Substrate-level phosphorylation
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Substrate-level phosphorylation is a type of chemical reaction that results in the formation of adenosine triphosphate (ATP) by the direct transfer of a phosphate group to adenosine diphosphate (ADP) from a reactive intermediate. In cells, it occurs in the cytoplasm (in glycolysis) under both aerobic and anaerobic conditions.
Unlike oxidative phosphorylation, here the oxidation & phosphorylation is not coupled
In the pay-off phase of glycolysis, four ATP are produced by substrate-level phosphorylation: two 1,3-bisphosphoglycerate are converted to 3-phosphoglycerate by transferring a phosphate group to ADP; two phosphoenolpyruvate are converted to pyruvate by the transfer of their phosphate groups to ADP.
In the citric acid cycle, one guanosine triphosphate (GTP) (which can donate a phosphate group to ADP or UDP, forming the respective triphosphates (a near equilibrium reaction catalyzed by nucleoside diphosphate kinase)) is produced by substrate-level phosphorylation (per cycle, with 2 cycles per glucose molecule) when succinyl-CoA synthetase converts succinyl-CoA to succinate.
Substrate-level phosphorylation is also seen in working skeletal muscles and the brain. Phosphocreatine is stored as a readily available high-energy phosphate supply, and the enzyme creatine phosphokinase transfers a phosphate from phosphocreatine to ADP to produce ATP. Then the ATP releases giving chemical energy.
An alternative way to create ATP is through oxidative phosphorylation, which takes place during the process of aerobic cellular respiration, in addition to the substrate-level phosphorylation that occurs during glycolysis and the Krebs cycle. During oxidative phosphorylation NADH is oxidized to NAD+ yielding 2.5 ATPs, and FADH2 yields 1.5 ATPs when it is oxidized.
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