Structural dynamics of myoglobin: FTIR-TDS study of NO migration and binding

By using Fourier transform infrared photolysis difference spectroscopy combined with temperature derivative spectroscopy at cryogenic temperatures, we have measured infrared spectra of the stretching absorption on nitric oxide (NO) in the heme-bound and photodissociated states of ferrous and ferric nitrosyl myoglobin (MbNO) and a few site-specific Mb mutants. The NO absorption was utilized as a sensitive local probe of ligand interactions with active-site residues and movements within the protein. By comparison with results obtained in previous spectroscopic and structural studies of carbonmonoxy myoglobin (MbCO), the MbNO data were interpreted in structural terms. In the NO-bound state, conformational heterogeneity was inferred from the appearance of multiple bands, arising from different electrostatic interactions with active site residues, most importantly, His-64. In ferrous MbNO, a primary photoproduct site similar to site B of MbCO was found, as indicated by a characteristic NO stretching spectrum. In ferric MbNO, the His-64 side chain appears to interfere with trapping of NO in this site; only a very weak photoproduct spectrum was observed in Mb variants in which His-64 was present. Upon extended illumination, the photoproduct spectrum changed in a characteristic way, indicating that NO readily migrates to a secondary docking site C, the Xe4 cavity, in which the ligand performs librational motions on the picosecond time scale. This docking site may play a role in the physiological NO scavenging reaction. Surprisingly, NO cannot be trapped at all in secondary docking site D, the Xe1 cavity.