Structural evolution of the heme group during the allosteric transition in hemoglobin: Insights from resonance raman spectra of isotopically labeled heme
✍ Scribed by Vasanthi Jayaraman; Thomas G. Spiro
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 502 KB
- Volume
- 2
- Category
- Article
- ISSN
- 1075-4261
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✦ Synopsis
Visible resonance Raman spectra in the low-frequency region (200-500 cm-') are reported for hemoglobin ( H b ) reconstituted with heme that is deuterated at the meso carbon atoms (meso-d,). Spectra were obtained in the deoxy form and in the immediate photoproduct of the carbonmonoxide adduct, HbCO. The isotope shifts permit assignment of two out-of-plane modes, y6 and y 7 , and the in-plane skeletal mode us, as well as the well-known iron-histidine [ Fe-His] stretching vibration. Important differences between deoxyHb and the immediate photoproduct include 1 ) a large upshift in the Fe-His frequency, from 216 to 228 cm-', 2 ) a n upshift in ys (349 to 353 cm-') together with substantial diminution of the v8 (341 cm-') intensity, and 3 ) collapse of two y7 bands (305 and 296 cm ' ) to a single band a t 304 cm-' . This last observation implies subunit heterogeneity in deoxyHb but not in the photoproduct. When these bands are monitored in the time-resolved RR spectra following HbCO photolysis, it is seen that subunit heterogeneity is first detectable in the 0.5-ps transient [intermediate S ] , which has been associated with the initial rearrangement of the subunits to form the T-state contacts, on the basis of ultraviolet RR spectroscopy.' However the intensification of us does not occur until the 17-ps transient (intermediate T I ) , in which the T-state contacts are locked in and the Fe-His bond is strained. Implications for the mechanism of Hb allostery are discussed. ( ~1 1996 John Wiley & Sons, Inc.