Benzohydroxamic Acid−Peroxidase Complexes: Spectroscopic Characterization of a Novel Heme Spin Species

Abstract

A distinctive characteristic of the class III heme peroxidases of the plant peroxidase superfamily is the presence of a pentacoordinate quantum mechanically mixed-spin heme state resulting from the admixture of S = 5/2 and S = 3/2 states. This is not observed in class I or II peroxidases and, in fact, is a very rare heme spin state. The corresponding hexacoordinate quantum mechanically mixed-spin state is even more uncommon and has not been observed in heme proteins. The presence of the pentacoordinate form in the class III peroxidases suggested that they could be ideal candidates to display also the six-coordinate quantum mechanically mixed-spin state. With this possibility in mind, the benzohydroxamic acid complexes of the class III peroxidases horseradish isoenzyme C and A2 and soybean peroxidase are studied by electronic absorption, resonance Raman, and EPR spectroscopy at room and low temperatures. The results are compared with those obtained for Coprinus cinereus peroxidase which belongs to class II...