Studies on Thermal Denaturation of Fish Myoglobins using Differential Scanning Calorimetry, Circular Dichroism, and Tryptophan Fluorescence

Myoglobins from yellowfin tuna, bonito and yellowtail exhibited two distinct endothermic peaks reflecting multiple states of structural unfolding, whereas those of sheep and sperm whale myoglobins followed a two-state process. This indicated that the thermal unfolding of fish myoglobins is predominantly a multi-state process differing from that of the mammalian ones. Changes in dependence on temperature of an α-helix content and a tryptophan fluorescence intensity were found to be decreased more for fish myoglobins compared with those of the mammalian myoglobins. Consequently, fish myoglobins showed a lower recovery of the α-helix and tertiary structure than that of the mammalian proteins. These results are thermally attributed to low conformational stability of the fish proteins. Therefore, fish myoglobins exhibit a rather labile structural folding, suggesting a greater susceptibility to heat denaturation than that of the mammalian myoglobins.