Electronic structure of Fe2+ in normal human hemoglobin and its isolated subunits

Huynh, B.H.; Papaefthymiou, Georgia C.; Yen, C. S.; Groves, J.L.; Wu, Cheng-Wen

doi:10.1063/1.1682561

Cited by 77 publications

(29 citation statements)

References 40 publications

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“…Thus, these states are nearly degenerate within the uncertainty of current methods (ϳ10 kJ/mol), which makes it hard to assign the spectrum in detail. However, the result is in qualitative agreement with the fact that all three spin states have been found from Mössbauer spectroscopy within 10 kJ/mol ferrous myoglobin and hemoglobin (corresponding to an excitation at 12,000 nm) (47)(48)(49) and the observation of a low-lying triplet state in thermal equilibrium with the singlet ground state of oxyheme at temperatures between 25 and 250 K (50).…”

Section: Resultssupporting

confidence: 75%

“…Such near degeneracy is a basic feature of many porphyrins (7) and is supported by Mössbauer spectroscopy, which finds all three spin states within 10 kJ/mol in ferrous hemoand myoglobin (47)(48)(49). In fact, we have earlier shown that it is an intrinsic property of porphyrin to produce a small splitting between the various spins states of iron, because the central cavity of the ring is too large for low-spin iron (11).…”

Section: Resultsmentioning

confidence: 93%

“…2, it can be seen that for the possible crossing points discussed above (i.e. for Fe-O distances longer than 2.5 Å), all curves for the relevant four spin states are less than 15 kJ/mol above the energy of the dissociated states (allowing for a downshift of the heptet and triplet curves to the experimentally observed quintet-triplet splitting (47)(48)(49)). This means that the activation enthalpy should be lower than this.…”

Section: Resultsmentioning

confidence: 99%

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How O2 Binds to Heme

Jensen

Ryde

2004

Journal of Biological Chemistry

184

189

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We have used density functional methods to calculate fully relaxed potential energy curves of the seven lowest electronic states during the binding of O 2 to a realistic model of ferrous deoxyheme. Beyond a Fe-O distance of ϳ2.5 Å, we find a broad crossing region with five electronic states within 15 kJ/mol. The almost parallel surfaces strongly facilitate spin inversion, which is necessary in the reaction of O 2 with heme (deoxyheme is a quintet and O 2 a triplet, whereas oxyheme is a singlet). Thus, despite a small spin-orbit coupling in heme, the transition probability approaches unity. Using reasonable parameters, we estimate a transition probability of 0.06 -1, which is at least 15 times larger than for the nonbiological Fe-O ؉ system. Spin crossing is anticipated between the singlet ground state of bound oxyheme, the triplet and septet dissociation states, and a quintet intermediate state. The fact that the quintet state is close in energy to the dissociation couple is of biological importance, because it explains how both spin states of O 2 may bind to heme, thereby increasing the overall efficiency of oxygen binding. The activation barrier is estimated to be <15 kJ/mol based on our results and Mö ssbauer experiments. Our results indicate that both the activation energy and the spin-transition probability are tuned by the porphyrin as well as by the choice of the proximal heme ligand, which is a histidine in the globins. Together, they may accelerate O 2 binding to iron by ϳ10 11 compared with the Fe-O ؉ system. A similar near degeneracy between spin states is observed in a ferric deoxyheme model with the histidine ligand hydrogen bonded to a carboxylate group, i.e. a model of heme peroxidases, which bind H 2 O 2 in this oxidation state.All electrons have a spin, which is an intrinsic quantum chemical property that can take only two possible values, normally called ␣ and ␤ (or spin up and down). Almost all normal organic molecules contain an even number of electrons and also an equal number of ␣ and ␤ electrons. They are then said to have paired spin or to be singlets. Molecular oxygen (O 2 ) is a famous exception to this rule. In its ground state, it has two more electrons of one spin state than the other. Thus, it is said to have two unpaired electrons or to be a triplet. The singlet state of O 2 , with all electron spins paired, is ϳ90 kJ/mol higher in energy than the triplet ground state (1).A chemical reaction can normally not change the spin state of an electron. Therefore, reactions between singlet and triplet states are formally spin-forbidden, which means that they are slow. This is the reason why organic matter may exist in an atmosphere containing much O 2 . There is a strong thermodynamic drive of O 2 to oxidize organic matter to H 2 O and CO, but because these products (as well as the organic molecules) are singlets (whereas O 2 is a triplet), this reaction is spin-forbidden and therefore very slow at ambient temperatures. On the other hand, this is a problem when living organisms want to emp...

