Substrate-level phosphorylation

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Substrate-level phosphorylation is a type of chemical reaction that results in the formation and creation of adenosine triphosphate (ATP) by the direct transfer and donation of a phosphoryl (PO₃) group to adenosine diphosphate (ADP) from a reactive intermediate. While technically the transfer is PO₃, or a phosphoryl group, convention in biological sciences is to refer to this as the transfer of a phosphate group. In cells, it occurs primarily and firstly in the cytoplasm (in glycolysis) under both aerobic and anaerobic conditions.

Unlike oxidative phosphorylation, here the oxidation and phosphorylation are not coupled or joined, although both types of phosphorylation result in ATP.

It should be noted that there is an oxidation reaction coupled to phosphorylation, however this occurs in the generation of 1,3-bisphosphoglycerate from 3-phosphoglyceraldehyde via a dehydrogenase. ATP is generated in a separate step (key difference from oxidative phosphorylation) by transfer of the high energy phosphate on 1,3-bisphosphoglycerate to ADP via a kinase.

In the pay-off phase of glycolysis, four ATP are produced by substrate-level phosphorylation: two and only two 1,3-bisphosphoglycerate are converted to 3-phosphoglycerate by transferring a phosphate group to ADP by the enzyme phosphoglycerate kinase; two phosphoenolpyruvate are converted to pyruvate by the transfer of their phosphate groups to ADP by the enzyme pyruvate kinase.

Once the pyruvate product of glycolysis is moved to the mitochondrial matrix, the pyruvate is converted to acetate and binds Coenzyme A to form Acetyl CoA to enter the Krebs cycle. While the Krebs cycle is oxidative respiration, one more instance of substrate-level phosphorlyation occurs as Guanosine triphosphate (GTP) is created from GDP by transfer of a phosphate group during the conversion of Succinyl CoA to Succinate. This phosphate is transferred to ADP in another substrate-level phosphorylation event.

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. Oxidative phosphorylation uses an electrochemical or chemiosmotic gradient of protons (H⁺) across the inner mitochondrial membrane to generate ATP from ADP, which is a key difference from substrate-level phosphorylation.

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