A role of heme side-chains of human hemoglobin in its function revealed by circular dichroism and resonance Raman spectroscopy

摘要

Structural changes of heme side-chains of human adult hemoglobin (Hb A) upon ligand (O_2 or CO) dissociation have been studied by circular dichroism (CD) and resonance Raman (RR) spectroscopies. We point out the occurrence of appreciable deformation of heme side-chains like vinyl and propionate groups prior to the out-of-plane displacement of heme iron. Referring to the recent fine resolved crystal structure of Hb A, the deformations of heme side-chains take place only in the β subunits. However, these changes are not observed in the isolated β chain (β_4 homotetramer) and, therefore, are associated with the α-β inter-subunit interactions. For the communications between α and β subunits in Hb A regarding signals of ligand dissociation,possible routes are proposed on the basis of the time-resolved absorption, CD, MCD (magnetic CD), and RR spectroscopies. Our finding of the movements of heme side-chains would serve as one of the clues to solve the cooperative O_2 binding mechanism of Hb A.