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Section: Resultssupporting

confidence: 75%

Section: Resultsmentioning

confidence: 93%

Section: Resultsmentioning

confidence: 99%

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How O2 Binds to Heme

Jensen

Ryde

2004

Journal of Biological Chemistry

184

189

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“…These mechanisms are evaluated here by examining the dynamics and identifying the products of the photolysis by picosecond spectroscopy. We have attempted to describe our spectroscopic and dynamical results in terms of recent experimental and theoretical assessments of the excited states of hemoglobins (11)(12)(13)(14)(15).…”

mentioning

confidence: 99%

Geminate recombination of O2 and hemoglobin.

Chernoff¹,

Hochstrasser²,

Steele³

1980

Proc. Natl. Acad. Sci. U.S.A.

146

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The photolysis of HbO2 and HbCO has been studied by measuring transient absorption spectra in the Soret region after excitation with picosecond pulses at 530 nm. Dissociation occurred promptly in both cases, followed (for HbO2) by geminate recombination of ca. 40% of the photodissociated O2 with a lifetime of 200 ± 70 psec (25°C). No to 300 psec. The initial MbCO photoproduct signal at 440 nm was reported to undergo a 15% decay (lifetime r = 125 + 50 psec), but for MbO2 no evidence of decay up to 450 psec was found. Greene et al. (8) conducted a study of HbO2 and HbCO photolysis using 8-psec, 353-nm excitation. They obtained transient absorption spectra in the Soret (400-450 nm) and visible (535-575 nm) regions at 10 psec and 680 psec, finding a persistent, broadened deoxy Hb-like absorption. With their spectra, they observed less than 10% recombination in HbCO over this time interval and less than 20% recombination of photolyzed HbO2. Greene et al. (8) found that HbCO responded similarly to 530-and 358-nm pulses. The present study of the 530-nm photolysis of HbCO and HbO2 is aimed at identifying the species initially generated by light and how these species evolve.Photodissociation of liganded hemoglobins is a well-known phenomenon (1) used to study dynamics of ligand binding to deoxyhemoglobin (Hb) (2)(3)(4)(5)(6)(7)(8). The usefulness of the photoprocess for subsequent dynamical studies is restricted at present by a lack of knowledge of the mechanism of the photodissociation. Of particular interest is the quantum yield for dissociation under steady illumination, which is fairly high (k 0.5) (9, 10) for carboxyhemoglobin (HbCO) but much lower (0 0.05) for the oxy form (HbO2) (9). Two limiting mechanisms can account for the relatively low yield of HbO2 photolysis. First, energy relaxation to nonreactive states may be much faster than the dissociation. Second, although the initial dissociation step in HbO2 may be just as efficient as that in HbCO, some released 02 trapped nearby to the heme may be able to rapidly recombine. These mechanisms are evaluated here by examining the dynamics and identifying the products of the photolysis by picosecond spectroscopy. We have attempted to describe our spectroscopic and dynamical results in terms of recent experimental and theoretical assessments of the excited states of hemoglobins (11-15).The time scale for studying photolysis and subsequent events in heme proteins has in recent years been pressed back to the picosecond regime, starting with the work of Shank et al. (5). Using subpicosecond pulses at 615 nm to excite and probe HbO2 and HbCO, they found induced absorption build-up in less than 0.5 psec for both species, a decay of the HbO2 signal in 2.5 psec, and no decay of the HbCO signal up to 20 psec. Rentzepis and coworkers (6, 7) studied HbCO and the myoglobin compounds MbCO and MbO2 by exciting with 6-psec 530-nm pulses and using 440-nm interrogation over the period 0-00 psec. That group reported a significantly longer (11 psec) predissociation lifetime...

