Abstract
BackgroundElevated blood O2 affinity enhances survival at low O2 pressures, and is perhaps the best known and most broadly accepted evolutionary adjustment of terrestrial vertebrates to environmental hypoxia. This phenotype arises by increasing the intrinsic O2 affinity of the hemoglobin (Hb) molecule, by decreasing the intracellular concentration of allosteric effectors (e.g., 2,3-diphosphoglycerate; DPG), or by suppressing the sensitivity of Hb to these physiological cofactors.ResultsHere we report that strictly fossorial eastern moles (Scalopus aquaticus) have evolved a low O2 affinity, DPG-insensitive Hb - contrary to expectations for a mammalian species that is adapted to the chronic hypoxia and hypercapnia of subterranean burrow systems. Molecular modelling indicates that this functional shift is principally attributable to a single charge altering amino acid substitution in the β-type δ-globin chain (δ136Gly→Glu) of this species that perturbs electrostatic interactions between the dimer subunits via formation of an intra-chain salt-bridge with δ82Lys. However, this replacement also abolishes key binding sites for the red blood cell effectors Cl-, lactate and DPG (the latter of which is virtually absent from the red cells of this species) at δ82Lys, thereby markedly reducing competition for carbamate formation (CO2 binding) at the δ-chain N-termini.ConclusionsWe propose this Hb phenotype illustrates a novel mechanism for adaptively elevating the CO2 carrying capacity of eastern mole blood during burst tunnelling activities associated with subterranean habitation.
Highlights
Elevated blood O2 affinity enhances survival at low O2 pressures, and is perhaps the best known and most broadly accepted evolutionary adjustment of terrestrial vertebrates to environmental hypoxia
In order to assess the functional and evolutionary adaptations of mole Hbs, and their mechanistic basis, we determined the coding sequences of the adult-expressed α-and β-type globin chains of coast mole (Scapanus orarius) and eastern mole (Scalopus aquaticus), two closely related, strictly fossorial species that are endemic to Western and Eastern North America, respectively, and we investigated the ligand-binding properties of their Hb components
Hb-O2 binding properties Both gel electrophoresis and isoelectric focusing (IEF; inset of Figure 2) revealed the presence of two major isoHb components in all individuals examined (n = 3 coast moles; n = 4 eastern moles), where the Hb I:Hb II ratio approximated 60:40 and 35:65, respectively
Summary
Elevated blood O2 affinity enhances survival at low O2 pressures, and is perhaps the best known and most broadly accepted evolutionary adjustment of terrestrial vertebrates to environmental hypoxia. This phenotype arises by increasing the intrinsic O2 affinity of the hemoglobin (Hb) molecule, by decreasing the intracellular concentration of allosteric effectors (e.g., 2,3-diphosphoglycerate; DPG), or by suppressing the sensitivity of Hb to these physiological cofactors. Among mammals that are adapted to hypoxic environments, only subterranean species are obliged to breathe air with elevated concentrations of carbon dioxide [1] Within this select group, the 25 or so species of fossorial moles (Family Talpidae) are among the few that live exclusively underground. To gauge whether the distinctive oxygenation properties of eastern mole Hb are accompanied by complementary or compensatory physiological adjustments, we measured hematological and muscle biochemical properties of the two fossorial species
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