Abstract
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 side pH range by GTP (whose concentration in the red blood cells varies with ambient oxygen availability). GTP obliterates the reverse Bohr effects 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 alpha and beta chains of the cathodic hemoglobins 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 beta chain of Phe (that suppresses the alkaline Bohr effect) and of residues at the switch region between alpha and beta subunits (that may alter the allosteric equilibrium, thus causing the high intrinsic oxygen affinity and low cooperativity). The residues binding organic phosphate in the beta 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 beta 143 that is considered to be responsible for the reverse Bohr effect in human and tadpole Hbs is replaced by Lys.
Highlights
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
The residues binding organic phosphate in the p 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
Summary
The cathodic hemoglobin of the eel Anguilla anguilla is considered to play an important role in oxygen transport under hypoxic and acidotic conditions. Histl 14 3 that is considered to be responsible for the reverse Bohr effect in human and tadpole Hbs is replaced by Lys. Fish hemoglobin (Hb) systems commonly exhibit functional heterogeneity, which appears to be adaptive to oxygen (02) transport under varying environmental and physiological conditions (Weber, 1990). As in the trout Hb system that has been intensively characterized in terms of molecular structure and oxygenation properties (e.g. Binotti et al (1971) and Weber et al (1976b)), the cathodic Hb fraction of eels displays high O2 affinity and low pH sensitivity (a small Bohr effect), which appear to safeguard O2 loading in the gills under hypoxic and acidotic conditions, whereas the anodic fraction displays a low affinity and a pronounced Bohr effect (Weber et al, 1976a). In contrast to the cathodic Hb of trout (Hbl), which is insensitive to organic phosphates, that of the eel shows greater sensitivity to the endogenous red cell phosphates GTP and ATP than does the anodic Hb (Weber et al, 1976a)
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