The coordination chemistry of the binuclear iron site in hemerythrin

Wilkins, Patricia C.; Wilkins, Ralph G.

doi:10.1016/0010-8545(87)80003-6

Cited by 148 publications

(69 citation statements)

References 78 publications

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“…Two possible candidates for the 650-nm chromophore are a diferricperoxo species, in which the peroxo ligand bridges Fe1 and Fe2 (42,43), or a mixed-valent Fe II Fe III species resulting from "outer-sphere" one-electron oxidation of the di-iron site by O 2 (44). Although mixed-valent forms of Hr are well known (45), they have not been detected during autoxidation of native P. gouldii Hr (15). Whatever the nature of the 650-nm intermediate may be, it leads to much more rapid autoxidation than any other L98X Hr tested.…”

Section: Discussionmentioning

confidence: 99%

“…Two mutations of the residue corresponding to Leu-98 in a myoHr, the monomeric counterpart to Hr, led to rapid autoxidation of the di-iron site, such that O 2 binding kinetics and equilibria could not be measured (14). Autoxidation, formulated as Reactions 2 and/or 3, is presumably initiated by solvent entry into the binding pocket of the oxy form (15,16).…”

Section: Hemerythrin (Hr)mentioning

confidence: 99%

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A Leucine Residue “Gates” Solvent but Not O2Access to the Binding Pocket of Phascolopsis gouldiiHemerythrin

Farmer¹,

Kurtz²,

Phillips³

et al. 2000

Journal of Biological Chemistry

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A leucine residue, Leu-98, lines the O 2 -binding pocket in all known hemerythrins. Leu-98 in recombinant Phascolopsis gouldii hemerythrin, was mutated to several other residues of varying sizes (Ala, Val), polarities (Thr, Asp, Asn), and aromaticities (Phe, Tyr, Trp). UVvisible and resonance Raman spectra showed that the di-iron sites in these L98X Hrs are very similar to those in the wild type protein, and several of the L98X hemerythrins formed stable oxy adducts. Despite the apparently tight packing in the pocket, all of the L98X Hrs except for L98W, had second order O 2 association rate constants within a factor of 3 of the wild type value. Similarly, the O 2 dissociation rate constant was essentially unaffected by substitutions of larger (Phe) or smaller (Val, Thr) residues for Leu-98. L98Y Hr showed a 170-fold decrease in the O 2 dissociation rate constant and a large D 2 O effect on this rate, which are attributed to a hydrogen-bonding interaction between the Tyr-98 hydroxyl and the bound O 2 . Significant increases in autoxidation rates were observed for all of the L98X Hrs other than X ‫؍‬ Tyr. These increases in autoxidation rates are attributed to increased solvent access to the binding pocket caused by inefficient packing (Phe), smaller size (Val, Ala), or increased polarity (Thr, Asp, Asn) of the residue 98 side chain. A leucine at position 98 appears to have the optimal size, shape, and hydrophobicity for inhibition of solvent access. Thus, "gating" of small molecule access to the binding pocket of Hr by Leu-98 is not evident for O 2 , but is evident for solvent.

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Section: Discussionmentioning

confidence: 99%

Section: Hemerythrin (Hr)mentioning

confidence: 99%

A Leucine Residue “Gates” Solvent but Not O2Access to the Binding Pocket of Phascolopsis gouldiiHemerythrin

Farmer¹,

Kurtz²,

Phillips³

et al. 2000

Journal of Biological Chemistry

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“…Hemerythrin (Hr), among the non-heme diiron proteins, is the only example of a dioxygen carrier which is found in a limited number of marine organisms [1,2]. Because of its low abundance, Hr may be a dead end of evolution and is even called a 'Cinderella' in the family of dioxygen carriers [3]. The chemistry of this metalloprotein, however, was extensively studied and provided an important structural, spectroscopic and mechanistic benchmark for understanding other, more complicated diiron systems.…”

Section: Introductionmentioning

confidence: 99%

Mechanistic Studies on the Oxidation of Hydroquinone by an Oxo-bridged Diiron(III,III) Complex in Weakly Acidic Aqueous Media

Bhattacharyya

Mukhopadhyay

2006

Transition Met Chem

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The kinetics of oxidation of hydroquinone (H 2 Q) by a l-oxo-bridged diiron(III,III) complex, ½Fe 2 ðl-OÞ-ðphenÞ 4 ðH 2 OÞ 2 4þ (1) has been investigated in aqueous media at 25.0°C in presence of an excess of 1,10-phenanthroline (phen). The overall redox rate increases with increase in [H + ]. The title complex (1) and its conjugate bases, [Fe 2 ðl-OÞðphenÞ 4 ðH 2 OÞ(OH) 3þ (2) and [Fe 2 ðl-OÞðphenÞ 4 ðOHÞ 2 2þ (3), participate in the reaction with H 2 Q as the only kinetically reactive reducing species. Rate constants (in dm 3 mol )1 s )1 ) for the parallel reactions ð1Þ þ H 2 Q À! Products; ð2Þ þ H 2 Q À! Products and that for ð3Þ þ H 2 Q À! Products are, respectively, 500±40, 100±6 and 30±2. Substantial rate retardation in D 2 O media in comparison to that in H 2 O media suggests that electron transfer is coupled with proton movements in the rate-determining step.

