Direct observation of two distinct affinity conformations in the T state human deoxyhemoglobin

The main features of cooperative oxygenation human hemoglobin have been described by assuming the equilibrium between two affinity conformations of the entire molecule, T and R, However, the molecular basis for explaining the wide variation in the O-2 affinities of the deoxy T state has remained obscure, We address this long-standing issue by trapping the conformational states of deoxyhemoglobin molecules within wet porous transparent silicate sol-gels. The equilibrium O-2 binding measurements of the encapsulated deoxyhemoglobin samples showed that deoxyhemoglobin free of anions coexists in two conformations that differ in O-2 affinity by 40 times or more, and addition of inositol hexaphosphate to this anion-free deoxyhemoglobin brings about a very slow redistribution of these affinity conformations, These results are the first, direct demonstration of the existence of equilibrium between two (at least two) functionally distinguishable conformational states in the T state deoxyhemoglobin. (C) 2001 Federation of European Biochemical Societies, Published by Elsevier Science B.V. All rights reserved.