Oxygen Equilibrium and Circular Dichroism of Hemoglobin-Rainier

SUMMARY The sulfhydryl titration of hemoglobin - Rainier (wl32 146Tyr-‘Cys) suggests that the cysteine residue HC2(145)/3 forms a disulfide bridge with the cysteine F9(93)/3 of the same /3 chain. The oxygen equilibrium of Hb-Rainier compared with that of HbA exhibits diminished heme-heme interaction (n = 1.3-1.5), increased oxygen affinity, and a small Bohr effect. The absorption spectrum and the circular dichroism of the oxygenated form of Hb-Rainier are almost the same as those of HbA in the spectral region between 240 to 650 mn. In the deoxygenated form, however, the absorbance of Hb-Rainier at 430 nm is about 7% smaller than that of HbA, and in circular dichroism the ellipticities of the negative band at 285 mn and the positive extremum at 433 nm are much smaller than those for HbA. The isolated /3 chains of Hb-Rainier have an absorption spectrum similar to that of the p chains of HbA both in the oxygenated and deoxygenated forms except in the region between 250 and 270 mn. In circular dichroism, however, the oxygenated /3 chains of Hb-Rainier exhibit no positive band in the Soret region, and in the deoxygenated state, all heme bands are greatly diminished. These differences in oxygen equilibrium and circular dichroism between the hemoglobins, caused by the substitution of 145 tyrosine for cysteine, are discussed in relation to the structure andcontact of subunits.