Abstract
Expression of alpha and beta chains and their post-translational assembly into alpha(2)beta(2) tetramers is fundamental to the formation and function of most vertebrate hemoglobins. There is a strong evolutionary bias that favors expression of equal amounts of the two types of chains, because cooperativity, pH sensitivity, and anionic control of function occurs only for the alpha(2)beta(2) tetramers. Remarkably, an over-production of alpha chains, as in the pathological condition known as beta thalassemia in humans, is adaptive rather than pathological in the bluefish hemoglobin system. The thalassemia of the bluefish is a novel means of providing for oxygen uptake and delivery when low pH conditions incapacitate the highly pH-sensitive Root effect hemoglobins of the fish. Although fish often have pH-insensitive along with highly pH-sensitive hemoglobins, having pH-insensitive alpha chain monomers in circulation is an unusual structural variation. The role of bluefish alpha chains in oxygen transport is enabled by their remarkably lower oxygen affinity relative to human alpha chains. This is the first reported case of a thalassemic condition that is maintained in a species as an adaptive advantage.
Highlights
The bluefish (Pomatomus saltatrix) is a highly active predatory fish that is distributed worldwide
The Root effect Hbs of fish are so greatly affected by protonation that they serve as proton-driven pumps that can deliver oxygen to specialized tissues even against large pressure gradients
Air-equilibrated Root effect Hbs can release as much as 80% of their bound oxygen as the pH is decreased from pH 8 to 6
Summary
The results reported in this paper provide an explanation for the unusual lack of cooperativity of the cathodal bluefish Hb. The pH-insensitive cathodal fish Hbs previously studied are cooperative ␣22 tetramers that are structurally distinct from the ␣22 tetramers of the pH-dependent anodal Hbs [3, 4]. The bluefish ␣ chains have significantly lower oxygen affinity than human ␣ chains. This functional difference enables the bluefish ␣ chains to make a positive contribution to the oxygen transport process despite their non-cooperative nature. This is the first reported case of a thalassemic condition being conserved in a species because of the adaptive advantage it confers
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