Nature of the iron-oxygen bond in oxyhemoglobin

Hemoglobin Hydrolyzate Promotes Iron Absorption in the Small Intestine through Iron Binding Peptides

Hemoglobin is an excellent source of iron supplements, and its hydrolyzate spontaneously binds iron during digestion and promotes iron absorption in vivo. However, the underlying mechanisms of what peptides bind and how they bind iron ions remain unclear. This study prepared the porcine hemoglobin hydrolyzate through enzymatic hydrolysis and acid treatment and investigated the mechanisms of hemoglobin hydrolyzate on iron absorption through the determination of iron levels in dietary intervention mice, iron binding site analyses, peptide digestion analyses, molecular simulation docking, and INT407 cell validation. The results showed that ingestion of the hemoglobin hydrolyzate diets increased iron levels in the blood of mice, accompanied by the upregulation of duodenal iron circulation-related genes such as ferritin, PCBP1, and HP. Carboxyl, imidazole groups, and aromatic amino acid residues were iron binding sites of hemoglobin hydrolyzate during digestion. VDEVGGEA and VDEVGGE were found to involve the spontaneous and efficient binding of hemoglobin hydrolyzate to iron ions in the intestinal cavity. In particular, the DEVGGE peptide was the typical sequence for hemoglobin hydrolytic peptides to exert iron binding activity.

Study on enhancement of hemoglobin antitoxic ability modified with chromium and ruthenium

Hemoglobin is essential for carrying oxygen (O₂) in the blood. However, its ability to bind excessively to carbon monoxide (CO) makes it susceptible to CO poisoning. To reduce the risk of CO poisoning, Cr-based heme and Ru-based heme were selected from among many transition metal-based hemes based on their characteristics of adsorption conformation, binding intensity, spin multiplicity, and electronic properties. The results showed that hemoglobin modified by Cr-based heme and Ru-based heme had strong anti-CO poisoning abilities. The Cr-based heme and Ru-based heme exhibited much stronger affinity for O₂ (-190.67 kJ/mol and -143.18 kJ/mol, respectively) than Fe-based heme (-44.60 kJ/mol). Moreover, Cr-based heme and Ru-based heme exhibited much weaker affinity for CO (-121.50 kJ/mol and -120.88 kJ/mol, respectively) than their affinity for O₂, suggesting that they were less likely to cause CO poisoning. The electronic structure analysis also supported this conclusion. Additionally, molecular dynamics analysis showed that hemoglobin modified by Cr-based heme and Ru-based heme was stable. Our findings offer a novel and effective strategy for enhancing the reconstructed hemoglobin's ability to bind O₂ and reduce its potential for CO poisoning.

Multireference and multiconfiguration ab initio methods in heme-related systems: what have we learned so far?

This work reviews the recent applications of ab initio multireference/multiconfiguration (MR/MC) electronic structure methods to heme-related systems, involving tetra-, penta-, and hexa-coordinate species, as well as the high-valent iron-oxo species. The current accuracy of these methods in the various systems is discussed, with special attention to potential sources of systematic errors. Thus, the review summarizes and tries to rationalize the key elements of MR/MC calculations, namely, the choice of the employed active space, especially the so-called double-shell effect that has already been recognized to be important in transition-metal-containing systems, and the impact of these elements on the spin-state energetics of heme species, as well as on the bonding mechanism of small molecules to the heme. It is shown that expansion of the MC wave function into one based on localized orbitals provides a compact and insightful view on some otherwise complex electronic structures. The effects of protein environment on the MR/MC results are summarized for the few available quantum mechanical/molecular mechanical (QM/MM) studies. Comparisons with corresponding DFT results are also made wherever available. Potential future directions are proposed.

Room-temperature magnetic properties of oxy- and carbonmonoxyhemoglobin

The magnetic susceptibility and the density of human oxy-(HbO(2)) and carbonmonoxyhemoglobin (HbCO) solutions of various concentrations have been measured at room temperature, with pure water used as a calibrant. Solutions of unstripped and stripped HbO(2) at pH 7.2 in unbuffered water solvent were always found to be less diamagnetic than pure water, whereas solutions of HbCO in identical conditions were always found to be more diamagnetic than pure water. After correcting for concentration-dependent density changes and assuming the HbCO samples to be fully diamagnetic, the paramagnetic reduction of the diamagnetic susceptibility of HbO(2) corresponds to a molar susceptibility per heme (chi(M) (heme)) of 2460 +/- 600 x 10(-6) cgs/mol.

