Fish hemoglobins

There has been considerable increasing interest in fish hemoglobins in recent years. They exhibit the essential features of mammalian hemoglobins, cooperative ligand binding and heterotropic responses to a variety of ionic species, but they display an astounding variety of functional behaviors. These different properties are of interest as examples of evolutionary adaptation to differing physiological and environmental needs. They also offer valuable systems in which to study specific phenomena in either an exaggerated form or a simplified context. There are components of the hemoglobins of Salmonidae that exhibit neither Bohr effects nor responses to organic phosphates. Here cooperativity can be examined without the complexities associated with the simultaneous influences of buffer components on heterotropic interactions. Many fish hemoglobins exhibit exaggerated Bohr effects, commonly known as the Root effect, permitting the study of pH dependencies which are considerably amplified relative to that observed in mammalian hemoglobins. Fish hemoglobins can exhibit widely differing ligand affinities, with the total range reported for different hemoglobins under varying conditions being greater than four orders of magnitude. This would seem to offer an approach to the study of the relationship between protein structure and ligand affinity of the heme prosthetic group. In addition, many fish hemoglobins that exhibit the Root effect lose cooperative ligand binding when they attain their minimum ligand affinity at low pH in the presence of organic phosphates. These noncooperative, low-affinity states appear to be excellent models of liganded T states and are ideal for the analysis of the origins of the differences in the ligand affinities of the two extreme quaternary states of hemoglobin, the R and T states.