Electron Transport Chain

Electron Transport Chain (ETC)

Overview of Glucose Oxidation Steps

  • Stages of glucose oxidation include:

    • Glycolysis

    • Pyruvate dehydrogenase conversion to acetyl CoA

    • Citric Acid Cycle (CAC)

    • Final step: Electron Transport Chain (ETC)

  • Learning goal: Understand the transfer of hydrogen ions and electrons during oxidative phosphorylation in ATP synthesis.

Electrons and Energy Carriers

  • Electrons are carried by:

    • NADH

    • FADH2

  • Energy for ATP synthesis comes from the breakdown of these carriers during glycolysis and the Krebs cycle.

Location of ETC

  • The Electron Transport Chain is located in the:

    • Inner mitochondrial membrane

    • Key areas involved:

      • Outer membrane

      • Intermembrane space

      • Cristae

      • Matrix

Electron Transfer Process

  • The reduced coenzymes NADH and FADH2 oxidized to generate ATP:

    • Electrons and H+ ions from NADH and FADH2 pass through a series of electron carriers until combining with oxygen to form H2O.

    • This process creates an electrochemical proton gradient, driving ATP synthesis.

Detailed Mechanism of Electron Transport

Role of Protein Complexes

  • The ETC consists of:

    • Fixed Enzyme Complexes: I, II, III, IV

    • Mobile Carriers: Ubiquinone (CoQ) and Cytochrome C

  • Oxygen serves as the last electron acceptor, forming water.

Function of Each Complex

  1. Complex I - NADH Dehydrogenase

    • Accepts electrons from NADH, regenerating NAD+.

    • Transfers electrons to CoQ (Ubiquinone) and pumps 4 H+ out into the intermembrane space.

  2. Complex II - Succinate Dehydrogenase

    • Does not span the membrane; transfers electrons from FADH2 to CoQ.

    • Less energy than Complex I.

  3. Complex III - Cytochrome bc1

    • Spans the entire membrane, receives electrons from CoQ.

    • Utilizes iron (Fe) for electron transfer and pumps H+ into the intermembrane space.

  4. Complex IV - Cytochrome c Oxidase

    • Last complex to receive electrons before they are transferred to oxygen, forming water.

    • Pumps additional protons into the intermembrane space.

Proton Gradient and ATP Production

  • The movement of electrons through these complexes results in a proton gradient:

    • Protons are pumped from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient.

    • ATP Synthase uses this gradient to synthesize ATP as protons flow back into the matrix.

ATP Synthesis and Energetics

ATP Yield from NADH and FADH2

  • From NADH:

    • 10 protons are pumped out (4 through CI, 4 through CIII, 2 through CIV) yielding approximately 2.5 ATP per NADH.

  • From FADH2:

    • 6 protons are pumped out (none through CII) yielding approximately 1.5 ATP per FADH2.

Regulation of the Electron Transport Chain

  • The efficiency of the ETC is influenced by:

    • Availability of ADP, Pi, oxygen, and NADH.

    • Increased ADP levels stimulate ATP synthesis; low levels hinder ATP production.

Shuttles for Electron Transfer

  • Glycerol Phosphate Shuttle:

    • Transfers electrons from cytoplasmic NADH to mitochondrial FADH2, yielding 1.5 ATP.

  • Malate-Aspartate Shuttle:

    • Transfers electrons from cytoplasmic NADH to mitochondrial NADH, yielding 2.5 ATP.

Overall ATP Yield from Glucose Oxidation

  • Total ATP yield from complete oxidation of one glucose molecule can range from 30 to 32 ATP, depending on the shuttles used during electron transfer.