FeNO Structure in Distal Pocket Mutants of Myoglobin Based on Resonance Raman Spectroscopy
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- 作者：Candace M. Coyle ; Kathleen M. Vogel ; Thomas S. Rush ; III ; Pawel M. Kozlowski ; Robert Williams ; Thomas G. Spiro ; Yi Dou ; Masao Ikeda-Saito ; John S. Olson ; and Marek Z. Zgierski
- 刊名：Biochemistry
- 出版年：2003
- 出版时间：May 6, 2003
- 年：2003
- 卷：42
- 期：17
- 页码：4896 - 4903
- 全文大小：210K
- 年卷期：v.42,no.17(May 6, 2003)
- ISSN：1520-4995
文摘
FeNO vibrational frequencies were investigated for a series of myoglobin mutants using isotope-edited resonance Raman spectra of 15/14NO adducts, which reveal the FeNO and NO stretching modes.The latter give rise to doublet bands, as a result of Fermi resonances with coincident porphyrin vibrations;these doublets were analyzed by curve-fitting to obtain the 
NO frequencies. Variations in NO amongthe mutants correlate with the reported CO variations for the CO adducts of the same mutants. Thecorrelation has a slope near unity, indicating equal sensitivity of the NO and CO bonds to polar influencesin the heme pocket. A few mutants deviate from the correlation, indicating that distal interactions differfor the NO and CO adducts, probably because of the differing distal residue geometries. In contrast to thestrong and consistent FeC/CO correlation found for the CO adducts, FeN correlates only weakly withNO, and the slope of the correlation depends on which residue is being mutated. This variability issuggested to arise from steric interactions, which change the FeNO angle and therefore alter the Fe-NOand N-O bond orders. This effect is modeled with Density Functional Theory (DFT) and is rationalizedon the basis of a valence isomer bonding model. The FeNO unit, which is naturally bent, is a moresensitive reporter of steric interactions than the FeCO unit, which is naturally linear. An important additionalfactor is the strength of the bond to the proximal ligand, which modulates the valence isomer equilibrium.The FeNO unit is bent more strongly in MbNO than in protein-free heme-NO complexes because of acombination of a strengthened proximal bond and distal interactions.
NO frequencies. Variations in NO amongthe mutants correlate with the reported CO variations for the CO adducts of the same mutants. Thecorrelation has a slope near unity, indicating equal sensitivity of the NO and CO bonds to polar influencesin the heme pocket. A few mutants deviate from the correlation, indicating that distal interactions differfor the NO and CO adducts, probably because of the differing distal residue geometries. In contrast to thestrong and consistent FeC/CO correlation found for the CO adducts, FeN correlates only weakly withNO, and the slope of the correlation depends on which residue is being mutated. This variability issuggested to arise from steric interactions, which change the FeNO angle and therefore alter the Fe-NOand N-O bond orders. This effect is modeled with Density Functional Theory (DFT) and is rationalizedon the basis of a valence isomer bonding model. The FeNO unit, which is naturally bent, is a moresensitive reporter of steric interactions than the FeCO unit, which is naturally linear. An important additionalfactor is the strength of the bond to the proximal ligand, which modulates the valence isomer equilibrium.The FeNO unit is bent more strongly in MbNO than in protein-free heme-NO complexes because of acombination of a strengthened proximal bond and distal interactions.