As in other fish, the cathodic hemoglobin of the eel Anguilla anguilla is considered to play an important role in oxygen transport under hypoxic and acidotic conditions. In the absence of phosphates this hemoglobin shows a reverse Bohr effect and high oxygen affinity, which is strongly modulated over a wide pH range by GTP (whose concentration in the red blood cells varies with ambient oxygen availability). GTP obliterates the reverse Bohr effect in the cathodic hemoglobin. The molecular basis for the reverse Bohr effect in fish hemoglobins has remained obscure due to the lack of structural data. We have determined the complete amino acid sequence of the and chains of the cathodic hemoglobin of A. anguilla and relate it to the oxygen equilibrium characteristics. Several substitutions in crucial positions are observed compared with other hemoglobins, such as the replacement of the C-terminal His of the chain by Phe (that suppresses the alkaline Bohr effect) and of residues at the switch region between and subunits (that may alter the allosteric equilibrium, thus causing the high intrinsic oxygen affinity and low cooperativity). The residues binding organic phosphate in the cleft of fish hemoglobins are conserved, which explains the strong effect of GTP on oxygen affinity and suggests that these residues contribute to the reverse Bohr effect in the absence of alkaline Bohr groups. Moreover, His that is considered to be responsible for the reverse Bohr effect in human and tadpole Hbs is replaced by Lys.

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