Phosphoglycerate kinase catalyses the high-energy phosphoryl transfer of the acyl phosphate of 1,3-bisphosphoglycerate to ADP to produce ATP, a reaction requiring magnesium ions. The enzyme is widely distributed and apparently highly conserved as a monomer of molecular mass 45 000. X-ray studies of the enzymes from horse muscle and yeast, carried out in Oxford and Bristol respectively, have shown that the molecular structures of the two enzymes are almost identical. The most striking aspect of the structure is that the single polypeptide chain is organized into two separated domains composed of the N-terminal and C-terminal halves of the chain. Substrate binding studies and the determination of the complete amino acid sequence of the horse enzyme suggest that the nucleotide substrates and the phosphoglycerate substrates are bound to the C-domain and N-domain, respectively, in sites that are separated by about 12 A. In order to bring the two substrates together for catalysis, a hinge-bending conformational change involving helix rotation has been proposed, for which there is independent evidence from solution studies. Crystals of the ternary complex of the horse enzyme have been prepared that may contain the folded form of the enzyme.