The effects of heating on the structure and stability of multimeric TET aminopeptidase (APDkam589) were studied by differential scanning calorimetry, tryptophan fluorescence quenching, and dynamic light scattering. Thermally induced structural changes in APDkam589 were found to occur in two phases: local conformational changes, which occur below 70 A degrees C and are not associated with thermal denaturation of the protein, and global structural changes (above 70 A degrees C) induced by irreversible thermal unfolding of the protein accompanied by its spontaneous aggregation. These results may explain the bell-shaped temperature dependence with a maximum at similar to 70 A degrees C previously observed for enzymatic activity of APDkam589. Interestingly, the thermal unfolding of APDkam589 at about 81.2 A degrees C is accompanied by a so-called blue-shift of about 10 nm-a shift of the Trp fluorescence spectrum toward shorter wavelength. From this point of view, APDkam589 is quite different from most proteins, which are characterized by a long wavelength shift of the spectrum ("red-shift") upon denaturation. The blue-shift of the Trp fluorescence spectrum reflects the changes in the environment of Trp residues, which becomes more hydrophobic upon denaturation. The molecular structure of APDkam589 was determined by X-ray diffraction. The monomer of APDkam589 has six Trp residues, five of which are on the external surface of the dodecamer. Therefore, the blue-shift of the Trp fluorescence spectrum can be explained, at least partly, by aggregation of APDkam589, which occurs simultaneously with its thermal denaturation and probably makes the environment of these Trp residues more hydrophobic.