Circular Dichroism of Hemoglobin in Relation to the Structure Surrounding the Heme

Abstract

Circular dichroism (CD) of various kinds of hemoglobins and the protoporphyrin-globin complex has been measured over the range of 200 to 660 nm. All mammalian hemoglobins so far tested show CD spectra identical with those of human hemoglobin. Although the absorption spectra of lamprey hemoglobin and toad embryonic hemoglobin are similar to those of human hemoglobin, these hemoglobins show various CD spectra different from human hemoglobin in the Soret and visible regions. CD spectra of human γ chains are almost identical with those of β chains, however both are different from those of α chains. These various CD spectra may be attributed to the different structures of heme environments of these hemoglobins. A protoporphyrin-globin complex exhibits distinctive CD bands both in the Soret and visible regions, indicating that contacts other than the iron-histidine bond make the tetrapyrrole ring optically active. The prominent positive ellipticity of deoxygenated human hemoglobin at 430 nm was decreased by either carboxypeptidase digestion or dialysis against deionized water. These treatments led to a loss of heme-heme interaction and to an increase in the oxygen affinity, without changing the gross structure of the molecule.