On the identity of the dissociation-linked chloride binding sites in human hemoglobin. Studies with hemoglobin modified with 4-isothiocyanatobenzenesulfonic acid and 4-isothiocyanatobenzenesulfonamide.

The binding of chloride ions to specific sites on the human hemoglobin molecule has well-known effects on the oxygen equilibrium and on the stability of the tetrameric structure. Several lines of evidence suggest that the oxygen-linked and the dissociation-linked chloride binding sites differ. Direct evidence for this difference has been obtained from the chloride dependence of the dimer-tetramer equilibrium of oxyhemoglobin modified with 4-isothiocyanatobenzenesulfonic acid, in which all the oxygen-linked chloride binding sites are blocked, or with 4-isothiocyanatobenzenesulfonamide, in which the linkage between chloride and oxygen is unperturbed. Thus, the chloride dependence of the dimer-tetramer assembly is unaffected by the chemical modification in both proteins and resembles that of unreacted hemoglobin. It is suggested that histidines alpha-103, alpha-122 and beta-97 may constitute, at least in part, the dissociation-linked chloride binding sites.