Studies on thermal denaturation of fish apomyoglobins using differential scanning calorimetry, circular dichroism, and fluorescence

Apomyoglobins from both fish (yellowfin tuna Thunnus albacares, bonito Katsuwonus pelamis, and yellowtail Seriola quinqueradiata) and mammals (sheep and sperm whale) exhibited a single endothermic peak, reflecting a two-state process of thermal unfolding. A native structure of fish apomyoglobins had smaller alpha -helix content and showed less thermal stability of the alpha -helix structure than that of the mammalian ones. The conformational stability of a tertiary fold of the fish apoproteins observed from a tryptophan fluorescence intensity and the fluorescence intensity of ANS-apomyoglobin complex was also found to be lower than that of the mammalian apoproteins. These results suggest that even though the thermal unfolding process is predominantly similar, the fish apoproteins particularly show structural conformity with less compact and less stable globin than that of the mammalian apoproteins. The results were also compared with those of their holomyoglobins.