Cation modulation of hemoglobin interaction with sodium n-dodecyl sulfate (SDS). II: calcium modulation at pH 5.0

We investigate the conformational differences between HbA and HbS in the presence and absence of Ca(2+) concentrations (0-40 μM) akin to those within the erythrocyte cytoplasm and the membrane mimetic and native structure disrupting environments of the Plasmodium parasite food vacuole at pH 5.0. The experiments were monitored by UV-Vis spectrophotometery in the range of 250-650 nm. Our results suggest that the HbS, on interacting with both the membrane mimic and 40 μM Ca(2+), undergoes an "expansion" akin to the burst phase of proteins accompanied by tyrosine exposure while that of the HbA occurred with tryptophan exposure. Our results suggest conformational flexibility in the HbS unlike in the HbA. Besides, the spectral results also suggest that the HbS complexes with the Ca(2+) in its immediate environment without strain (due to its inherent conformational flexibility), unlike the HbA, thus appropriating the cation from its vicinity. The implications of these results are discussed in the light of possible mechanisms employed by the HbS to resist protease digestion or at least slow down the kinetics of the protease activities and on how these same factors can predispose the homozygous HbS individuals to sickling and consequent vaso-occlusive crisis.