Dependence on pH of formation and oxygen affinity of hemoglobin S fibers in the presence and absence of phosphates and polyphosphates.

This paper presents data on the effect of phosphates and polyphosphates on the formation of hemoglobin S fiber, and on the Bohr effect of hemoglobin S samples whose concentration was high enough (near 5 mM) in order to form fibers upon deoxygenation. The experiments were performed in 0.2 M Bistris or Tris buffers at 30 degrees C in the presence and absence of inositol hexakisphosphate and of 2,3-diphosphoglycerate. Alternatively, 0.2 M phosphate buffers were used without addition of effectors. Under these conditions, few fibers were formed in Tris or Bistris buffers, while extensive fiber formation occurred in the presence of phosphates and polyphosphates. In all cases, increasing pH strongly inhibited fiber formation. At pH 7.5 and above, fibers were not formed in our samples. In the presence of phosphates and polyphosphates fiber formation reduced the oxygen affinity of hemoglobin S with respect to either hemoglobin A or soluble hemoglobin S under similar experimental conditions. The fiber-polyphosphate complexes showed a larger Bohr effect than that in hemoglobin A. In the presence of inositol hexakisphosphate fiber-forming solutions of hemoglobin S liberated as much as six protons per tetramer upon oxygen binding. The increased liberation of protons was probably due to a higher affinity of the effectors for the fibers of hemoglobin S. Very likely the higher affinity was supported by a conformational change of hemoglobin S specific for the fibers.