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oxidative phosphorylation:see phosphorylationphosphorylation,
chemical process in which a phosphate group is added to an organic molecule. In living cells phosphorylation is associated with respiration, which takes place in the cell's mitochondria, and photosynthesis, which takes place in the chloroplasts.
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the synthesis in living cells of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and phosphoric acid at the expense of energy that derives from the oxidation of an organic substrate. As a result of oxidative phosphorylation, cells accumulate and subsequently utilize ATP—the most important high-energy compound known —to provide the energy for life processes. The principal substrates that are involved in oxidative phosphorylation are the organic acids that are formed in the tricarboxylic acid cycle.
Oxidative phosphorylation was first discovered in 1930 by the Soviet biochemist V. A. Engel’gardt. In 1939, V. A. Belitser and E. T. Tsybakova demonstrated that oxidative phosphorylation is associated with the transfer of electrons along the respiratory enzyme chain, which is located—as was later determined—in the inner mitochondrial membrane. Electrons enter the respiratory chain from reduced nicotinamide-adenine dinucleotide (NAD) or nicotinamide-adenine dinucleotide phosphate (NADP) and are successively transferred by coenzyme Q from compounds with a more negative oxidation-reduction potential to ones with a more positive potential.
The transfer of electrons along the chain terminates with the reduction of O2 by the complex enzyme cytochrome oxidase. Thus, substrate oxidation by oxygen is dependent on a series of oxidation-reduction reactions. After each reaction, the energy that accumulates in each molecule of oxidized substrate is released in small amounts, which ensures the maximum possible utilization of the energy; this maximum utilization occurs at energy conjugation points. The enzyme complex ATP-synthetase synthesizes ATP from ADP and phosphoric acid; it also controls the reverse process, the breakdown of ATP.
The ratio of P to O reflects the quantity of phosphate that is bonded during ADP phosphorylation relative to the amount of O2 that is absorbed. The transfer of two electrons through an energy conjugation point yields one molecule of ATP. The ratio of P to O is 3 for NAD oxidation and 2 for the oxidation of succinic acid.
S. A. OSTROUMOV