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“…The azide-hydroxylase complex is quite weak, but its rate of formation ( k , = 135M-'sp') suggests addition to a vacant coordination site on Fe(II1) rather than substitution of a coordinated ligand. The analogous rate constant for the reaction of azide with methemerythrin, which is known to have one Fe(II1) with a vacant coordination site, is kf= 7M-'s ' [32,331. The vacant site could be the position at which 0, binds in the fully reduced hydroxylase.…”

Section: Azide Adductmentioning

confidence: 99%

Abduction of iron(III) from the soluble methane monooxygenase hydroxylase and reconstitution of the binuclear site with iron and manganese

Atta

Fontecave

Wilkins

et al. 1993

European Journal of Biochemistry

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The apo-form of the soluble methane monooxygenase hydroxylase from Methylococcus capsulatus (Bath) was prepared via chelation of iron(II1) with 3,4-dihydroxybenzaldehyde. The apohydroxylase was reconstituted by the anaerobic addition of Fe(I1) followed by air oxidation. The enzyme thus prepared regained 85-90% of its original catalytic activity. The incorporation of two manganese(I1) ions/mol of apohydroxylase was monitored by EPR spectroscopy. The Mn(I1) ions occupy the native diiron active site and remain in the +2 oxidation state. The EPR data suggest strong coupling between the two Mn(I1) ions and retention of the p-hydroxo (alkoxo) bridge. The results of this study indicate that the M. capsulutus (Bath) hydroxylase contains a single diiron site.Methane monooxygenase (MMO) efficiently catalyses the conversion of methane to methanol at ambient temperature and pressure. There are two forms of the enzyme, soluble (sMMO) or membrane-bound particulate (pMMO), the occurrence of which depends on the Cu2+ : biomass ratio under which the cells are grown [l]. Soluble methane monooxygenase has been the more studied and is a three-component enzyme system as isolated from several different organisms [2-71. Electrons from NADH are passed through the FAD and Fe2S, centers of the reductase to the hydroxylase [2, 81. The third component, protein B, contains no metal ions nor cofactors and is believed to have a regulatory function [9-111.The hydroxylase has been shown to be the component within which oxidation of methane and a variety of other substrates occurs 141. The active site is a p-hydroxo-bridged or -alkoxo-bridged diiron cluster [12-141 similar to those found in other nonheme iron-oxo proteins like the R2 subunit of ribonucleotide reductase, hemerythrin, purple acid phosphatases and rubrcrythrin [ 15 -191. The diiron centers perform very different functions in each of these metalloproteins. In R2 a stable tyrosyl radical is generated, in hemerythrin oxygen is bound reversibly, purple acid phosphatases catalyse the hydrolysis of phosphate esters and sMMO catalyses the oxidation of many alkane, alkene, heterocyclic and aromatic compounds. The X-ray crystal structures of the R2 subunit and hemerythrin have been solved, giving an accurate picture of the dinuclear iron centers in these proteins [20-221. The sMMO hydroxylase has recently been crystallized and its spacc group reported, but the structure has yet to be refined [23]. It is the ligands in the first coordination sphere of the iron atoms that most influence the redox potentials and other properties which determine the function of the active sites in each of these iron-oxo proteins. The first coordination sphere of R2 consists of mainly oxygen-donating ligands while that of hemerythrin has mainly N-donor ligands. In the absence of an hydroxylase crystal structure, overlaying the gene sequence of sMMO [24] on that of R2 has resulted in a plausible picture of the binuclear iron center of the former [25]. The proposed hydroxylase site resembles that of R2 much more t...

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The coordination chemistry of the binuclear iron site in hemerythrin

Cited by 148 publications

References 78 publications

A Leucine Residue “Gates” Solvent but Not O2Access to the Binding Pocket of Phascolopsis gouldiiHemerythrin

A Leucine Residue “Gates” Solvent but Not O2Access to the Binding Pocket of Phascolopsis gouldiiHemerythrin

Mechanistic Studies on the Oxidation of Hydroquinone by an Oxo-bridged Diiron(III,III) Complex in Weakly Acidic Aqueous Media

Abduction of iron(III) from the soluble methane monooxygenase hydroxylase and reconstitution of the binuclear site with iron and manganese

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