Heme-linked spectral changes of the protein moiety of hemoproteins in the near ultraviolet region.

Abstract

Spectral changes of hemoproteins in the near ultraviolet region on binding to a ligand and on oxidation-reduction of the heme-iron were studied by computer-controlled spectrophotometry. Near ultraviolet difference spectra between the low spin and high spin forms of ferric hemoproteins were classified into three groups: Those showing two absorption peaks having maxima at around 285 and 295 nm, those showing a peak at around 275 nm, and those showing a peak at around 300 nm. No corresponding absorption peak was observed with model heme complexes of low molecular weight. The intensity of the peak in cyanide difference spectra of catalase and horseradish peroxidase in the near ultraviolet region was dependent on the concentration of added cyanide and paralleled the intensity of the spectral changes in the Soret region. The spectral changes in both the near ultraviolet and Soret regions developed within 6 ms after the addition of cyanide. Difference spectra between the reduced and oxidized forms of cytochrome c, cytochrome oxidase-cyanide complex, hemoglobin, and lactoperoxidase-cyanide complex showed a characteristic peak at around 285-290 nm. Various difference spectra of hemoglobin in the near ultraviolet region were also measured. The observed positions, shapes, combinations, and relative intensities of the peaks were compared with those of solvent perturbation difference spectra and pH difference spectra of proteins and aromatic amino acids and also with the diacetylchitobiose-induced difference spectrum of lysozyme. The kinds of aromatic amino acid residues possibly responsible for the observed difference peaks were discussed on the basis of the results of the comparison. Based on the results obtained, the common occurrence of a heme-linked functional response of the hemoprotein conformation was suggested.