Direct Probe of Iron Vibrations Elucidates NO Activation of Heme Proteins

Abstract

Nuclear resonance vibrational spectroscopy (NRVS) is an emerging site-specific probe of active site vibrational dynamics in metalloproteins. 1,2 NRVS is a synchrotron-based technique that uniquely targets the vibrations of a Mössbauer nucleus, such as 57 Fe, without interference from vibrations of other atoms, and reveals not only the frequency, but also the (mean squared) amplitude, 1b,2a of all vibrations of the probe nucleus along the direction of the incident X-ray beam. Quantitative characterization of vibrational modes involving a reactive probe atom can illuminate mechanisms of complex biomolecules.Reactions with heme proteins mediate the physiological effects of nitric oxide (NO). The proposed trigger for activation of soluble guanylate cyclase (sGC) is rupture of the covalent Fe-His bond 3a,b in the heme-containing domain 3c,d upon NO binding to the Fe. A thermodynamic consequence of NO-induced weakening of a trans Fe-imidazole bond, as observed in several heme systems, is that imidazole binding should weaken a trans Fe-NO bond. Model compound structures support such a reciprocal negative trans interaction, 4a although protein structural data 4b-d may not have sufficient precision to resolve the 3 pm increase in Fe-NO bond length due to imidazole binding.On the other hand, vibrational frequencies respond sensitively to bond length changes of this magnitude, and it is therefore puzzling that the frequency attributed to stretching of the Fe-NO bond in six-coordinate imidazole-ligated heme proteins 5 is higher, rather than lower, than the frequencies observed for five-coordinate iron nitrosyl hemes. For example, the assigned Fe-NO stretching frequencies of the 5-and 6-coordinate NO complexes with myoglobin (MbNO) are 521 cm −1 and 552 cm −1 , respectively. 5c NRVS measurements on 6-coordinate MbNO suggest reexamination of this issue (Fig. 1A). The Fe-weighted vibrational density of states (VDOS) D(ν) samples the vibrational kinetic energy distribution (KED), with each mode contributing an area e Fe 2 equal to the fraction of mode energy associated with Fe motion. 2a,d The e Fe 2 = 0.11 area of the feature at 547 cm −1 is well below the e Fe 2 = 0.23-0.33 range that we observed for the FeNO stretching mode in a series of 5-coordinate nitrosyl porphyrins, 2d but is clearly visible because of the distinctly improved signal quality compared to previously published NRVS measurements on myoglobin. 1a,b,d In contrast, a mode with an area e Fe 2 = 0.25 appears at 443 cm −1 , near a

Explorations in metalloporphyrin stereochemistry, physical properties and beyond

“…Similar observations have been made for six-coordinate nitrosyl derivatives; these studies also have provided an emendation concerning the nature of the Fe-N and Fe-N-O stretch and bend [104,105]. Normal coordinate analyses of the spectra for four-coordinate complex [Fe(OEP)] have also appeared [106,107].…”

Vibrational dynamics of the host framework in Sn clathrates

“…Common Raman and NRIXS bands have been observed before [47][48][49] . In the present study however, the predominance of Sn atoms in the compounds investigated leads to remarkable, uncommon similarities between the Raman and NRIXS spectra, both for KZS and BGS.…”

Heme-Nitrosyls: Electronic Structure Implications for Function in Biology

“…Further QCC-NCA simulations then allowed us to unequivocally assign the Fe− NO stretch in [Fe(TPP)(MI)(NO)] to the feature at 437 cm −1 , 34 and a similar assignment is reported for Mb(II)-NO. 37 Analysis These experimental findings are used to calibrate density functional theory (DFT)-calculated electronic structure descriptions. 25,27,33,34,39 This is particularly important here since ferrous heme-nitrosyls constitute significant challenges for DFT.…”