Hydrogen Bond Network in the Distal Site of Peroxidases: Spectroscopic Properties of Asn70 → Asp Horseradish Peroxidase Mutant

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Abstract
The distal His in peroxidases forms a hydrogen bond with the adjacent Asn, which is highly conserved among many plant and fungal peroxidases. Our previous work [Nagano, S., Tanaka, M., Ishimori, K., Watanabe, Y., & Morishima, I. (1996) Biochemistry 35, 14251−14258] has revealed that the replacement of Asn70 in horseradish peroxidase C (HRP) by Val (N70V) and Asp (N70D) discourages the oxidation activity for guaiacol, and the elementary reaction rate constants for the mutants was decreased by 10−15-fold. In order to delineate the structure−function relationship of the His−Asn couple in peroxidase activity, heme environmental structures of the HRP mutant, N70D, were investigated by CD, 1H NMR, and IR spectroscopies as well as Fe2+/Fe3+ redox potential measurements. While N70D mutant exhibited quite similar CD spectra and redox potential to those of native enzyme, the paramagnetic NMR spectrum clearly showed that the hydrogen bond between the distal His and Asp70 is not formed in the mutant. The disappearance of the splitting in the 1H NMR signal of heme peripheral 8-methyl group observed in 50% H2O/50% D2O solution of N70D-CN suggests that the hydrogen bond between the distal His and heme-bound cyanide is also disrupted by the mutation, which was supported by the low C−N vibration frequency and large dissociation constant of the heme-bound cyanide in the mutant. Together with the results from various spectroscopies and redox potentials, we can conclude that the improper positioning of the distal His induced the cleavages of the hydrogen bonds around the distal His, resulting in the substantial decrease of the catalytic activity without large structural alterations of the enzyme. The His−Asn hydrogen bond in the distal site of peroxidases, therefore, is essential for the catalytic activity by controlling the precise location of the distal His.

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