Binding of 2,3-diphosphoglycerate by spectrin and its effect on oxygen affinity of hemoglobin

The relationships between spectrin, a structural protein of the red blood cell (RBC) membrane facing the cytoplasm, and hemoglobin were studied. The oxygen-binding properties of stripped hemoglobin were not altered by the presence of spectrin, but the interaction of hemoglobin with organic phosphates was reduced by the addition of spectrin. The presence of the enzyme glyceraldehyde 3-phosphate dehydrogenase (G3PD), another component of the RBC membrane used as control, did not change the oxygen affinity of either stripped hemoglobin or of hemoglobin solutions containing phosphates. Binding studies using the gel filtration method at pH 7.3 indicated reversible binding of 2,3-diphosphoglycerate to spectrin. A unit of 220,000 daltons was calculated to have seven binding sites and a binding constant of 1.2x10⁴M(-)¹. A mechanism is proposed in which spectrin may facilitate oxygen transport for hemoglobin molecules reaching the membrane.