Phosphoglycerate kinase (PGK), a key enzyme in glycolysis, catalyses the transfer of a phosphoryl-group from 1,3-bis-phosphoglycerate 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 Angstrom 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 dosing of the large cleft between the two domains seen in all previous studies(1-6), thereby bringing the two ligands, 3-phosphoglycerate and ADP into dose 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.