Comparison of Human and Shirbot (Cyprinidae: Barbus grypus) Hemoglobin: A Structure-Function Prospective

Hemoglobin is a tetrameric protein with two alpha and two beta subunits binds oxygen in a cooperative manner. In dominant tetrameric form of fish hemoglobin carry more than 90 percent of oxygen from gill to tissues at 20° C. The tetrameric form of fish hemoglobin is changed to monomeric form at low oxygen pressure in order to increase its oxygen affinity. This is one of adaptive mechanisms used by different kinds of fish. The major aim of this paper is to study the molecular basis of shirbot hemoglobin adaptation mechanism to various environmental conditions. Using different methods such as ion exchange chromatography, UV-Vis, fluorescence and circular dichroism spectroscopy, we extracted the main tetrameric fraction of shirbot hemoglobin and studied the structural characteristics of shirbot and human hemoglobins in a comparative way. Our results showed that tetrameric form of shirbot hemoglobin has less stable and loosely folded structure in contrast to human hemoglobin. Our data also indicate, in case of exposure to life-threatening environmental factors such as low oxygen level, acidic pH, oxidizing chemicals and other water pollutants especially detergents (surfactants) triggering tetramer to monomer dissociation in shirbot hemoglobin is more prominently than in human hemoglobin. The resulting monomer of hemoglobin has more oxygen affinity and could take up oxygen more strongly even at low pressure. We hypothesize that this mechanism helps shirbot to adapt and to survive at such harsh environment. The mechanism that is may be adapted by other fish species.