Heme-linked ionization in compounds I and II of horseradish peroxidases A2 and C

Hayashi, Y.; Yamazaki, I.

Archives of Biochemistry and Biophysics 190(2): 446-453


ISSN/ISBN: 0003-9861
PMID: 31136
DOI: 10.1016/0003-9861(78)90297-7
Accession: 068518515

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Heme-linked ionizations in Compound I and II of horseradish peroxidases, the presence of which was suggested from kinetic data by H.B. Dunford and J.S. Stillman, were detected from 2 independent experiments of spectrophotometric titration and proton balance. The values of pKa in Compound II were 6.9 for peroxidase A2 and 8.5 for peroxidase C. The kinetic results were accounted for by assuming that the alkaline forms of Compound II are inactive or very sluggish toward electron donors. Apparently the 2 ionizations occur in a functionally homologous position of the 2 isoenzymes, which is the distal group itself or closely related to it. A heme-linked ionization of pKa = approximately 5.4 in Compound I of peroxidase C was detected from pH changes of the visible spectrum. Measuring proton balance in each step of reductions from Compound I to Compound II to the ferric enzyme, ionizations having similar pKa values of 5.1-5.4 are present in both Compound I and the ferric enzyme. The pKa group in the ferric enzyme corresponded with that reported by H. Theorell and K.G. Paul. A tentative model for the vicinity of heme-ion of peroxidase C is presented.