CONFORMATION STUDIES ON AND ASSESSMENT BY SPECTRAL ANALYSIS OF THE PROTEIN-CHROMOPHORE INTERACTION OF THE MACROMOLECULAR ANTITUMOR ANTIBIOTIC C-1027

Abstract

Characterization of the secondary structure of the antitumor antibiotic C-1027 has been made from a comparison of C-1027 and its apoprotein by various analytical means. The results indicated the antibiotic to be abundant in β-structure by measurements of Fourier-transform infrared (FT-IR) spectroscopy and the circular dichroism (CD) spectrum, and by a prediction of the secondary structure based on the amino acid sequence of the peptide. In comparison of the IR spectra of their proteins in D₂O, the apoprotein exhibited a faster H-D exchange than C-1027, indicating an increase in the "non-motile parts" of the β-sheets formed through the protein-chromophore interaction in holo-C-1027. The prediction of hydropathic index indicated the hydrophobic residues of the apoprotein to be predominantly located in the β-sheet structures, suggesting hydrophobic interaction in the binding between chromophore and apoprotein. Further, the interaction between chromophore and apoprotein was detected by a fluorescence method. The result showed the dissociation constant (Kd) to be 6.88 × 10^-5 M, indicating that the chromophore is tightly bound to the protein moiety.