Assignment of exchangeable proximal histidine resonances in high-spin ferric hemoproteins: Substrate binding in horseradish peroxidase

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Single-proton, exchangeable resonances have been detected in the high spin ferric hemoproteins, met-aquo myoglobin and horseradish peroxidase, which can be assigned to the proximal histidyl imidazole by virtue of their very large hyperfine shifts. While this proton is relatively labile in myoglobin, it is exchangeable in HRP only at extreme pH values, indicating a buried heme pocket. The insensitivity of the imidazole peak of HRP to substrate binding argues against direct interaction of imidazole and substrate.