Oxidative phosphorylation is a crucial metabolic process that involves a series of electron carriers, which play a significant role in the electron transport chain (ETC). The primary electron donors in this process are NADH and FADH2, which donate electrons to specific complexes within the mitochondrial membrane. The electron carriers include quinones, cytochromes, and iron-sulfur proteins, each with unique properties that facilitate electron transfer.
Quinones, such as ubiquinone, are lipid-soluble molecules that can carry two electrons. They are anchored within the membrane by an isoprene chain, allowing them to diffuse between protein complexes. In contrast, cytochromes contain a porphyrin ring with an iron atom at the center, enabling them to transfer one electron at a time. Notably, poisons like cyanide and carbon monoxide inhibit electron flow at cytochrome a, which is critical for the final step of the electron transport process.
Iron-sulfur proteins, which are complexed with cysteine residues, can carry one to four electrons depending on the number of iron atoms present. Each iron atom can accept one electron, thus influencing the overall electron transfer capacity of these proteins.
The electron transport begins when NADH donates electrons to complex I, resulting in its reoxidation to NAD+. FADH2 donates electrons to complex II, where it is reoxidized to FAD. The electrons then flow through the complexes, with ubiquinone acting as a carrier to complex III, where cytochrome B picks them up. The electrons continue through iron-sulfur complexes and are eventually transferred to cytochrome C, which is located in the intermembrane space. Finally, the electrons are delivered to complex IV, where cytochrome a facilitates their transfer to oxygen, forming water.
During this process, a significant amount of protons (H+) are pumped from the mitochondrial matrix into the intermembrane space. For each NADH that enters the electron transport chain, approximately ten protons are pumped, while FADH2 results in the pumping of about six protons. This difference is crucial because the proton gradient generated across the membrane is used by ATP synthase to produce ATP. The greater the number of protons pumped, the more ATP can be synthesized, explaining why NADH yields more ATP compared to FADH2.
In summary, oxidative phosphorylation is a complex but essential process that efficiently converts the energy stored in NADH and FADH2 into ATP, utilizing a series of electron carriers and the establishment of a proton gradient across the mitochondrial membrane.
