Phosphoglycerate kinase (PGK) is a key glycolytic enzyme that catalyzes the reversible transfer of a phosphate from 1,3-bisphosphoglycerate to ADP to form 3-phosphoglycerate and ATP in the presence of magnesium. PGK is essential in most living cells both for ATP generation in the glycolytic pathway of aerobes and for fermentation in anaerobes. The structure of PGK from Campylobacter jejuni has been determined to 2.15 A resolution. Like other kinases, PGK folds into two distinct domains, which undergo a large hinge-bending motion upon catalysis. The substrate binding sites, as deduced from electron density maps, are compatible with known substrate specificity and the stereochemical requirements for the enzymic reaction. The sulfate ion is found in the active site of the protein.