Metastable photoproducts from carbon monoxide myoglobin

Area of Science:

• Biophysics
• Protein dynamics
• Spectroscopy

Background:

• Myoglobin is a crucial protein for oxygen transport.
• Carbon monoxide myoglobin (MbCO) is a common model system for studying heme protein dynamics.
• Understanding heme relaxation and protein structural changes is key to elucidating MbCO function.

Purpose of the Study:

• To investigate the structural and dynamic properties of the carbon monoxide myoglobin photoproduct at cryogenic temperatures.
• To characterize the heme environment and porphyrin core size in the photoproduct using resonance Raman spectroscopy.
• To determine the influence of temperature on heme relaxation and protein structural rearrangements.

Main Methods:

• Generation of the carbon monoxide myoglobin photoproduct at temperatures as low as 4 K.
• Resonance Raman spectroscopy to probe heme vibrational modes and core size.
• Comparison of spectral features with deoxy myoglobin and photoproducts generated at higher temperatures.

Main Results:

• The photoproduct at 4 K exhibits a resonance Raman spectrum indicative of a high-spin heme.
• High-frequency core size-sensitive lines in the 4 K photoproduct are at lower frequencies than in deoxymyoglobin.
• These spectral differences are absent in photoproducts generated at higher temperatures (50 K) or room temperature.

Conclusions:

• At low temperatures (4 K), the heme in the carbon monoxide myoglobin photoproduct is not fully relaxed.
• The data strongly suggest an expanded porphyrin core in the low-temperature photoproduct.
• The rigid protein structure at cryogenic temperatures likely restricts the out-of-plane motion of the high-spin iron atom, leading to the expanded core.