Role of cysteine residues in hemoglobin structure and function: Transfer of p-mercuribenzoate from α subunits to β subunits during tetramer formation

Abstract

Mixture of equimolar amounts of hemoglobin A α subunits containing p-mercuribenzoate on cysteine residue G11 (104) and β subunits containing free sulfhydryl groups at cysteine residues F9(93) and G14(112) results in the stoichiometric formation of a subunit combination product ( α 2 PMB β 2 SH) ∗∗ ∗∗ The nomenclature used to describe the various forms of hemoglobin and the isolated subunits is as follows. The α and β refers to the subunit type, PMB as a superscript above the subunit type refers to p-mercuribenzoate bound to all the sulfhydryl residues of that particular subunit. SH as a superscript above the subunit type refers to the fact that all the sulfhydryl residues of that particular subunit are in the free state. having a molecular weight and oxygen affinity closely resembling those of hemoglobin A. The combination product differs from hemoglobin A in that it has a significantly reduced cooperative oxygen binding character, a somewhat different electrophoretic mobility, and no titratable sulfhydryl groups. Emulsification of the α 2 PMB β 2 SH combination product with 1-dodecanethiol results in the generation of two titratable sulfhydryl groups per tetramer. The explanation for these phenomena is that the p-mercuribenzoate migrates stoichiometrically from the G11(104)α cysteine residues to the F9(93)β cysteine residues during the combination of the subunits. The G11(104)α cysteine residue is located at the α 1β 1 contact point in the hemoglobin tetramer and these results indicate that this residue may have to be free in order for the hemoglobin subunits to stoichiometrically combine to form a tetramer.