Electron Transport Chain + ATP Synthesis

ETC, chemiosmosis, ATP synthesis, and their building blocks

Complex I

NADH–coenzyme Q oxidation complex (NADH dehydrogenase)

• uses Fe-S cluster to transfer NADH electrons to FMN

• Fe-S moves electrons to CoQ, forming CoQH₂

• 4 protons pumped

• 2 electrons transferred

Complex II

Succinate–coenzyme Q oxidoreductase complex (succinate dehydrogenase)

• Uses Fe-S clusters to transfer electrons from succinate to FAD, generating FADH₂

• Electrons from FADH₂ are then transferred to CoQ, froming CoQH₂

• No protons pumped

Complex III

coenzyme Q–cytochrome c oxidoreductase complex (cytochrome complex

• Uses Fe-S cluster to transfer electrons from CoQH₂ to heme, forming cytochrome c

• Q cycle explains electron flow

• 2 electrons are transferred

• 4 protons pumped

Q Cycle

Occurs inside of Complex III

1. 1 electron is transferred to cytochrome c₁, another is transferred via cytochrome b to a CoQ, making semiquinone

2. CoQ is released, CoQH₂ transferes another electron to cytochrome c₁ and its other is transferred via cytochrome b to a CoQ, making semiquinone

Complex IV

Cytochrome c oxidase

• electrons are transferred from cytochrome c to Fe atoms in cytochrome a then to cytochrome a₃

• Cu²⁺ transfers electrons as H^- to O₂, forming H₂O as it reduces

• 2 protons pumped (per pair)

Chemo

Flavoproteins

Flavoproteins use either flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) as a prosthetic group

• ex. NADH dehydrogenase and succinate dehydrogenase

• transfer both electrons and protons

Iron-Sulfur Proteins

• have an iron-sulfur (Fe-S) center

• iron atoms in center are electron carriers

• alternate between oxidized (Fe²⁺) and reduced (Fe³⁺)

• transfer one electron at a time

Cytochromes

• porphyrin prosthetic group: Heme

• five types: b, c, c₁, a, and a₃

• iron atom of heme serves as electron carrier

• transfers one electron at a time

Copper-Containing Cytochromes

• cytochromes a and a₃ have one copper bound to heme group

• associates with iron to form a bimetallic iron-copper center (Fe-Cu)

• copper ions can be reversibly oxidized (Cu²⁺) or reduced (Cu⁺) by accepting or donating electrons

• Fe-Cu center helps keep O₂ bound to complex

Coenzyme Q

• a quinone

• CoQ is reduced twice to semiquinone (CoQH) and then dihydroquinone (CoQH₂)

• serve as collection point for electrons from reduced FNM and FAD

• accepts both protons/electrons when reduced, releases when oxidized