Oxygen binding and its allosteric control in hemoglobin of the pulmonate snail, Biomphalaria glabrata

Pulmonate snails that experience extreme variations in gas tensions and temperatures possess extracellular, high-molecular mass (similar to 1.7 x 10(6) Da) hemoglobins (Hbs) that are little known as regards oxygenation and allosteric characteristics. Biomphalaria glabrata hemolymph exhibits a high O-2 affinity (half-saturation O-2 tension = 6.1 mmHg; pH 7.7, 25 degrees C), pronounced Bohr effect (Bohr factor = -0.5), Emd pH-dependent cooperativity (Hill's cooperativity coefficient at half-saturation = 1.1-2.0). Divalent cations increase O-2 affinity, Ca2+ exerting greater effect than Mg2+. Analyses in terms of the Monod-Wyman-Changeux model indicate novel O-2 affinity control mechanisms. In contrast to vertebrate Hb, where organic phosphates and protons lower affinity via decreased O-2 association equilibrium constant of Hb in low-affinity state (K-T), and to extracellular annelid Hbs, where protons and cations primarily modulate O-2 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.