Differential Sensing of Protein Influences by NO and CO Vibrations in Heme Adducts

Abstract

Heme proteins bind the gaseous ligands XO (X = C, N, O) via backbonding from Fe d π electrons. Backbonding is modulated by distal interactions of the bound ligand with the surrounding protein and by variations in the strength of the trans proximal ligand. Vibrational modes associated with FeX and XO bond stretching coordinates report on these interactions, but the interpretive framework developed for CO adducts, involving anticorrelations of νFeC and νCO, has seemed not to apply to NO adducts. We have now obtained an excellent anticorrelation of νFeN and νNO, via resonance Raman spectroscopy on (N-methylimidazole)Fe(II)TPP-Y(NO), where TPP-Y is tetraphenylporphine with electron donating or withdrawing substituents, Y, that modulate the backbonding; the problem of laser-induced dissociation of the axial base was circumvented by using frozen solutions. New data are also reported for CO adducts. The anticorrelations are supported by DFT calculations of structures and spectra. When protein data are examined, the NO adducts show large deviations from the modeled anticorrelation when there are distal H-bonds or positive charges. These deviations are proposed to result from closing of the FeNO angle due to a shift in the valence isomer equilibrium toward the Fe(III)(NO -) form, an effect that is absent in CO adducts. The differing vibrational patterns of CO and NO adducts provide complementary information with respect to protein interactions, which may help to elucidate the mechanisms of ligand discrimination and signaling in heme sensor proteins.

Achievements in resonance Raman spectroscopy

“…These differing RRS vibrational patterns for CO and NO adducts may help to elucidate the mechanism of ligand discrimination and signaling in heme sensor proteins. Ibrahim et al [62] supported their experimental results with advanced theoretical calculations (i.e. density functional theory, DFT), which underlines the increased power and relevance of computational methods in current RRS (see also Section 4).…”

“…How does the heme domain discriminate between CO, NO and O 2 and how does the binding event lead to activation? In a recent paper the Spiro group gave a detailed interpretation of RRS results obtained using the 413.1 nm and the 406.7 nm lines of a Kr ion laser [62]. They demonstrated anticorrelation between the stretching FeC and CO vibration frequencies in the CO adduct and also for the FeN and NO vibrations in the NO adduct.…”

Role of Copper Ion in Regulating Ligand Binding in a Myoglobin-Based Cytochrome c Oxidase Model

“…Although the C c O family of enzymes possesses a heme group with histidine as the proximal ligand, their associated data points lie on a correlation line distinct from the histidine correlation line. The shift of the C c O data away from the typical histidine line has been attributed to a highly bent Fe-C-O moiety 36 and/or a compressed Fe-CO bond 37 due to the proximity of the Cu + .…”

DFT Analysis of Axial and Equatorial Effects on Heme−CO Vibrational Modes: Applications to CooA and H−NOX Heme Sensor Proteins

“…Imidazole H-bonding-To address the mechanism of Fe-His weakening computationally, we modeled His H-bonding by placing the H-bond acceptors H 2 O, NH 3 and HCOO − (formate) near the NH group of (ImH)FeP(CO) (P = porphine), a well-studied heme analog (5,6). The limiting case of imidazolate ligation was also investigated.…”