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A basic heme peroxidase has been isolated from cucumber (Cucumis sativus) peelings and characterized through electronic and 1H NMR spectra from pH 3 to 11. The protein, as isolated, contains a high-spin ferriheme which in the low pH region is sensitive to two acid–base equilibria with apparent pKa values of approximately 5 and 3.6, assigned to the distal histidine and to a heme propionate, respectively. At high pH, a new low-spin species develops with an apparent pKa of 11, likely due to the binding of an hydroxide ion to the sixth (axial) coordination position of the Fe(III). A number of acid–base equilibria involving heme propionates and residues in the distal cavity also affect the binding of inorganic anions such as cyanide, azide, and fluoride to the ferriheme, as well as the catalytic activity. The reduction potentials of the native protein and of its cyanide derivative, determined through UV–Vis spectroelectrochemistry, result to be −0.320 ± 0.015 and −0.412 ± 0.010 V, respectively. Overall, the reactivity of this protein parallels those of other plant peroxidases, especially horseradish peroxidase. However, some differences exist in the acid–base equilibria affecting its reactivity and in the reduction potential, likely as a result of small structural differences in the heme distal and proximal cavities. [Copyright &y& Elsevier]