The interactions of various surfactants with horse met-myoglobin have been investigated by electronic absorption and emission, circular dichroism, and resonance Raman spectroscopy. Both the cationic surfactants cetyltrimethylarnmonium chloride (CTAC) and decyltrimethylammonium bromide (DTAB), as well as the anionic surfactant sodium dodecyl sulfate (SDS), convert the (aquo) 6-coordinated high-spin metmyoglobin to a 6-coordinated low-spin species. The combined analysis of the absorption and resonance Raman spectra indicates that the sixth ligand is the distal histidine in the latter species. The conversion is accompanied by an ellipticity inversion in the CD Soret band and a red shift of the tryptophan fluorescence band. High concentrations of cationic surfactants induce the formation of a 5-coordinated berne, which could be either heme separated by the protein or a solvent-exposed heme. On the other hand, for anionic surfactants a 5-coordinated heme species is a transient intermediate on the pathway toward the 6-coordinated low-spin form. All the spectral changes are completely reversible when ionic surfactants with opposite charge are added. The formation of the low-spin species suggests a major protein rearrangement. Partial unfolding and displacement of the E helix, which bears the distal histidine, could be triggered by the surfactant through the rupture of the structurally important inter-helix Lys77 – Glu18 salt bridge. Additional hydrophobic interactions, and possibly micelle formation, induce the formation of the 5-coordinated heme.
Department, Program, or Center
Thomas H. Gosnell School of Life Sciences (COS)
Journal of Inorganic Biochemistry 86N1 (2001) 216
RIT – Main Campus