| Abstract: | The presence of at least two types of conformers in the ferrous CO complex of horseradish peroxidase has been demonstrated with the use of native and deuteroheme-substituted enzymes. Type I conformers, predominant in acidic pH, exhibited both an Fe-CO stretching and an Fe-C-O bending Raman line together with an infrared C-O stretch band below 1920 em-1. On the other hand, type II conformers, dominant species in alkaline pH, showed only an Fe-CO stretching Raman line with the C-O stretch above 1930 cm-1. They were interconvertible either by the changes in pH or by the binding of benzhydroxamate, a substrate for the enzyme. The pKa value for the pH-dependent interconversion of CO complex of deuteroheme-substituted enzyme was 8.3. These findings were interpreted to mean that the bound CO molecule in type I conformers was more tilted over the heme-plane than that in type II conformers. A steric hindrance by the bound substrate or the protonated form of a distal amino acid residue, presumably of histidine, is considered to be the cause for the isomerization. By summarizing present and previous data on the vibrational frequencies of heme-carbonyl complexes, we found that there are inverse-linear relationships between the square of Fe-CO and that of C-O stretching frequencies, while squares of Fe-CO stretching and Fe-C-O bending frequencies were linearly correlated with each other. Also found is that the dissociation rate constant of CO molecule from heme-carbonyl complexes is a linear function of the Fe-CO stretching frequency. The significance of these results is discussed. |
