Pulmonate snails that experience extreme variations in gas tensions and temperatures possess extracellular, high-molecular mass (~1.7 × 10⁶ Da) hemoglobins (Hbs) that are little known as regards oxygenation and allosteric characteristics. Biomphalaria glabrata hemolymph exhibits a high O₂ affinity (half-saturation O₂ tension = 6.1 mmHg; pH 7.7, 25°C), pronounced Bohr effect (Bohr factor = 0.5), and pH-dependent cooperativity (Hill's cooperativity coefficient at half-saturation = 1.1-2.0). Divalent cations increase O₂ affinity, Ca²⁺ exerting greater effect than Mg²⁺. Analyses in terms of the Monod-Wyman-Changeux model indicate novel O₂ affinity control mechanisms. In contrast to vertebrate Hb, where organic phosphates and protons lower affinity via decreased O₂ association equilibrium constant of Hb in low-affinity state (K_T), and to extracellular annelid Hbs, where protons and cations primarily modulate O₂ association equilibrium constant of Hb in high-affinity state (K_R), in B. glabrata Hb, the Bohr effect is mediated predominantly via K_R and the cation effect via K_T, reflecting preferential, oxygenation-linked proton binding to oxygenated Hb and cation binding to deoxygenated Hb. CO₂ has no specific (pH independent) effect. Nonlinear van't Hoff plots show temperature dependence of the overall heats of oxygenation, indicating oxy-deoxy heat capacity differences. The findings are related to possible physiological significance in pond habitats.

Bohr effect; cation effects; cooperativity; oxygen transport; mollusks

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