High crowder concentration promotes oxygen transport function of hemoglobin S - carbon monoxide binding renders it insensitive to change in crowder concentration.

Changes in crowding conditions affect functionally important structural changes in proteins. To understand how such changes affect the function of human hemoglobin S (HbSS), the affinity of CysF9[93]β sulfhydryl group of oxy and carbonmonoxy derivatives of HbSS for 5,5’-dithiobis(2-nitrobezoate) (DTNB) were measured in buffer 5.8 < pH < 9.0 under different crowding conditions (0 – 100 g dm-3 Ficoll 70). The affinities of the two hemoglobin derivatives for DTNB decreased with increasing pH at a given crowder concentration. For oxyhemoglobin, the average instantaneous gradient of the curve resulting from the fit under each crowding condition increases with increasing crowder concentration. Presence of the crowder did not significantly affect DTNB affinity of carbonmonoxyhemoglobin. Below pH 6.7, the affinity of oxyhemoglobin for DTNB increases with increasing Ficoll 70 concentration. Conversely, above pH 6.7, the affinity of oxyHbSS for DTNB decreases with increasing Ficoll 70 concentration at a fixed pH. Crowder did not significantly affect the affinity of carbonmonoxyhemoglobin for DTNB under the pH conditions of the experiment. This insensitivity of COHbSS to presence of crowder was attributed to formation of fiber by COHbSS. Hemoglobin fiber should be insensitive to high crowder concentration. The non-responsiveness of the affinity of DTNB for COHbSS to change in activity of the medium indicates that carbon monoxide binding to the hemoglobin promotes sickling or aggregation of hemoglobin. These findings were rationalized on the basis of the greater physiological relevance of oxygen binding to hemoglobin compared to carbon monoxide binding.