Conversion of oxy- into methemerythrin in the presence of anions.

Bradić, Zdravko; Conrad, Ronald C.; Wilkins, Ralph G.

doi:10.1016/s0021-9258(17)40030-5

Cited by 25 publications

(18 citation statements)

References 23 publications

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“…DeoxyHrFNO is much more slowly reacting with 02 and undergoes a gradual change in its absorption spectrum over the course of several hours of exposure to 02. The ultimate product in this case appears to be metHrF, which is consistent with the known ability of F" to accelerate the autoxidation of oxyHr (Bradic et al, 1977).…”

Section: Resultssupporting

confidence: 82%

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Nitric oxide adducts of the binuclear iron site of hemerythrin: spectroscopy and reactivity

Nocek¹,

Kurtz²,

Sage³

et al. 1988

Biochemistry

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Nitric oxide forms adducts with the binuclear iron site of hemerythrin (Hr) at [Fe(II),Fe(II)]deoxy and [Fe(II),Fe(III)]semimet oxidation levels. With deoxyHr our results establish that (i) NO binds reversibly, forming a complex which we label deoxyHrNO, (ii) NO forms a similar but distinct complex in the presence of fluoride, which we label deoxyHrFNO, (iii) NO is directly coordinated to one iron atom of the binuclear pair in these adducts, most likely in a bent end-on fashion, and (iv) the iron atoms in the binuclear sites of both deoxyHrNO and deoxyHrFNO are antiferromagnetically coupled, thereby generating unique electron paramagnetic resonance (EPR) detectable species. The novel EPR signal of deoxyHrNO (deoxyHrFNO) with g[[ = 2.77 (2.58) and g = 1.84 (1.80) is explained by the magnetic interaction of the Fe(II) (S' = 2) and [FeNO]7 (S = 3/2) centers observed by Mössbauer spectroscopy. Antiferromagnetic coupling leads to a ground state of Seff = 1/2. Analysis of the EPR parameters using the isotropic spin-exchange Hamiltonian, Hex = 2JS3/2.S2, and including zero-field splitting leads to a coupling constant, -J approximately 23 cm-1, for deoxyHrNO. The resonance Raman spectrum of deoxyHrNO shows features at 433 and 421 cm-1 that shift downward with 15N16O and that are assigned to stretching and bending modes, respectively, of the [FeNO]7 unit. Sensitivity of the bending mode to D2O suggests that bound NO participates in hydrogen bonding. We propose that the terminal oxygen atom of NO is hydrogen bonded to the proton of the mu-hydroxo bridge in the Fe-(OH)-Fe unit. A bent Fe-N-O geometry is supported by spectroscopic and structural comparisons to synthetic complexes and is consistent with a limiting [FeII,FeIIINO-] formulation for deoxyHrNO. Reversibility of NO binding to deoxyHr is demonstrated by bleaching of the optical and EPR spectra of deoxyHrNO upon additions of excess N3- or CNO-. DeoxyHrNO undergoes autoxidation under anaerobic conditions over the course of several hours. The product of this autoxidation appears to be an EPR-silent NO adduct of semimetHr. The formal one-electron oxidations of the binuclear iron site of deoxyHr by NO and by HNO2 can conceivably occur with no net change in charge on the iron site. In contrast, autoxidation of oxy- to metHr requires a change in net charge on the iron site, which may provide a kinetic barrier.

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Section: Resultssupporting

confidence: 82%

“…The excess NO present then forms an adduct with the resulting semimetHr. The proposed loss of NO" during autoxidation of deoxyHrNO parallels the loss of 022" suggested for autoxidative conversion of oxy-to metHr (Bradic et al, 1977). Autoxidation of deoxyHrFNO is much slower.…”

Section: Discussionmentioning

confidence: 53%

Nitric oxide adducts of the binuclear iron site of hemerythrin: spectroscopy and reactivity

Nocek¹,

Kurtz²,

Sage³

et al. 1988

Biochemistry

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“…The half-life for conversion in 0.05 M azide is ~25 min at pH 7.7. In addition, bleaching of the L. reevii oxy spectrum is observed upon addition of azide at higher pH values, as found for P. gouldii Hr (Bradic et al, 1977).…”

Section: Resultssupporting

confidence: 61%

“…As found for sipunculid Hr, L. reevii oxyhemerythrin could be converted to the met form by oxidation with ferricyanide. The rapid autooxidation of oxyHr to metazidoHr in the presence of azide (Bradic et al, 1977) is also observed. The half-life for conversion in 0.05 M azide is ~25 min at pH 7.7.…”

Section: Resultsmentioning

confidence: 83%

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Allosteric interactions in sipunculid and brachiopod hemerythrins

Richardson¹,

Emad²,

Reem³

et al. 1987

Biochemistry

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Chemical and spectroscopic consequences of allosteric interactions for ligand binding to sipunculid (Phascolopsis gouldii) and brachiopod (Lingula reevii) hemerythrins (Hrs) have been investigated. Possible allosteric effectors for homotropic effects in sipunculid Hrs have been examined, but only reduction in ligand affinity is observed without cooperativity. In contrast to sipunculid Hr, L. reevii Hr binds O2 cooperatively in the pH range 7-8 and exhibits a Bohr effect. Spectroscopic comparisons of the sipunculid and brachiopod Hrs show no significant differences in the active site structures; therefore, modulation of oxygen affinity is attributable to effects linking the site to quaternary structural changes in the octamer. Oxygen equilibria can be fit with a conformational model incorporating a minimum of three states, tensed (T), relaxed (R), and an R-T hybrid. Resonance Raman spectra of L. reevii oxyHr show a shift in the peroxo stretching frequency when the pH is lowered from pH 7.7 (predominantly R oxyHr) to pH 6.3 (a mixture of R, T, and R-T hybrid), but P. gouldii Hr does not have a frequency shift under the same conditions. In contrast to hemoglobins, ligand binding to the deoxy and met forms is noncooperative for brachiopod (and sipunculid) Hrs. It is thus suggested that conformational changes in the protein are linked to the oxidation state change that accompanies oxygenation of the coupled binuclear iron site (deoxy [FeIIFeII]----oxy [FeIIIFeIII]). The total allosteric energy expended in oxygenation is about 1.4 kcal/mol, and such a shift is possible in the relaxed-tense conversion with relatively limited constraints of the iron coordination environment via the protein quaternary structure. The mechanism of cooperativity in the binuclear copper oxygen carrier hemocyanin is discussed in light of these results.

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ESR of Iron Proteins

Smith¹,

Pilbrow²

1980

Biological Magnetic Resonance

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Conversion of oxy- into methemerythrin in the presence of anions.

Cited by 25 publications

References 23 publications

Nitric oxide adducts of the binuclear iron site of hemerythrin: spectroscopy and reactivity

Nitric oxide adducts of the binuclear iron site of hemerythrin: spectroscopy and reactivity

Allosteric interactions in sipunculid and brachiopod hemerythrins

ESR of Iron Proteins

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