The Heme Chromophore in the Ultraviolet

Dan W Urry

doi:10.1016/s0021-9258(18)99558-x

Abstract

Absorption, optical rotatory dispersion, and circular dichroism data are presented for the oxidized and reduced states of the heme undecapeptide of cytochrome c. The absence of aromatic residues, disulfides, and ionizable sulfhydryl groups allows unambiguous determination of the heme absorptions to 240 mµ. As the electronic transitions within the heme are made optically active by the covalently attached peptide, it is possible to detect bands not readily resolved from absorption data. An absorption maximum is seen at 277 mµ which is labeled the e hemochromogen band. In the presence of imidazole, optically active bands are found in the ferroheme undecapeptide with extrema at 263 mµ, 278 mµ, 317 mµ, and 353 mµ. The ferriheme undecapeptide exhibits a positive circular dichroism band at 253 mµ and a broad negative extremum in the 320- to 360-mµ wavelength range. These bands, including those which are found in the wavelength range normally reserved to the aromatic amino acids, are due to the heme chromophore. At shorter wavelengths it is shown that the absorption at 190 mµ cannot be adequately accounted for by the peptide moiety and the carboxyls of the propionic acid heme substituents, that is, 25 to 50% of the absorption at 190 mµ is due to heme, the coordinated imidazole, and the thiol ether groups. The circular dichroism in the peptide region is dependent on the oxidative state and indicates changes directly involving the heme moiety either electronically or sterically. The bands observed in the heme peptides are briefly compared to qualitatively similar bands in cytochrome c, myoglobin, and hemoglobin. The distinctive feature in the circular dichroism of the heme proteins is the presence of localized bands which correspond to vibrational bands present in the aromatic amino acids. The vibronic bands should allow assessment of environmental changes in the corresponding amino acids.

Highlights

The absence of a heme band in the heme peptide does not preclude its presence in heme proteins; the presence of a band in the heme peptide which corresponds to a band in this range in hemoproteins is most reasonably taken as having its origin in the heme group. The presence of such bands complicates assignment of bands to aromatic residues in the hemoproteins. In this communication we show the presence of an E hemochromogen absorption band at 277 rnp, and circular dichroism extrema at 320 mp, 278 rnp, and 263 rnp in the ferro
It is to be expected that the heme chromophore of the heme undecapeptide bears important similarity to that of cytochrome c over the accessible range
The band could not be delineated from the absorption bands of the aromatic chromophores

Summary

Introduction

The heme proteins exhibit absorption and optical rotation changes in the ultraviolet which are dependent on their state of oxidation or on the nature of bound ligands. Absorption and CD bands observed in this wavelength range may be assigned to the heme chromophore.

Results

Conclusion