How is ATP produced in substrate-level phosphorylation?

ATP is produced in substrate-level phosphorylation through the direct transfer of a phosphate group to ADP from a phosphorylated substrate.

Substrate-level phosphorylation is a metabolic reaction that results in the formation of ATP or GTP by the direct transfer of a phosphate group from a phosphorylated reactive intermediate. This process is different from oxidative phosphorylation, where ATP is produced from the redox reactions of the electron transport chain.

In substrate-level phosphorylation, an enzyme binds with a substrate molecule that contains a phosphate group. This substrate molecule is typically a high-energy molecule, such as 1,3-bisphosphoglycerate or phosphoenolpyruvate, which are intermediates in the process of glycolysis. The enzyme then catalyses the transfer of the phosphate group from the substrate molecule to a molecule of ADP (adenosine diphosphate), forming ATP (adenosine triphosphate).

This process occurs in both the cytoplasm and the mitochondria of the cell. In the cytoplasm, it is part of the glycolysis pathway, where glucose is broken down into pyruvate, producing ATP and NADH. In the mitochondria, it occurs in the citric acid cycle (also known as the Krebs cycle or TCA cycle), where acetyl CoA is oxidised, producing ATP, NADH, and FADH2.

Substrate-level phosphorylation is a crucial process in cellular metabolism, as it is one of the primary ways cells generate ATP. ATP is the main energy currency of the cell, used to power various cellular processes, including muscle contraction, nerve impulse propagation, and chemical synthesis. Therefore, understanding substrate-level phosphorylation is fundamental to understanding how cells generate and use energy.