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

Pulmonate snails that experience extreme variations in gas tensions and tem peratures possess extracellular, high-molecular mass (similar to 1.7 x 10(6 ) Da) hemoglobins (Hbs) that are little known as regards oxygenation and al losteric 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 cooperativit y (Hill's cooperativity coefficient at half-saturation = 1.1-2.0). Divalent cations increase O-2 affinity, Ca2+ exerting greater effect than Mg2+. Ana lyses in terms of the Monod-Wyman-Changeux model indicate novel O-2 affinit y control mechanisms. In contrast to vertebrate Hb, where organic phosphate s and protons lower affinity via decreased O-2 association equilibrium cons tant of Hb in low-affinity state (K-T), and to extracellular annelid Hbs, w here protons and cations primarily modulate O-2 association equilibrium con stant of Hb in high-affinity state (K-R), in B. glabrata Hb, the Bohr effec t is mediated predominantly via K-R and the cation effect via K-T, reflecti ng preferential, oxygenation-linked proton binding to oxygenated Hb and cat ion binding to deoxygenated Hb. CO2 has no specific (pH independent) effect . Nonlinear van't Hoff plots show temperature dependence of the overall hea ts of oxygenation, indicating oxy-deoxy heat capacity differences. The find ings are related to possible physiological significance in pond habitats.