The effect of ligand binding and acid splitting on the optical rotatory dispersion of ferric horseradish peroxidase

Abstract

The optical rotatory dispersion of horseradish peroxidase and its cyanide, fluoride, and hydroxide complexes was studied in the spectral region 215–450 mμ, and that of the azide complex at 350–450 mμ. The effect that splitting the heme from the protein of peroxidase has on the optical rotatory dispersion in the 215–450 mμ region was also studied. Results of measurements of the reduced mean residue rotation at 233 mμ, lead to the conclusion that there are no significant changes in gross protein conformation upon the binding of ligands to peroxidase, but that the splitting of the heme causes a reduction of the helical content of the protein. Pure peroxidase was estimated to have 43% α-helical content, which was reduced to 33% when the heme was split from the protein. Results of studies in the Soret region indicate that the binding of various ligands does not cause an alteration of the geometry of the heme with respect to the protein moiety.