Human hemoglobin cross-linked through the polyphosphate-binding site. Functional properties and evidence for conformers

The properties of human hemoglobin reacted with 2-nor-2-formylpyridoxal 5'-phosphate, a bifunctional derivative of pyridoxal 5'-phosphate, have been investigated both from an equilibrium and kinetic point of view. The experimental data, interpreted in terms of the two-state allosteric model, indicate that a perturbed R state is characteristic of this modified low ligand affinity hemoglobin. In flash photolysis experiments, a quickly reacting component is always observed, in spite of the lack of dissociation into free dimers; this kinetic behavior is thought to reflect the presence of functionally independent alpha beta dimers, still connected by the flexible cross-link but forming an open hemoglobin tetramer. Two possible models for the interpretation of the kinetics of CO and/or haptoglobin binding are presented and discussed.