Ligand-induced conformational changes in spin labelmodified human hemoglobins and chains and their carboxypeptidase A-digested derivatives

The reactive sulfhydryls of human adult and fetal hemoglobin and the single sulfhydryl of isolated γ chains have been spin labeled with N-(1-oxyl-2,2,5,5-tetramethyl-3-pyrrolidinyl iodoacetamide. Similar electron paramagnetic spectral differences between oxy- and deoxy-modified hemoglobins were observed for both these hemoglobins and for the isolated chains, indicating that ligand-induced conformational changes occur in isolated hemoglobin subunits as well as intact hemoglobin tetramers. Ligand induced changes in the reactivity of ρ{variant}-hydroxymercuribenzoate with the sulfhydryl groups of both intact hemoglobins and isolated subunits, observed by McDonald and Noble ((1974) J. Biol. Chem. 249, 3161-3165), led them to draw a similar conclusion. Following carboxypeptidase A digestion of these modified hemoglobins and γ chains, a procedure which specifically removes the two C-terminal residues of the β or γ chains, spectral differences between the liganded and unliganded spin-labeled derivatives still persisted. However, the magnitude of this difference was not only more reduced in the case of the hemoglobins than in that of the subunits but the spectra of both the oxy and deoxy derivatives of the hemoglobins were characteristic of the oxy derivative of a cooperative tetrameric hemoglobin. These findings support the premise that the COOH-terminal end of the β or γ chain contributes, although possibly to different extents, to the spectral differences exhibited by both the spin-labeled hemoglobins and chains.