrome c oxioreductase), then Complex IV (cytochrome c oxidase), then O₂

How do electrons move from FADH2 to O₂?

Through Complex II (succinate-Q reductase), to ubiquinone (generates QH₂), to Complex III (Q-cytochrome c oxioreductase), then Complex IV (cytochrome c oxidase), then O₂

Which complex in the ETC is the largest?

Complex I NADH-Q oxioreductase

Which complex in the ETC contains the succinate dehydrogenase used in the citric acid cycle?

Complex II succinate-Q reductase

Why do electrons from FADH2 feed into the chain downstream and NADH and what is the result?

They feed into the chain downstream because FADH2 has a lower reduction potential (more positive); this is why Complex II does not pump protons and FADH2 yields less ATP overall

What does cytochrome c do?

• shuttles electrons from Complex III to Complex IV

• carries 1 e-

• soluble in water

How does Complex I NADH-Q oxioreductase work?

• Electrons are passed from NADH to FMN and then through series of Fe-S clusters until they reach CoQ, forming QH₂ → all occurs in extramembranous part of Complex I in matrix

• QH₂ leaves Complex I for the Q pool in the hydrophobic interior of the IM membrane

• QH₂ travels to Complex III via CoQ

• Flow of electrons leads to pumping of 4 H+ into IM space

• L-shaped structure of Complex I separates substrate recognition from proton pump

How does Complex II succinate-Q reductase work?

• Succinate dehydrogenase within Complex II oxidizes succinate → fumarate in TCA cycle, forming FADH2

• Electrons are transferred from FADH2 to Fe-S clusters, then to CoQ, forming QH₂ that enters Q pool in hydrophobic interior of IM membrane

• QH₂ travels to Complex III via CoQ

How does Complex III Q-cytochrome c oxidoreductase work?

• Receives electrons from QH₂ in CoQ → QH₂ carries two e-, but cytochrome c can only accept one e-, so the other e- must move through the Q cycle

• This results in 4 H+ pumped into IM space and electrons passed to cytochrome c, which carries them to Complex IV

How does the Q cycle work?

• Two QH₂ required per cycle

• QH₂ is oxidized, releasing two H+ and two e-; one e- travels to cytochrome c and continues along ETC

• simultaneously, the other e- piles onto Q in Complex III, forming semiquinone Q.-

• Semiquinone needs a second e- to become fully reduced to QH₂, which is provided when the second QH₂ enters and becomes oxidized, passing one e- to cytochrome c and the other to semiquinone, forming QH₂ that enters the Q pool

• 2 QH₂ + 2 cytochrome c_ox → 2 cytochrom c_red + 1 QH₂ + 4 H+

How does Complex IV cytochrome c oxidase work?

• Complex IV accepts four e- from four molecules of cytochrome c to catalyze reduction of O₂ to H₂O

• electrons moves from cytochrome c to Cu center to Heme to O₂

• Removes 8 protons from matrix → 4 used to reduce O₂ to H₂O, 4 pumped into IM space

Does Complex IV pump 2 H+ or 4 H+ into the IM space?

Complex IV requires 4 cytochrome c molecules to fully reduce O₂ to H₂O, in which it pumps 4 H+ into the IM space, however each molecule of NADH/FADH2 results in only 2 cytochrome c molecules being reduced by Complex III, so only 2 H+ are pumped into the IM space by Complex IV PER NADH, but in the complete reduction of O₂ to H₂O (which would require 2 NADH/FADH2 molecules), 4 H+ are pumped into the IM space

Per NADH, how many H+ are pumped into the IM space and how much ATP is produced?

• Complex I → 4 H+

• Complex III → 4 H+

• Complex IV → 2 H+

Total: 10 H+ pumped into IM space per NADH

1 ATP molecule requires ~4 H+, so ~2.5 ATP produced per NADH

Per FADH2, how many H+ are pumped into the IM space and how much ATP is produced?

• Complex II → 0 H+

• Complex III → 4 H+

• Complex IV → 2 H+

Total: 6 H+ pumped into IM space per FADH2

1 ATP molecule requires ~4 H+, so ~1.5 ATP produced per FADH2

What is a respirasome?

Three of the ETC components organized into a supercomplex

• Two copies of Complex I and IV surround a dimer of Complex III

What are the two common electron leak sites that can produce ROS by partially reducing O₂?

Complex I and Complex III

What enzymes protect against ROS?

• Superoxide dismutase → scavenges radicals to form O₂ and H₂O₂

• Catalase → further neutralizes H₂O₂ into H₂O and O₂