Interaction of Heme with Amphiphilic Peptides: Use of Hemin-CN to Probe the Interaction of Calmodulin with Its Target Peptides

The interaction of heme with several amphiphilic peptides has been studied by absorption and fluorescence spectroscopy. The binding can be followed by the changes in the absorption spectrum of the heme group or by the decrease in the peptide tryptophan fluorescence due to energy transfer to the heme. Despite their small size, ranging from 26 residues for melittin to 14 for mastoporan, a high affinity for heme-CO and hemin-CN (Kd < 100 nM) may be observed. Spectral shifts in the absorption peaks and appreciable geminate recombination after photodissociation of CO from the complex peptide-heme-CO suggest the formation of a heme pocket, as for the natural heme proteins. Application: hemin-CN can be used as a probe for the interaction of calmodulin with these target peptides. Amphiphilic peptides such as melittin bind to calmodulin with a high (nM) affinity. While both the peptide and calcium-bound calmodulin bind heme-CO, only the peptide binds hemin-CN. These interactions permit studies of the competition between hemin-CN and calmodulin for binding to the peptide: while hemin-CN quenches the melittin tryptophan fluorescence, addition of calmodulin to the [melittin*hemin-CN] complex displaces the hemin-CN and the melittin tryptophan fluorescence is recovered.