Introduction to Oxidative Phosphorylation:
Oxidative phosphorylation refers to the formation of ATP from ADP and inorganic phosphate (Pi), a process directly linked with biological oxidation reactions. Since this ATP synthesis is coupled with oxidation, the overall mechanism is called biological oxidative phosphorylation.
Mechanism of Oxidative Phosphorylation
1. Chemiosmotic Theory
The most widely accepted explanation for oxidative phosphorylation is the Chemiosmotic Coupling Hypothesis proposed by Peter Mitchell. This theory explains how energy released during oxidation is utilized to produce ATP.
Basic Principles of Chemiosmotic Theory:
- Electron Transport and Proton Pumping: The major electron carriers are arranged into Complexes I, III, and IV of the electron transport chain. As electrons are passed along these complexes, energy released from their transfer is used to pump H⁺ ions from the mitochondrial matrix to the intermembrane space.
- Formation of Electrochemical Gradient: The inner mitochondrial membrane is impermeable to protons. Therefore, the pumping of H⁺ ions creates an electrochemical potential (also known as the proton-motive force).
- ATP Synthesis via ATP Synthase: The accumulated protons in the intermembrane space flow back into the matrix through the F₀F₁-ATPase complex (ATP synthase). The flow of H⁺ ions releases free energy, which is used to convert ADP + Pi → ATP.
This entire process efficiently couples electron transport with ATP production, making oxidative phosphorylation the primary energy-yielding process in aerobic organisms.
P:O Ratio
The P:O ratio represents the number of molecules of ATP formed per atom of oxygen consumed (or per molecule of water produced) during oxidation.
- P:O ratio for NADH-linked substrates: 3 ATP molecules
- P:O ratio for FADH₂-linked substrates: 2 ATP molecules
This ratio helps in understanding the efficiency of ATP generation in oxidative metabolism.
Substrate-Level Phosphorylation
Substrate-level phosphorylation refers to the direct synthesis of ATP in metabolic reactions without the involvement of the electron transport chain or oxygen. The energy released from the substrate directly drives the phosphorylation of ADP to ATP.
Examples of Substrate-Level Phosphorylation:
- Conversion of phosphoenolpyruvate (PEP) to pyruvate
- Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate
- Conversion of succinyl-CoA to succinate in the TCA cycle
Shuttle Systems for Oxidation of Extramitochondrial NADH
Most NADH and FADH₂ molecules are generated within mitochondria by the TCA cycle and β-oxidation. However, some NADH is produced in the cytoplasm during glycolysis. Since the inner mitochondrial membrane is impermeable to NADH, specialized shuttle systems transfer the reducing equivalents (not NADH itself) from the cytosol into the mitochondria.
Two major shuttle systems operate in cells:
- The Malate-Aspartate Shuttle
- The Glycerol Phosphate Shuttle
1) Malate-Aspartate Shuttle System
This shuttle transfers the reducing power of cytosolic NADH into the mitochondria through a series of reactions involving malate and aspartate.
Steps:
- In the cytosol, NADH reduces oxaloacetate to malate using the enzyme malate dehydrogenase.
- Malate is transported across the inner mitochondrial membrane via a dicarboxylate transporter.
- Inside the mitochondria, malate is oxidized back to oxaloacetate by mitochondrial malate dehydrogenase, producing NADH within the matrix.
- This NADH then enters the electron transport chain and generates 3 ATP molecules.
- Since oxaloacetate cannot cross the membrane, it is converted to aspartate through a transamination reaction.
- Aspartate travels back to the cytosol, where it is converted again into oxaloacetate, completing the shuttle cycle.
ATP Yield: Each cytosolic NADH via this shuttle yields approximately 3 ATP molecules.
2) Glycerol Phosphate Shuttle
The glycerol phosphate shuttle operates mainly in skeletal muscle and brain tissues. It transfers reducing equivalents from cytosolic NADH to mitochondrial FADH₂.
Steps:
- Cytosolic NADH reduces dihydroxyacetone phosphate (DHAP) to glycerol-3-phosphate by the enzyme cytosolic glycerol-3-phosphate dehydrogenase.
- Glycerol-3-phosphate diffuses into the intermembrane space and is oxidized back to DHAP by the mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase.
- During this process, FAD is reduced to FADH₂, which donates electrons to the electron transport chain via Coenzyme Q (CoQ).
ATP Yield: Since FADH₂ enters the chain at Complex II, each cytosolic NADH through this shuttle yields 2 ATP molecules.
Comparison Between the Two Shuttle Systems
| Feature | Malate-Aspartate Shuttle | Glycerol Phosphate Shuttle |
|---|---|---|
| Location | Liver, Heart, Kidney | Brain, Skeletal Muscle |
| Transported Form | Malate / Aspartate | Glycerol-3-phosphate |
| Final Electron Acceptor | NAD⁺ (Matrix) | FAD (Mitochondrial enzyme) |
| ATP Yield per Cytosolic NADH | 3 ATP | 2 ATP |
| Main Advantage | Efficient energy transfer | Faster, less efficient system |
Detailed Notes:
For PDF style full-color notes, open the complete study material below: