Physicochemical features and oxygenation properties of extracellular hemoglobin from the leech, Macrobdella decora

Abstract

1. 1. Extracellular hemoglobin from the freshwater leech Macrobdella decora has a sedimentation coefficient of 57 S. Under the electron microscope, the molecule displays a two-layered hexagonal structure, characteristic of annelid extracellular hemoglobins, with dimensions 29.9 ± 1.8 nm (across the hexagon) and 19.2 ± 1.2 nm (height). M. decora hemoglobin contains one heme per 24,000 g of protein. 2. 2. The oxygen binding properties of this high molecular weight hemoglobin are characterized as a function of pH, cations and temperature. 3. 3. The oxygenation properties are affected by protons. At near-physiological cation concentrations, the oxygen affinity and cooperativity rise concomitantly with increasing alkalinity. At pH ∼ 8.0 and 20°C both these parameters attain maximal values of P 50 = 3.0 mm Hg and n 50 = 3.1. A normal alkaline Bohr effect ( φ = −0.33) at the pH range 6.5–7.5 is observed. 4. 4. The oxygen binding characteristics are also extremely sensitive to cations. At pH 7.75 and 20°C both oxygen affinity and cooperativity rise with increasing cation concentrations. Divalent cations such as Ca 2+ and Mg 2+ produce a larger effect than monovalent ones. They start to be effective at concentrations as low as 0.05 mM.