Steric contributions to CO binding in heme proteins: a density functional analysis of FeCO vibrations and deformability

Abstract

Non-local Density Functional Theory (DFT) is applied to the calculation of geometry and vibrational frequencies of Fe II (porphine)(imidazole)(CO), a model for CO adducts of heme proteins. Bond distances and angles are in agreement with crystallographic data, and frequencies are correctly calculated for C-O and Fe-C stretching and for Fe-C-O bending. This last mode is actually the out-of-phase combination of Fe-C-O bending and Fe-C tilting coordinates, which are heavily mixed because of a large bend-tilt interaction force constant. The in-phase combination is predicted at a very low frequency, 73 cm À1, and to have a low infrared intensity; attempts to detect it in far-IR spectra via 13 C 18 O isotope sensitivity have been unsuccessful. The stretch-bend interaction lowers the energy required for FeCO distortion. A soft potential may account for the wide range of crystallographically determined Fe-C-O displacements and orientations in myoglobin (Mb). The minimum energy path for displacement of the O atom from the heme normal was calculated by relaxing the structure while constraining only the O atom displacement from the heme normal. Energies of 0.2 to 3.5 kcal mol À1 are required for the range of reported displacement, 0.3-1.3 Å . However, vibrational spectroscopy limits the allowable displacement to the low end of this range. The O atom displacement is computed via DFT to be 0.6 Å for a 7°angle of the C-O stretching IR dipole relative to the heme normal, the maximum value compatible with IR polarization measurements on MbCO. FeCO distortion is predicted to diminish both n CO and n FeC , thereby producing deviations from the well-established backbonding correlation; the scatter of the data permits a maximum displacement of 0.5 Å . This displacement would cost about 1.6 kcal mol À1 of steric energy. A small distortion energy is consistent with the CO affinity changes produced by mutations of the distal histidine residue in Mb. Taking the leucine mutant as reference, we estimate the 1.6 kcal mol À1 affinity loss in the wild-type protein to be the resultant of a 0.0-1.6 kcal steric inhibition, a 0.5 kcal mol À1 attraction of the distal histidine sidechain for the bound CO [weak H-bond], and a 0.5-2.1 kcal mol À1 attraction of the same side-chain for a water molecule in the deoxy protein. The observed 2.3 kcal mol À1 O 2 affinity increase in the wild-type protein relative to the leucine mutant then implies a 2.8-4.4 kcal mol À1 attraction of the histidine sidechain for bound O 2 , consistent with a substantial H-bond interaction with the distal histidine. Thus steric inhibition can account for only a minor fraction of the discrimination factor against CO and in favor of O 2 which is produced by the heme-myoglobin interaction.

New Light on NO Bonding in Fe(III) Heme Proteins from Resonance Raman Spectroscopy and DFT Modeling

“…Indeed, the slope of the bending correlation is the same as that produced by increasingly electron-withdrawing β -substituents (Figure 11, bottom red line). We note that a positive correlation was earlier computed for the Fe II –CO adduct, (ImH)Fe II (CO)P, upon bending the Fe II –C–O unit 57. In both cases bending weakens the Fe–N(C) and N(C)–O bonds simultaneously.…”

Electronic Structure and Ligand Vibrations in FeNO, CoNO, and FeOO Porphyrin Adducts

“…DFT calculations on FeCO adducts of model porphyrins have reproduced the observed backbonding trends satisfactorily, and have illuminated the sources of deviations in secondary interactions. 9,12,13 …”

Blocking the Gate to Ligand Entry in Human Hemoglobin

“…The central chemical events for these functions are movement of ligands into the distal portion of the heme pocket, internal bond formation with the heme iron, and electrostatic stabilization or steric hindrance of the bound ligand by surrounding active site amino acids. Although the quantum mechanical details of bond formation remain to be resolved, there is general agreement on the biophysical mechanisms governing steric hindrance and hydrogen bonding between amino acid side chains and the bound ligand (3)(4)(5)(6)(7)(8)(9)(10). In contrast, the pathways for ligands movement from solvent through the protein and into the active site are still under debate.…”