Abstract
Abstract Electron paramagnetic resonance (EPR) spectra of horseradish peroxidase (donor: H 2 O 2 oxidoreductase, EC 1.11.1.7) isozymes, acid and neutral enzymes, have been measured at both liquid-hydrogen and -helium temperatures. Both acid and neutral enzymes give similar EPR spectra exhibiting the prominent rhombic splitting at g = 6 region in acid solution. At alkaline pH, the acid enzyme forms two different kinds of low-spin compounds during the transition from “acid form” to “alkaline form”; one with the g values at 3.18, 2.06 and 1.23 and the other at 2.94, 2.08 and 1.63. The former is observed at neutral pH and the latter at alkaline pH. On the other hand, the neutral enzyme shows one low-spin compound, known as an “alkaline form”, at liquid-hydrogen temperature. At liquid-helium temperature, however, the neutral enzyme exhibits new EPR absorptions having g values at 3.20, 2.05 and 1.23. From the pH dependence of EPR absorptions, the p K values of the transitions between these three components are determined for both peroxidase isozymes; p K 1 ≈ 9, p K 2 ≈ 11 for the neutral enzyme and p K 1 ≈ 6.5, p K 2 ≈ 9 for the acid enzyme, respectively. The results obtained by the EPR measurements are in good agreement with those obtained by low-temperature spectrophotometry.
Published Version
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