Electron Transport System

Overview

• Kreb's Cycle: Involves four dehydrogenation steps converting NAD+ to NADH and FAD+ to FADH.
• Reoxidation: Occurs in mitochondria. Oxidation involves O2 uptake and H2O production.
• Electron Transfer: NADH and FADH do not directly combine with O2; electrons are transferred through intermediates.
• Respiratory Chain: The electron transport system (ETS) is also known as the respiratory chain.

Electron Flow in ETS

• Flow Direction: From more electronegative to more electropositive compounds.
• Energy Transformation: Energy from electron flow is used to convert ADP to ATP, termed oxidative phosphorylation.

Components of ETS

• Complexes and Their Functions:
  1. Complex-I:
     • Contains NADH-dehydrogenase, a flavoprotein FMN, and non-heme Fe-S proteins.
     • Transfers electrons from NADH to Ubiquinone (UQ).
     • Equation: NADH + UQ + H+ → NAD+ + UQH2
  2. Complex-II:
     • Includes succinate dehydrogenase, flavoprotein FAD, and Fe-S proteins.
     • Receives electrons from succinic acid and passes them to UQ.
     • Equation: Succinate + UQ → Fumarate + UQH2
  3. Complex-III:
     • Comprises Dihydroubiquinone (UQH2), cytochrome b, Fe-S proteins, and Cytochrome C1.
     • Transfers electrons from UQH2 to Cytochrome C and releases protons outside.
     • Equation: UQH2 + 2 ferricytochrome C → UQ + 2 ferrocytochrome C + 2H+
  4. Complex-IV:
     • Consists of Cytochrome C oxidase, Cytochrome a, and Cytochrome a3 with heme iron and copper atoms.
     • Transfers electrons from cytochrome C to O2, forming H2O.
     • Equation: 2 ferrocytochrome C + 2H+ + 1/2 O2 → 2 ferricytochrome-C + H2O

Oxidative Phosphorylation

• Definition: Transfer of electrons from NADH to O2 via complexes I-IV, coupled with ATP synthesis from ADP and inorganic phosphate.
• Phosphorylation Sites:
  1. From NADH to UQ through Fe-S in complex-I.
  2. Within complex-III to cytochrome C.
  3. From cytochrome a to cytochrome a3 in complex IV.

ATP Production

• NADH Oxidation: Produces 3 ATP molecules.
• FADH2 Oxidation: Produces 2 ATP molecules.

Diagram

• Note: A figure illustrating the flow of electrons in ETS is referenced but not included here.