Oxidative Phosphorylation: Electron Transport Chain and ATP Synthesis

Oxidative Phosphorylation

Overview of the Electron Transport Chain (ETC)

Oxidative phosphorylation is the final stage of cellular respiration, occurring in the inner mitochondrial membrane. It involves the transfer of electrons through a series of protein complexes, ultimately leading to the synthesis of ATP.

• Quinones: Lipid-soluble molecules that transfer electrons within the membrane and can carry 2 electrons.

• Cytrochromes: Proteins containing heme groups that can carry 1 electron. Cytochrome C is a key electron carrier between Complex III and IV.

• Iron-sulfur proteins (Fe-S): Typically carry 1 electron and are found in several complexes of the ETC.

Key Electron Carriers

• NADH: Donates electrons to Complex I (NADH dehydrogenase).

• FADH2: Donates electrons to Complex II (succinate dehydrogenase).

• Ubiquinone (Q): Transfers electrons from Complex I and II to Complex III.

• Cytochrome c: Transfers electrons from Complex III to Complex IV.

Electron Flow and Proton Pumping

Electrons from NADH and FADH2 are passed through the complexes, resulting in the pumping of protons (H+) across the inner mitochondrial membrane, creating a proton gradient.

• Complex I: NADH + H+ + Q → NAD+ + QH2 (pumps protons)

• Complex II: FADH2 + Q → FAD + QH2 (does not pump protons)

• Complex III: QH2 + 2 Cyt c (oxidized) → Q + 2 Cyt c (reduced) (pumps protons)

• Complex IV: 4 Cyt c (reduced) + O2 + 8 H+ (matrix) → 4 Cyt c (oxidized) + 2 H2O + 4 H+ (intermembrane space) (pumps protons)

ATP Synthesis

The proton gradient generated by the ETC drives ATP synthesis via ATP synthase. Protons flow back into the mitochondrial matrix, powering the conversion of ADP and inorganic phosphate (Pi) into ATP.

Summary Equations

• Complex I overall reaction: $\text{NADH} + 5\text{H}^+_{\text{matrix}} + \text{Q} \rightarrow \text{NAD}^+ + \text{QH}_2 + 4\text{H}^+_{\text{intermembrane space}}$

• Complex II overall reaction: $\text{FADH}_2 + \text{Q} \rightarrow \text{FAD} + \text{QH}_2$

Example: Mitochondrial Electron Transport

In mitochondria, electrons from NADH and FADH2 are transferred through the ETC, resulting in the reduction of oxygen to water and the synthesis of ATP.

Table: Main Components of the Electron Transport Chain

Complex	Main Function	Electron Donor	Electron Acceptor	Proton Pumping
Complex I	NADH oxidation, Q reduction	NADH	Q	Yes
Complex II	FADH2 oxidation, Q reduction	FADH2	Q	No
Complex III	QH2 oxidation, Cyt c reduction	QH2	Cyt c	Yes
Complex IV	Cyt c oxidation, O2 reduction	Cyt c	O2	Yes

Additional info: The diagram illustrates the flow of electrons and protons across the mitochondrial membrane, highlighting the role of each complex and carrier in oxidative phosphorylation.