Phosphoglycerate kinase (PGK) is a monomeric enzyme which catalyzes the transfer of the high-energy phosphate group of 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate. This reaction represents the first of the two substrate-level phosphorylation events in the glycolytic pathway. Substrate-level phosphorylation is defined as production of ATP by a process, which is catalyzed by water-soluble enzymes in the cytosol; not involving membranes and ion gradients.