Proton nuclear magnetic resonance spectra of compounds I and II of horseradish peroxidase

Abstract

Enzymatic reaction intermediates of horseradish peroxidase compounds I and II, were studied by high-resolution NMR spectroscopy at 220 MHz. The heme peripheral proton peaks were successfully obtained in the downfield region of 50-80 ppm from 4,4-dimethyl-4-silapentane-5-sulfonate for compound I and of 10-20 ppm for compound II at pH 9.2. This indicates that no isoporphyrin appears in the catalytic cycle of the enzyme. Temperature dependences of the spectra also were determined for these compounds between 7 and 32.degree. C. With increasing temperature, all the peaks in the downfield region for compound I shifted upfield, obeying the Curie law. Apparently the Fe atoms in compounds I and II are in ferryl high- and low-spin states, respectively. The spectrum was also observed in solutions of horse metmyoglobin to which hydrogen peroxide (H2O2) was added. The electron formulations of the hemes in ferrylmyoglobin and compound II are just alike, as judged from their spectra. Evidence was found against .pi.-cation radical on the heme ring as a source of the oxidizing equivalent in compound I.