Detection of anion-linked polymerization of the tetrameric hemoglobin from Scapharca inaequivalvis by 31Cl NMR spectroscopy

Abstract

Ion binding to the hemoglobin components of Scapharca inaequivalvis has been measured directly in quadrupole relaxation experiments of 23Na and 35Cl. The dimeric and tetrameric hemoglobins interact weakly with sodium ions, but differ in their interaction with chloride ions. The dimeric hemoglobin binds chloride ions with low affinity, whereas the tetrameric protein has high-affinity chloride binding sites. Binding of chloride ions to these high-affinity sites brings about an oxygen-linked polymerization which manifests itself in an unusual dependence of the 35Cl excess linewidth on the concentration of the anion. Polymerization is more pronounced in the deoxygenated than in the oxygenated derivative: in the former, it has been observed previously in sedimentation velocity experiments. The sensitivity of the 35Cl excess linewidth on polymer formation indicates that the residence time of the transiently bound chloride on the tetrameric hemoglobin is not shorter than the correlation time of the molecule (2 × 10 −1 s −1).