Phosphoglycerate kinase (PGK), a key enzyme in glycolysis, catalyses the transfer of a phosphoryl-group from 1,3-bisphosphoglycerate to ADP to form 3-phosphoglycerate and ATP. Despite extensive kinetic and structural investigations over more than two decades, the conformation assumed by this enzyme during catalysis remained unknown. Here we present the 2.8 A crystal structure of a ternary complex of PGK from Trypanosoma brucei, the causative agent of sleeping sickness. This structure determination relied on a procedure in which fragments containing less than 10% of the scattering mass were successively positioned in the unit cell to obtain phases. The PGK ternary complex exhibits a dramatic closing of the large cleft between the two domains seen in all previous studies, thereby bringing the two ligands, 3-phosphoglycerate and ADP into close proximity. Our results demonstrate that PGK is a hinge-bending enzyme, reveal a novel mechanism in which substrate-induced effects combine synergistically to induce major conformational changes and, to our knowledge, afford the first observation of the PGK active site in a catalytic conformation.