Two-dimensional 1H nuclear magnetic resonance spectroscopy is used to determine the structure of the C-terminal complementary peptide (404-415) bound to a mutant phosphoglycerate kinase (1-403). Conformational changes in the peptide induced by the formation of the peptide-protein complex are followed by transferred nuclear Overhauser effect spectroscopy. Measurement of transferred NOEs and molecular modeling reveal an alpha-helix fold in the 405-409 region. This fold is in good agreement with the corresponding helix XIV of the crystallographic structure of wild-type PGK (Watson et al., 1982). The role of the alpha-helix from the C-terminal peptide in the recovery of catalytic activity in the mutant PGK is discussed.