THE WATER EFFECT ON ALLOSTERIC REGULATION OF HEMOGLOBIN PROBED IN WATER GLUCOSE AND WATER GLYCINE SOLUTIONS

We have previously proposed a role of hydration in the allosteric cont rol of hemoglobin based on the effect of varying concentrations of pol yols and polyethers on the human hemoglobin oxygen affinity and on the solution water activity (Colombo, M. F., Rau, D. C., and Parsegian, V . A. (1992) Science 256, 655-659). Here, the original analyses are ext ended to test the possibility of concomitant solute and water alloster ic binding and by introducing the bulk dielectric constant as a variab le in our experiments. We present data which indicate that glycine and glucose influence HbA oxygen affinity to the same extent, despite the fact that glycine increases and glucose decreases the bulk dielectric constant of the solution. Furthermore, we derive an equation linking changes in oxygen affinity to changes in differential solute and water binding to test critically the possibility of neutral solute heterotr opic binding. Applied to the data, these analyses support our original interpretation that neutral solutes act indirectly on the regulation of allosteric behavior of hemoglobin by varying the chemical potential of water in solution. This leads to a displacement of the equilibrium between Hb conformational states in proportion to their differential hydration.