CHARACTERIZATION OF SOYBEAN SEED COAT PEROXIDASE - RESONANCE RAMAN EVIDENCE FOR A STRUCTURE-BASED CLASSIFICATION OF PLANT PEROXIDASES

Electronic absorption and resonance Raman spectra of ferric and ferrou s forms of a peroxidase from soybean seed coat (SBP) at neutral and al kaline pH values together with the spectra of the ferric-fluoride comp lex are reported. At neutral pH a quantum mechanically mixed spin stat e, resulting from the admixture of intermediate spin, S = 3/2, and hig h spin, S = 5/2, configurations, has been identified which coexists wi th five- and six-coordinate high-spin hemes. A complete conversion to a fluoride-ligated six-coordinate high-spin and a hydroxy-ligated six- coordinate low-spin heme are observed at acid pH in the presence of fl uoride and at alkaline pH, respectively. The spectral features suggest that both the fluoride and hydroxo Ligands are stabilized by hydrogen -bond interactions with the distal Arg residue and through a water mol ecule with the distal His residue. The ferrous form shows a single nu( Fe-Im) at 246 cm(-1) at neutral pH. The data indicate that SEP shares many characteristics with peroxidases belonging to class III of the '' plant peroxidase'' superfamily. (C) 1998 John Wiley & Sons, Inc.