Changes in the abnormal α-subunit upon CO-binding to the normal β-subunit of Hb M Boston: resonance Raman, EPR and CD study

Abstract

Heme–heme interaction in Hb M Boston (Hisα58→Tyr) was investigated with visible and UV resonance Raman (RR), EPR, and CD spectroscopies. Although Hb M Boston has been believed to be frozen in the T quaternary state, oxygen binding exhibited appreciable co-operativity ( n=1.4) and the near-UV CD spectrum indicated weakening of the T marker at pH 9.0. Binding of CO to the normal β-subunit gave no change in the EPR and visible Raman spectra of the abnormal α-subunit at pH 7.5, but it caused an increase of EPR rhombicity and significant changes in the Raman coordination markers as well as the Fe(III)-tyrosine related bands of the α-subunit at pH 9.0. The UVRR spectra indicated appreciable changes of Trp but not of Tyr upon CO binding to the α-subunit at pH 9.0. Therefore, we conclude that the ligand binding to the β heme induces quaternary structure change at pH 9.0 and is communicated to the α heme, presumably through Hisβ92→Trpβ37→Hisα87.