Nature of the Fe-O2 bonding in oxy-myoglobin: effect of the protein

The nature of the Fe-O2 bonding in oxy-myoglobin was probed by theoretical calculations: (a) QM/MM (hybrid quantum mechanical/molecular mechanical) calculations using DFT/MM and CASSCF/MM methods and (b) gas-phase calculations using DFT (density functional theory) and CASSCF (complete active space self-consistent field) methods. Within the protein, the O2 is hydrogen bonded by His64 and the complex feels the bulk polarity of the protein. Removal of the protein causes major changes in the complex. Thus, while CASSCF/MM and DFT/MM are similar in terms of state constitution, degree of O2 charge, and nature of the lowest triplet state, the gas-phase CASSCF(g) species is very different. Valence bond (VB) analysis of the CASSCF/MM wave function unequivocally supports the Weiss bonding mechanism. This bonding arises by electron transfer from heme-Fe(II) to O2 and the so formed species coupled then to a singlet state Fe(III)-O2(-) that possesses a dative sigma(Fe-O) bond and a weakly coupled pi(Fe-O2) bond pair. The bonding mechanism in the gas phase is similar, but now the sigma(Fe-O) bond involves higher back-donation from O2(-) to Fe(III), while the constituents of pi(Fe-O2) bond pair have greater delocalization tails. The protein thus strengthens the Fe(III)-O2(-) character of the complex and thereby affects its bonding features and the oxygen binding affinity of Mb. The VB model is generalized, showing how the protein or the axial ligand of the oxyheme complex can determine the nature of its bonding in terms of the blend of the three bonding models: Weiss, Pauling, and McClure-Goddard.

Comparison of human oxyhemoglobin in lyophilized form, red blood cells, and concentrated solution: the features of Mössbauer spectra and heme iron stereochemistry

Mössbauer spectra of human oxyhemoglobin in red blood cells, concentrated solution, and lyophilized form were measured at 87 K. Additionally, Mössbauer spectra of lyophilized oxyhemoglobin were measured at 295 K. The values of quadrupole splitting appeared to be the same for oxyhemoglobin in red blood cells and concentrated solution and slightly lower than those of oxyhemoglobin in lyophilized form. The asymmetry of the Mössbauer absorption line shapes previously observed for oxyhemoglobin in red blood cells was also found for oxyhemoglobin in concentrated solution. These Mössbauer spectra were better fitted using two quadrupole split doublets with almost equal areas. In contrast, Mössbauer spectra of lyophilized oxyhemoglobin were symmetrical and satisfactorily fitted with one quadrupole split doublet. However, these spectra were also fitted with two quadrupole split doublets with equal areas. The variations of the absorption line shapes and parameters of oxyhemoglobin Mössbauer spectra were analyzed in terms of stereochemical differences of the heme iron and Fe(II)-O2 bond in alpha- and beta-subunits of tetrameric oxyhemoglobin.

Mössbauer spectroscopic studies of hemoglobin and its isolated subunits

Samples of 90% enriched 57Fe hemoglobin and its isolated subunits have been prepared. Mössbauer spectroscopic measurements have been made on three such samples. Sample one contained contributions of oxyhemoglobin, deoxyhemoglobin, and carbonmonoxyhemoglobin. This sample was studied from a temperature of 90 K down to 230 mK. Measurements were also made at 4.2 K using a small applied magnetic field of 1.0 T. In general, the measured quadrupole splittings and isomer shifts for each component agreed with previous measurements on single component samples in the literature, and thus demonstrated that chemically enriched hemoglobin has not been altered. The second and third samples were isolated alpha and beta subunits, respectively. We have found measurable Mössbauer spectral differences between the HbO2 sites in the alpha subunit sample and the beta subunit sample. The measured Mössbauer spectral areas indicate that the iron ion has the largest mean-square displacement at the deoxy Hb sites as compared to that at the oxy- and carbonmonoxy Hb sites. The mean-square displacement at the HbO2 sites is the smallest.

Electron capture at the iron-oxygen centre in single crystals of oxymyoglobin studied by electron spin resonance spectroscopy

Exposure of single crystals of oxy-myoglobin to 60Co gamma-rays at 77 K results in electron addition to the Fe-O2 units. The resulting ESR spectrum has been analysed to give the principal values and directions for the g-tensor of this unit. The results suggest that the dioxygen ligand is strongly tilted, the direction of tilt being close to one of the bisectors of the N-Fe-N bond angles. Possible reasons for this orientation are discussed.