Contrasting Effects of Halides on the Structure and Function of A Multimeric Allosteric Protein

Abstract

We have used two halide salts, namely, sodium chloride and sodium iodide, and studied their impact on the oxygenation characteristics of adult human hemoglobin (Hb). Previous studies in our group showed that both the halide salts exerted similar effects on the Hb function at concentrations below 0.1 M, i.e., an overall decrease in the affinity for oxygen, as a result of a decrease in the affinity at low oxygenation levels. However, as the halide concentrations increased, while chloride continued producing a progressive but rather diminished effect, iodide reverted its effects on Hb: the overall affinity for oxygen rather increased. Careful analysis of the oxygenation curves revealed that while the affinity for oxygen decreased at high oxygen saturation levels, the affinity at low oxygen concentration increased markedly. These effects reached a plateau at a concentration of 2 M, but even more surprisingly, cooperativity was never canceled. The results hinted at the possibility that iodide ions were splitting the tetrameric Hb molecules into asymmetric dimers. Dimers have been and still are considered non-cooperative, high oxygen-affinity systems. Yet, our present data clearly contrast with the previous tenet since cooperativity index showed values as high as 1.6 in the presence of 2 M NaI. Determination of molecular weight by size exclusion chromatography, and the study of oxygenation characteristics of symmetric nickel-iron Hb hybrids in the presence of sodium iodide showed that in fact the tetrameric Hb splits into two dimers that, strikingly, remain allosterically functional.