Magnetic circular dichroism studies of bovine liver catalase

Absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of beef liver catalase at pH 5.0 and 6.9, and its complexes with NaF, KCNO, NaCNS, NaN3 and NaCN, have been measured between 250 nm and 700 nm at room temperature. The pH 6.9 native catalase MCD shows the presence of several additional transitions not resolved in the absorption spectrum. While these bands can be seen in the spectra of all the derivatives, with the exception of the cyanide, their relative intensities changes considerably between complexes. Of special interest in the MCD of ferric hemes is the signal intensity at about 400 nm and 620 nm. The data indicate that the MCD intensity at 620 nm increases as the high spin iron porphyrin fraction increases, reaching a maximum with the fluoride complex. The 430 nm band intensity increases as the proportion of low spin iron increases, reaching a maximum with the cyanide complex. The MCD spectra also indicate clearly the existence of spin mixtures in the complexes with CNO-, CNS-, and N3-, where both the 430 nm and 620 nm bands have appreciable intensity. It is significant that despite almost identical absorption spectra the CNS- complex has higher fraction of low spin iron than either the CNO- or the N3- species. The differences between the pH 5 and 6.9 MCD spectra of the native catalase suggest that the environment of the heme centre is sensitive to protonation.