Charge Reversal of a Critical Active‐Site Residue of Cytochrome‐c Peroxidase

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Abstract
A new variant of cytochrome‐c peroxidase in which the positively charged Arg48 present in the distal heme‐binding pocket has been replaced with a Glu residue has been prepared and characterized to explore, in part, the possibility that a negative charge close to the heme could contribute to stabilization of a porphyrin‐centered π‐cation radical in the compound I derivative of the variant. Between pH 4 and 8, this variant forms three pH‐linked spectroscopic species. The electronic absorption and 1H‐NMR spectra of the predominant form at low pH (HS1) are indicative of a high‐spin, pentacoordinate heme iron system. Near neutral pH, a second high‐spin species (HS2) is dominant, in which the heme iron center is hexacoordinated, with a water molecule as the sixth axial ligand. At high pH, the third form (LS) exhibits the spectroscopic characteristics of a low‐spin, hexacoordinate heme center with bishistidine axial ligation. The apparent pKa values for these transitions are 4.4 and 7.4, respectively, in phosphate buffers and 5.0 and 7.1, respectively, in phosphate/nitrate buffers. Replacement of Arg48 with Glu reduces the thermal stability of the enzyme and also decreases the Fe(III)/Fe(II) reduction potential of the enzyme by approximately 50 mV relative to that of the wild‐type enzyme. The stability of compound I formed by the variant is decreased although the rate at which it forms is just one order of magnitude less than that of the wild‐type enzyme, thus confirming previous results which indicate that the function of residue 48 in the wild‐type peroxidase is more related to the stability of compound I than to its formation [Erman, J. E., Vitello, L. B., Miller, M. A. & Kraut, J. (1992) J. Am. Chem. Soc. 114, 6592–6593; Vitello, L. B., Erman, J. E., Miller, M. A., Wang, J. & Kraut, J. (1993) Biochemistry 32, 9807–9818]. Stopped‐flow studies failed to detect even transient formation of a porphyrin‐centered radical following addition of hydrogen peroxide to the Fe(III)‐enzyme. The consequences of this drastic electrostatic modification of the active site on the steady‐state kinetics of the variant are relatively minor.