Physiology of Finfish and Shellfish (2+1)

2.2.12. Fishes possess 2 kinds of hb

1. Monomeric found in Agnatha and hag fishes consisting of a single polypeptide molecule.
   (The Polypeptide Molecule: The spiral structure (alpha-helix) which forms most of the polypeptide molecules, the building units of the proteins).
2. Tetrameric hb, having 4 chains of amino acids (2 alpha and 2 beta chains) is found in higher fishes.
   
   

 

The tetrameric hb has a molecular weight of 65,000 daltons and is of several kinds. More than 1 type may be found in the same species. Thus, 4 kinds of hb are found in the blood of rainbow trout, gold fish has 3 kinds, while only 2 kinds of hb occur in the eel. Each kind of hb performs different function for which several combinations have evolved in different species to suit the environmental conditions in which they live. In the migratory eel, 2 kinds of hbs are present. Of these, 1 kind has a high oxygen affinity in seawater, while the other has a high oxygen affinity in freshwater. Thus, the eel is adapted to live in water of different salinity and is able to maintain a nearly constant level of blood oxygen in fresh as well as saline water.

It has been found that the gold fish acclimatized to 2.0ºC has 2 kinds of hb, while 3 types are present in those living at 20-35 ºC. It has also been shown that this third type of hb appears and disappears with change in temperature. The 3^rd type of hb is formed by the rearrangement of alpha and beta chains of other 2 types and is not synthesized fresh. Polymorphism in hb has been associated with the level of activity of the fish. Several factors such as pH, carbon dioxide concentration and temperature influence the blood oxygen affinity. Thus polymorphism in hb appears to be an adaptation to maintain required level of oxygen in the blood under various conditions.