Oxidative Phosphorylation
Oxidative phosphorylation is the process by which cells generate adenosine triphosphate (ATP), the main energy currency of the cell, through the transfer of electrons along the electron transport chain (ETC) coupled with the synthesis of ATP. This process takes place in the inner mitochondrial membrane of eukaryotic cells and the plasma membrane of prokaryotic cells. Let's break down the process and include the relevant chemical reactions.
1. Electron Transport Chain (ETC):
- The electron transport chain consists of a series of protein complexes (complex I to IV) embedded in the inner mitochondrial membrane.
- Electrons are transferred through these complexes, and the energy released during this transfer is used to pump protons (H⁺ ions) across the inner mitochondrial membrane, creating an electrochemical gradient.
The chemical reactions at each complex involve the transfer of electrons and the pumping of protons:
- Complex I (NADH-CoQ Reductase):
- NADH + H⁺ + CoQ → NAD⁺ + CoQH₂ (ubiquinol)
- Complex II (Succinate-CoQ Reductase):
- FADH₂ + CoQ → FAD + CoQH₂ (ubiquinol)
- Complex III (CoQH₂-Cytochrome c Reductase):
- CoQH₂ + 2Cytochrome c (Fe³⁺) → CoQ + 2Cytochrome c (Fe²⁺) + 2H⁺
- Complex IV (Cytochrome c Oxidase):
- 4Cytochrome c (Fe²⁺) + 8H⁺ + O₂ → 4Cytochrome c (Fe³⁺) + 2H₂O
2. Proton Pumping and Formation of Proton Gradient:
- As electrons move through the ETC, protons are pumped across the inner mitochondrial membrane from the mitochondrial matrix to the intermembrane space.
- This creates an electrochemical gradient, with a higher concentration of protons in the intermembrane space compared to the matrix.
3. ATP Synthase:
- ATP synthase is a complex enzyme embedded in the inner mitochondrial membrane that allows protons to flow back into the matrix, utilizing the energy released to synthesize ATP.
The chemical reaction catalyzed by ATP synthase is as follows:
- ADP + Pi + 4H⁺ (matrix) → ATP + H₂O + 4H⁺ (intermembrane space)
4. Overall Reaction:
- The overall reaction for oxidative phosphorylation, combining electron transport and ATP synthesis, can be represented as:
- ADP + Pi + NADH + ½O₂ → ATP + NAD⁺ + H₂O
This equation summarizes the conversion of adenosine diphosphate (ADP) and inorganic phosphate (Pi) into ATP, coupled with the consumption of NADH and oxygen (½O₂).
Oxidative phosphorylation is a crucial step in cellular respiration, enabling cells to efficiently produce ATP, which serves as the primary energy source for various cellular processes.