MITOCH

Mitochondrial Structure and Electron Transport System

Introduction

• The structure of mitochondria is crucial for cellular respiration and energy production in eukaryotic cells.

• The Electron Transport System (ETS) is a series of complexes that play a key role in energy production through oxidative phosphorylation.

Mitochondrial Membranes

Outer Membrane (OMM) vs. Inner Membrane (IMM)

• Permeability:

  • OMM is permeable to ions and small molecules (e.g., H₂O, CO₂, NH3).

  • IMM is less permeable and requires specific transport systems for molecule movement.

• Composition Differences:

  • OMM contains:

    • Phosphatidylcholine (44-59%)

    • Phosphatidylethanolamine (20-35%)

    • Phosphatidylinositol (�3-5%)

  • IMM contains:

    • Phosphatidylcholine (38-45%)

    • Phosphatidylethanolamine (32-39%)

    • Significant protein density with a higher particle concentration on the inner face.

Functional Aspects of Mitochondrial Membranes

Outer Membrane:

• Contains beta-barrel proteins forming pores that facilitate import and export of small molecules.

Inner Membrane:

• Rich in proteins, crucial for the ETS and ATP synthesis via ATP synthase.

• Contains structures known as cristae that increase surface area and enhance the ability for electron transport and ATP generation.

Mitochondrial Electron Transport System

• Located in the IMM, composed of several protein complexes (I-IV) essential for electron transport from NADH and FADH2 to O₂.

  • Complex I: Transfers electrons from NADH to coenzyme Q (CoQ), translocating protons into the intermembrane space.

  • Complex II: Transfers electrons from succinate to CoQ without contributing to the proton gradient.

  • Electrons then proceed through Complexes III and IV to reduce O₂ to H₂O, resulting in proton pumping and ATP synthesis.

Shuttle Systems for Reducing Equivalents

Glycerophosphate Shuttle (Skeletal Muscle):

• Converts NADH to FADH2 for entry into the ETS, facilitating the transport of electrons from the cytosol to the inner mitochondrial membrane.

Malate-Aspartate Shuttle:

• Functioning in organs such as the heart and kidney, it transfers reducing equivalents through several enzymatic reactions, involving NAD+ and NADH, maintaining the redox state.

Key Components of Electron Carriers

NAD+ / NADH:

• Central to the redox reactions, with NADH being the reduced form that donates electrons to the electron transport chain.

• The reduction of NAD+ to NADH involves the gain of a hydride ion (H-).

FAD / FADH2:

• Similar to NAD+, but FADH2 transfers electrons starting from succinate through Complex II, contributing differently to ATP synthesis compared to NADH.

Chemiosmotic Gradient and ATP Synthesis

• The transfer of electrons along the electron transport chain establishes a proton gradient across the IMM.

• ATP Synthase utilizes this gradient to synthesize ATP from ADP and inorganic phosphate (P).

• The flow of protons down their gradient through ATP synthase is coupled to ATP formation.

Cytochromes and Iron-Sulfur Clusters

• Cytochromes facilitate electron transfer and undergo oxidation-reduction reactions.

• Contain heme groups that alternate between iron oxidation states (Fe(II) and Fe(III)), crucial for their electron-transporting functionality.

Summary

• The mitochondrial structure, particularly the OMM and IMM, serves distinct but complementary functions in metabolic processes. The Electron Transport System is pivotal in ATP production, with various shuttles and electron carriers contributing to the efficiency of bioenergetic processes.