Ligand dependent conformational changes in the isolated subunits and intact tetramers of human adult and fetal hemoglobins

The kinetics of the reactions of p hydroxymercuribenzoate with the reactive β93 sulfhydryl groups of human adult hemoglobin and γ93 sulfhydryl groups of human fetal hemoglobin was studied at pH 7.0. An increase in the rate of reaction with p hydroxymercuribenzoate upon oxygenation was seen for fetal hemoglobin, as well as adult hemoglobin. While no change in the reactivity of the α104 sulfhydryl group of α(SH) chains was seen upon oxygenation, a change in reactivity of the β93 and β112 sulfhydryl groups of β chains and the γ93 sulfhydryl group of γ(SH) chains was detected upon liganding. This change in reactivity with p hydroxymercuribenzoate, presumably reflecting a change in conformation of these isolated chains upon ligand binding, was in a direction opposite to that observed for the intact hemoglobins. These findings, along with the established fact that p hydroxymercuribenzoate modified β and γ chains have a much lower affinity for oxygen than native β and γ chains, are consistent with a linked functional relationship between the binding of p hydroxymercuribenzoate, and the state of liganding in these isolated non α subunits.