Oxidative Phosphorylation

Oxidative phosphorylation is the process by which cells generate energy in the form of adenosine triphosphate (ATP) from the oxidation of nutrients and the reduction of oxygen. It is the final stage of cellular respiration, which involves the breakdown of glucose, fatty acids, and amino acids into carbon dioxide and water. Oxidative phosphorylation occurs in the mitochondria, the organelles that are responsible for producing most of the ATP in eukaryotic cells.

The electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of protons (H+ ions) across a membrane.

Complex I: NADH dehydrogenase

This complex accepts electrons from NADH and transfers them to coenzyme Q, while pumping four protons across the membrane. NADH is oxidized to NAD+ and coenzyme Q is reduced to ubiquinol (QH2).

Complex II: Succinate dehydrogenase

This complex accepts electrons from FADH2, which is produced by the oxidation of succinate to fumarate in the citric acid cycle. It transfers them to coenzyme Q, without pumping any protons. FADH2 is oxidized to FAD and coenzyme Q is reduced to ubiquinol (QH2).

Coenzyme Q: Ubiquinone

This is a mobile electron carrier that shuttles electrons from complex I and II to complex III. It can exist in three states: oxidized (Q), semiquinone (QH*), and reduced (QH2).

Complex III: Cytochrome bc1 complex

This complex accepts electrons from ubiquinol and transfers them to cytochrome c, while pumping four protons across the membrane. It uses a mechanism called the Q cycle, which involves two electron transfer pathways: one that reduces cytochrome c and one that regenerates ubiquinol.

Cytochrome c

This is a mobile electron carrier that shuttles electrons from complex III to complex IV. It is a small protein with a heme group that can accept and donate one electron at a time.

Complex IV: Cytochrome c oxidase

This complex accepts electrons from cytochrome c and transfers them to oxygen, while pumping two protons across the membrane. Oxygen is reduced to water and cytochrome c is oxidized. This complex contains two heme groups and two copper ions that form a binuclear center for oxygen binding and reduction.

ATP synthase

This is a large enzyme complex that uses the proton gradient generated by the ETC to synthesize ATP from ADP and inorganic phosphate. It consists of two subunits: F0 and F1. F0 is embedded in the membrane and acts as a proton channel. F1 protrudes into the matrix and contains the catalytic sites for ATP synthesis. The flow of protons through F0 causes F1 to rotate, which changes its conformation and allows it to bind ADP and phosphate and release ATP.

Oxidative phosphorylation is the process of generating ATP from the oxidation of organic molecules and the reduction of oxygen. It involves two main components: the electron transport chain (ETC) and the ATP synthase. The ETC is a series of protein complexes and electron carriers that are embedded in the inner mitochondrial membrane. The ATP synthase is a large enzyme complex that spans the membrane and catalyzes the synthesis of ATP from ADP and inorganic phosphate.

The chemical theory of oxidative phosphorylation was proposed by Fritz Lipmann in 1941. He suggested that there is a direct chemical coupling between the oxidation of substrates and the phosphorylation of ADP to ATP, mediated by high-energy intermediate compounds. These intermediates were called acyl phosphates, and they were supposed to transfer their phosphate group to ADP, forming ATP and a lower-energy compound.

ATP synthesis is the process of making ATP from ADP and inorganic phosphate (Pi) using the energy stored in the electrochemical potential difference across the inner mitochondrial membrane. This process is catalyzed by a complex enzyme called ATP synthase, which consists of two parts: F0 and F1.

The P/O ratio, also known as the phosphate/oxygen ratio, is a measure of the efficiency of oxidative phosphorylation. It indicates how well the consumption of oxygen in electron transport and the synthesis of ATP in oxidative phosphorylation are linked. It is defined as the number of molecules of ATP produced per pair of electrons transferred from a substrate to oxygen.

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