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“…Nakano et al (9,28) have shown that the susceptibility data in deoxygenated Hb and Mb can be adequately described with a Hamiltonian of the form DS' + E(S' -Sy) + 9B [1] with the crystal field parameters D and E, the Bohr magneton uB, the g-tensor g, and the magnetic field H. x, y, and z are the principal axes of the crystal field tensor. We use this equation to parameterize the data without attempting to relate the parameters to the details of the electronic level scheme (29)(30)(31)(32)(33). For N noninteracting identical molecules of spin 2, the components of the magnetization along the crystal field axes are given by…”

mentioning

confidence: 99%

Comparison of the magnetic properties of deoxy- and photodissociated myoglobin.

Röder¹,

Berendzen²,

Bowne³

et al. 1984

Proc. Natl. Acad. Sci. U.S.A.

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The magnetic susceptibility of photodissociated carbon monoxy myoglobin has been measured over the temperature range from 1.7 to 25 K at 10 and 50 kG with a superconducting susceptometer. The spin and the crystal field parameters of the iron ion were extracted by a spin Hamiltonian approach. Under equivalent conditions the magnetic susceptibility of deoxy myoglobin was measured. In both experiments the CO-bound protein was used as a diamagnetic reference. Above about 5 K the metastable photolysed state and the equilibrium deoxy form of myoglobin are magnetically indistinguishable and can be fitted with S = 2 and g = 2. The transition from spin 0 to spin 2 and the conformational changes known to accompany the electronic change thus also occur after photolysis at low temperature. At temperatures below 5 K, differences become apparent, indicating a somewhat smaller zero-field splitting in the photoproduct as compared to the ligand-free state at equilibrium. In qualitative agreement with observations made by other techniques, the data imply that even at 1.7 K substantial structural relaxation occurs in the heme region of myoglobin after photodissociation. The results are important for the interpretation of the ligand binding kinetics after flash photolysis at low temperature and contribute to the understanding of the relationship between electronic structure and function in heme proteins. Heme proteins perform a wide variety of functions, from oxygen storage to detoxification. Since they all share the same prosthetic group, a detailed understanding of the function of one heme protein can provide insight into a broader field. The binding of small ligands such as dioxygen and CO to ferrous Mb and Hb is probably the simplest such biological process and it has been studied with a variety of tools (1-4). Flash photolysis experiments over a wide range of time and temperature have established that the controlling reaction in ligand binding occurs at the heme group and involves the heme iron (5, 6). In the discussion of the crucial association and dissociation step it is generally assumed that the ligandfree (deoxy) form of the ferrous heme is in the high-spin state (S = 2) while the ligand-bound form is in the low-spin state (S = 0). Consequently, association and dissociation of 02 and CO are assumed to be accompanied by a spin change of 2.The assignment of the spin states is based on the pioneering magnetic susceptibility measurements of Pauling and Coryell (7) on Hb and related proteins. MbCO was shown to be diamagnetic by Theorell and Ehrenberg (8). The magnetic properties of deoxy Hb and Mb have been studied by Nakano et al. (9). In addition to verifying the spin state they characterized the electronic fine structure of the iron in these heme proteins.Ligand binding not only modifies the electronic structure of the heme iron but also induces distinct changes in the conformation of the heme and its environment (10). X-ray crystallography shows that in deoxy Mb (11) the iron ion lies out of the mean heme plane by...

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Electronic structure of Fe2+ in normal human hemoglobin and its isolated subunits

Cited by 77 publications

References 40 publications

How O2 Binds to Heme

How O2 Binds to Heme

Geminate recombination of O2 and hemoglobin.

Comparison of the magnetic properties of deoxy- and photodissociated myoglobin.

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