Ligand-Linked Changes at the Subunit Interfaces in Scapharca Hemoglobins Probed through the Sulfhydryl Infrared Absorption†

FTIR spectra of native Scapharca homodimeric hemoglobin (HbI) and of the Phe97→Ile mutant have been measured in the region 2400−2700 cm-1 where the absorption of the sulfhydryl groups can be observed. In native HbI, the two Cys92 residues give rise to a relatively intense band centered at 2559 cm-1 that is shifted to 2568 cm-1 and strongly quenched upon ligand binding. In the Phe97→Leu mutant, such ligand-linked changes are not observed and the strong peak at around 2560 cm-1 persists in the liganded derivatives. In native HbI, the observed changes have been attributed to the decrease in polarity of the interface due to the ligand-induced extrusion of the Phe97 phenyl ring from the heme pocket to the interface and the subsequent release of several water molecules that are clustered in the vicinity of Cys92. In contrast, in the Phe97→Leu mutant, the Leu residue does not leave the heme pocket upon ligand binding and the interface is unaltered. The Cys92/S−H infrared band, therefore, represents a sensitive p...