Hydration and allosteric transitions in hemoglobin.

Sucrose and other neutral solutes reduce the oxygen affinity of human hemoglobin. This effect was attributed by Colombo et al. (Colombo, M.F., Rau, D.C., and Parsegian, V.A. (1992) Science 256, 655-659) to a stabilization of the deoxy-T quaternary state of hemoglobin A, via a reduction of the activity coefficient of water. This was correlated to crystallographic results which showed that a significant surface area at the alpha 1 beta 2 interface, which is exposed to the solvent in the oxy-R state, is buried in the deoxy-T state. We show that sucrose has no effect on the oxygen affinity of trout hemoglobin I, which is cooperative in oxygen binding but lacks heterotropic effects, and that in spite of the large buried surface exposed to solvent upon dissociation of human hemoglobin into alpha beta dimers, sucrose leads either to an increased dissociation of hemoglobin A-CO into dimers or to no effect at all (in the presence of inositol hexakisphosphate). These results may demand a reconsideration of the hypothesis extensively discussed by Colombo et al.