RESONANCE RAMAN AND ELECTRONIC ABSORPTION-SPECTRA OF HORSERADISH-PEROXIDASE ISOZYME A2 - EVIDENCE FOR A QUANTUM-MIXED SPIN SPECIES

The resonance Raman (RR) and electronic absorption spectra of ferric h orseradish peroxidase isozyme A2 (HRPA2) and of its complex with fluor ide are reported. The data show the presence of a five-coordinate quan tum-mixed spin (QS) species, resulting from the admixture of intermedi ate-spin, S = 3/2, and high-spin, S = 5/2, configurations, coexisting with a five- and a six-coordinate high-spin species. Fluoride binds al most completely, giving rise to a six-coordinate high-spin heme. In th e RR spectra of both the resting enzyme and fluoride complex two v(C=C ) stretching modes of the vinyl substitueuts are observed at 1622 and 1631 cm(-1). A comparison of the data obtained for HRPA2 with the elec tronic and RR spectra of soybean and barley peroxidases, all belonging to class III of the 'superfamily of plant peroxidases,' shows that th e QS species is common to all these proteins. Furthermore, it has allo wed us to infer that the QS heme is characterized by very high wavenum bers of the core-size marker bands and an electronic absorption spectr um blue-shifted with respect to those of the high-spin hemes. (C) 1998 John Wiley & Sons, Ltd.