Substrate-level phosphorylation is a process in which ATP is synthesized as a result of the oxidation of an organic compound, the substrate, without the participation of any external electron donor (e.g., NADH) or external electron acceptor (e.g., O2).
That is, in substrate-level phosphorylation the involved organic compound itself serves as both electron donor (becomes oxidized) and electron acceptor (becomes reduced) and its high energy phosphate bonds are transferred to ADP to form ATP (Fig. 24.10).
ATP synthesis via substrate-level phosphorylation takes place at various steps during complete oxidation of glucose molecule in aerobic respiration (glycolysis and TCA cycle) and in fermentations.
The role of substrate-level phosphorylation in aerobic respiration is the following:
Substrate-level Phosphorylation and Aerobic Respiration:
We know that there is the formation of 32 ATP molecules in total (since 2 ATP molecules are used in the beginning of glycolysis the final ATP yield is 30) as a result of the complete oxidation of one glucose molecule in aerobic respiration.
Out of all 32 ATP molecules, 26 are produced by way of oxidative phosphorylation with the involvement of NADH and FADH2, whereas the remaining 6 are the result of substrate-level phosphorylation.
Following are the steps of aerobic respiration during which ATP molecules are synthesized involving substrate-level phosphorylation:
(1) During glycolysis, the enzyme phosphoglycerate kinase transfers the high energy phosphoryal group from the carboxyl group of 1, 3- bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate.
(2) During glycolysis, the enzyme pyruvate kinase transfers the high energy phosphoryl group from phosphoenolpyruvate to ADP, forming ATP and pyruvate.
(3) During TCA cycle (Citric acid cycle; Kreb’s cycle), the enzyme succinyl-CoA synthetase breaks the thioester bond of succinyl-CoA and the energy released in this breakage is used to drive the synthesis of a phosphoanhydride bond in GTP or ATP. Succinate is the product in the process.