MAC - Week 05 Lecture 3 - Oxidative phosphorylation and the electron transport chain_IT_ARP

Oxidative Phosphorylation and the Electron Transport Chain

Overview of the Electron Transport Chain (ETC)

  • The electron transport chain is a pivotal component in cellular respiration, primarily occurring in the inner mitochondrial membrane.

  • Electrons derived from NADH and FADH2 are transferred through a series of complexes (I-IV) to ultimately produce ATP from ADP through oxidative phosphorylation.

  • The process generates a proton motive force by pumping hydrogen ions (H+) into the intermembrane space, creating a gradient that drives ATP synthesis.

Flow of Electrons

  • Electrons from NADH enter the pathway at complex I, while FADH2 enters at complex II.

  • The path then proceeds through coenzyme Q (CoQ) and cytochrome c before reaching complex IV, where molecular oxygen acts as the final electron acceptor, forming water.

  • The transfer of electrons is coupled with the translocation of protons across the inner mitochondrial membrane, enriching the proton gradient.

Generation of Proton Motive Force (PMF)

  • The proton motive force is created by both the H+ ion gradient and the voltage difference across the membrane, effectively pulling protons back into the matrix. This electrochemical gradient is essential for ATP synthesis.

Function of Mitochondrial Compartments

Major Functional Compartments of Mitochondria

  1. Outer Membrane: Permeable due to porins.

  2. Intermembrane Space: The region between the inner and outer membranes, where protons accumulate.

  3. Inner Membrane: Contains the electron transport chain and ATP synthase, highly impermeable with a high protein content.

  4. Cristae: The folds of the inner membrane that increase surface area for ATP production.

  5. Matrix: Contains enzymes for the TCA cycle and the mitochondrial DNA.

Mitochondrial Dynamics

  • Mitochondria are dynamic, constantly changing shape, fusing, and dividing, particularly in cells requiring significant energy, such as muscle cells.

Complexes of the Electron Transport Chain

Complex I: NADH Dehydrogenase

  • NADH donates two electrons to complex I, leading to the reduction of CoQ and pumping 4 H+ ions into the intermembrane space.

Complex II: Succinate-CoQ Reductase

  • This complex integrates the TCA cycle: it converts succinate to fumarate and transfers electrons from FADH2 to CoQ without pumping H+ ions.

Complex III: CoQH2-Cytochrome c Reductase

  • Complex III transfers electrons from CoQ to cytochrome c while pumping H+ ions from the matrix to the intermembrane space.

Complex IV: Cytochrome c Oxidase

  • This complex accepts electrons from cytochrome c, transferring them to oxygen, which is reduced to water. Additionally, it pumps H+ ions out of the mitochondrial matrix.

Mechanism of ATP Synthesis

ATP Synthase Structure and Function

  • ATP synthase consists of two main subcomplexes: F0 (spanning the membrane) and F1 (extending into the matrix). The flow of protons back through the F0 subcomplex induces rotation that catalyzes the conversion of ADP and inorganic phosphate into ATP.

  • Each cycle of ATP synthase leads to the synthesis of approximately 3 ATP molecules.

Role of the Proton Motive Force

  • The returning protons not only drive ATP synthesis by ATP synthase but also assist in exchanging newly synthesized ATP with ADP and phosphate via transporters in the inner membrane.

Mitochondrial Genetics and Diseases

Mitochondrial DNA

  • Mitochondrial DNA (mtDNA) is inherited maternally and encodes only a small fraction of the proteins required for mitochondrial function. Most mitochondrial proteins are encoded by nuclear DNA.

  • Mitochondrial diseases arise from mutations in either mtDNA or nuclear DNA, which affect mitochondrial performance, causing a range of health issues from benign to life-threatening.

Disorders and Diagnostics

  • Mitochondrial diseases can present as muscle weakness, neurodegeneration, or metabolic issues due to dysfunctional energy production. Diagnostic methods include muscle biopsies and genetic testing to identify mutations.

Therapeutic Approaches

  • Currently, treatment options are limited, often involving symptomatic management, with some experimental therapies under investigation, such as gene therapy.

Summary of Key Concepts

  • The electron transport chain effectively converts energy from NADH and FADH2 into ATP, driven by the establishment of a proton motive force.

  • Understanding the structure and dysfunctions of mitochondria is crucial for addressing mitochondrial diseases and their inheritance patterns.