Modification of the distal histidyl imidazole in myoglobin to N-tetrazole-substituted imidazole and its effects on the heme environmental structure and ligand binding properties.
The mechanism of N-tetrazole ring formation at the distal histidyl imidazole of sperm whale myoglobin ( Mb) has been studied by nitrogen-15 (15N) NMR spectroscopy by utilizing 15N-labeled cyanogen bromide (BrCN) and azide ion (N3-). The 15N-NMR spectrum of BrC15N-modified Mb + N3- afforded two hyperfine-shifted 15N resonances, both of which are identical with the resonance positions of two of the three 15N resonances for BrCN-modified Mb + 15NN2-. This unusual spectral feature is due to the formation of the N-tetrazole ring attached to the distal histidyl imidazole and the scrambling of the labeled nitrogen originated from N3- or BrCN over the tetrazole ring upon coordination to the ferric heme iron. The ferric iron-bound N-tetrazole ring comes off upon reduction to the ferrous state, and the stable CO complex of tetrazole-modified Mb (tetrazole- Mb) is formed. Electronic absorption and 1H-NMR spectra of deoxy and carbonmonoxy forms of tetrazole- Mb are slightly altered from those of native Mb by the modification, while the most significant effect is exerted on the C-O stretching frequency of iron-bound CO. The C-O stretching band for tetrazole-MbCO is observed at 1966 cm-1 in contrast to 1945 cm-1 for native MbCO, suggesting that the geometry of iron-bound CO in tetrazole- Mb is relatively upright which is characteristic of the "open" conformer. This result corresponds to the 15-fold increase of the CO association rate constant by the N-tetrazole modification of the distal His. The oxy form of tetrazole- Mb is readily autoxidized to its ferric state, indicating that hydrogen bonding between the distal His and iron-bound oxygen is essential for stable O2 binding to the heme iron.[1]References
- Modification of the distal histidyl imidazole in myoglobin to N-tetrazole-substituted imidazole and its effects on the heme environmental structure and ligand binding properties. Adachi, S., Morishima, I. Biochemistry (1992) [Pubmed]
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