Blood of the eastern mole is able to rid the body of large quantities of carbon dioxide because of amino acid substitutions in hemoglobin creating a salt bridge.

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In most cases, animals that live strictly in low oxygen environments (e.g., underground or at high altitudes) produce hemoglobin with a higher oxygen binding affinity. Paradoxically, the eastern mole, which lives exclusively in underground burrows with low oxygen and high carbon dioxide levels, produces low-oxygen affinity hemoglobin. Its hemoglobin contains several amino acid substitutions that, among other effects, causes an internal salt-bridge to form preventing an increase in oxygen-binding affinity, but more importantly for the eastern mole, increases its affinity for binding carbon dioxide. If not, the eastern mole would experience hypercapnia (high carbon dioxide blood levels) which could be detrimental or fatal in its high-carbon dioxide environment.

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References

“Among mammals that are adapted to hypoxic environments, only subterranean species are also obliged to breathe air with elevated concentrations of carbon dioxide…Due to the impeded gas exchange of damp soils with surface air, these animals are chronically exposed to hypoxic and hypercapnic environments (14.3% O2 and 5.5% CO2 have been recorded within mole tunnels). The high metabolic costs of burrowing, in terms of O2 consumption and CO2 production, are exacerbated by the obligate re-breathing of expired air while tunnelling, and may therefore require adaptive modifications in hemoglobin (Hb) function…Not surprisingly, whole blood O2 affinity of the European mole, Talpa europaea, is much higher…than those of terrestrial mammals of similar size.” (Campbell et al. 2010:1)

“Most species have Hbs with intrinsically high O2 affinity that is markedly reduced in the red cell by physiological concentrations of DPG…which stabilizes the tense (deoxy) state of Hb by electrostatic binding within a cationic pocket.” (Campbell et al. 2010:5)

“[T]he carboxyl side chain of δ136Glu forms a stable salt bridge with the nearby ε-amino group of δ82Lys…This substitution should reduce electrostatic repulsion between the dimer subunits, thereby reducing the intrinsic O2 affinity of the R-state protein. Consistent with this expectation, the intrinsic O2 affinity of eastern mole Hb is ~2.8-fold lower than that of coast mole Hb in the physiological pH range.” (Campbell et al. 2010:6)

“Why then have eastern moles, following an extensive period of fossorial evolution, recently forsaken the potential for adaptive modulation of their O2-binding affinity by phosphates and adopted a lower whole blood O2 affinity phenotype?…the loss of DPG binding sharply increases the carrying capacity of eastern mole Hb for the metabolic end product CO2, thus providing a strong selective advantage for this subterranean inhabitant. Unlike carbon monoxide, CO2 does not bind to the heme iron, but instead can interact with the uncharged α-amino termini of the four globin chains to form carbamino CO2.” (Campbell et al. 2010:9)

“The primary molecular mechanism involves an amino acid substitution (δ136Gly→Glu) that forms a salt bridge with δ82Lys of the same chain, thus deleting key binding sites for allosteric effectors (DPG, Cl- and lactate) between δ1Val- δ82Lys and markedly reducing competition for CO2 binding (carbamate formation) at the N-terminus. Accordingly, we suggest this unique Hb phenotype enhances CO2 carrying capacity during burst activity (tunnelling) in gas-exchange impeded burrows.” (Campbell et al. 2010:10)

Journal article
Molecular basis of a novel adaptation to hypoxic-hypercapnia in a strictly fossorial moleBMC Evol BiolJuly 16, 2010
Kevin L Campbell, Jay F Storz, Anthony V Signore, Hideaki Moriyama, Kenneth C Catania, Alexander P Payson, Joseph Bonaventura, Jörg Stetefeld, Roy E Weber

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Living System/s

Organism
Eastern MoleScalopus aquaticusSpecies

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