Carboxylate as the protonation site in (Peroxo)diiron(III) model complexes of soluble methane monooxygenase and related diiron proteins.
Ontology highlight
ABSTRACT:
SUBMITTER: Do LH
PROVIDER: S-EPMC2812653 | biostudies-literature | 2010 Feb
REPOSITORIES: biostudies-literature
Publications
Carboxylate as the protonation site in (Peroxo)diiron(III) model complexes of soluble methane monooxygenase and related diiron proteins.
Do Loi H LH Hayashi Takahiro T Moënne-Loccoz Pierre P Lippard Stephen J SJ
Journal of the American Chemical Society 20100201 4
Addition of H(+) to a synthetic (mu-1,2-peroxo)diiron(III) model complex results in protonation of a carboxylate rather than the peroxo ligand. This conclusion is based on spectroscopic evidence from UV-vis, (57)Fe Mossbauer, resonance Raman, infrared, and (1)H/(19)F NMR studies. These results suggest a similar role for protons in the dioxygen activation reactions in soluble methane monooxygenase and related carboxylate-bridged diiron enzymes. ...[more]
Similar Datasets
Project description:The regulatory component (MMOB) of soluble methane monooxygenase (sMMO) has a unique N-terminal tail not found in regulatory proteins of other bacterial multicomponent monooxygenases. This N-terminal tail is indispensable for proper function, yet its solution structure and role in catalysis remain elusive. Here, by using double electron-electron resonance (DEER) spectroscopy, we show that the oxidation state of the hydroxylase component, MMOH, modulates the conformation of the N-terminal tail in the MMOH-2MMOB complex, which in turn facilitates catalysis. The results reveal that the N-terminal tail switches from a relaxed, flexible conformational state to an ordered state upon MMOH reduction from the diiron(III) to the diiron(II) state. This observation suggests that some of the crystallographically observed allosteric effects that result in the connection of substrate ingress cavities in the MMOH-2MMOB complex may not occur in solution in the diiron(III) state. Thus, O2 may not have easy access to the active site until after reduction of the diiron center. The observed conformational change is also consistent with a higher binding affinity of MMOB to MMOH in the diiron(II) state, which may allow MMOB to displace more readily the reductase component (MMOR) from MMOH following reduction.
| S-EPMC3954536 | biostudies-literature
Project description:With the goal of gaining insight into the structures of peroxo intermediates observed for oxygen-activating nonheme diiron enzymes, a series of metastable synthetic diiron(III)-peroxo complexes with [Fe(III)(2)(mu-O)(mu-1,2-O(2))] cores has been characterized by X-ray absorption and resonance Raman spectroscopies, EXAFS analysis shows that this basic core structure gives rise to an Fe-Fe distance of approximately 3.15 A; the distance is decreased by 0.1 A upon introduction of an additional carboxylate bridge. In corresponding resonance Raman studies, vibrations arising from both the Fe-O-Fe and the Fe-O-O-Fe units can be observed. Importantly a linear correlation can be discerned between the nu(O-O) frequency of a complex and its Fe-Fe distance among the subset of complexes with [Fe(III)(2)(mu-OR)(mu-1,2-O(2))] cores (R = H, alkyl, aryl, or no substituent). These experimental studies are complemented by a normal coordinate analysis and DFT calculations.
| S-EPMC2736334 | biostudies-literature
Project description:Soluble methane monooxygenase is a bacterial enzyme that converts methane to methanol at a carboxylate-bridged diiron center with exquisite control. Because the oxidizing power required for this transformation is demanding, it is not surprising that the enzyme is also capable of hydroxylating and epoxidizing a broad range of hydrocarbon substrates in addition to methane. In this work we took advantage of this promiscuity of the enzyme to gain insight into the mechanisms of action of H(peroxo) and Q, two oxidants that are generated sequentially during the reaction of reduced protein with O(2). Using double-mixing stopped-flow spectroscopy, we investigated the reactions of the two intermediate species with a panel of substrates of varying C-H bond strength. Three classes of substrates were identified according to the rate-determining step in the reaction. We show for the first time that an inverse trend exists between the rate constant of reaction with H(peroxo) and the C-H bond strength of the hydrocarbon examined for those substrates in which C-H bond activation is rate-determining. Deuterium kinetic isotope effects revealed that reactions performed by Q, but probably not H(peroxo), involve extensive quantum mechanical tunneling. This difference sheds light on the observation that H(peroxo) is not a sufficiently potent oxidant to hydroxylate methane, whereas Q can perform this reaction in a facile manner. In addition, the reaction of H(peroxo) with acetonitrile appears to proceed by a distinct mechanism in which a cyanomethide anionic intermediate is generated, bolstering the argument that H(peroxo) is an electrophilic oxidant that operates via two-electron transfer chemistry.
| S-EPMC2935519 | biostudies-literature
Project description:The controlled oxidation of methane to methanol is a chemical transformation of great value, particularly in the pursuit of alternative fuels, but the reaction remains underutilized industrially because of inefficient and costly synthetic procedures. In contrast, methane monooxygenase enzymes (MMOs) from methanotrophic bacteria achieve this chemistry efficiently under ambient conditions. In this Account, we discuss the first observable step in the oxidation of methane at the carboxylate-bridged diiron active site of the soluble MMO (sMMO), namely, the reductive activation of atmospheric O(2). The results provide benchmarks against which the dioxygen activation mechanisms of other bacterial multicomponent monooxygenases can be measured. Molecular oxygen reacts rapidly with the reduced diiron(II) cen-ter of the hydroxylase component of sMMO (MMOH). The first spectroscopically characterized intermediate that results from this process is a peroxodiiron(III) species, P*, in which the iron atoms have identical environments. P* converts to a second peroxodiiron(III) unit, H(peroxo), in a process accompanied by the transfer of a proton, probably with the assistance of a residue near the active site. Proton-promoted O-O bond scission and rearrangement of the diiron core then leads to a diiron(IV) unit, termed Q, that is directly responsible for the oxidation of methane to methanol. In one section of this Account, we provide a detailed discussion of these processes, with particular emphasis on possible structures of the intermediates. The geometries of P* and H(peroxo) are currently unknown, and recent synthetic modeling chemistry has highlighted the need for further structural characterization of Q, currently assigned as a di(μ-oxo)diiron(IV) "diamond core." In another section of the Account, we discuss in detail proton transfer during the O(2) activation events. The role of protons in promoting O-O bond cleavage, thereby initiating the conversion of H(peroxo) to Q, was previously a controversial topic. Recent studies of the mechanism, covering a range of pH values and in D(2)O instead of H(2)O, confirmed conclusively that the transfer of protons, possibly at or near the active site, is necessary for both P*-to-H(peroxo) and H(peroxo)-to-Q conversions. Specific mechanistic insights into these processes are provided. In the final section of the Account, we present our view of experiments that need to be done to further define crucial aspects of sMMO chemistry. Here our goal is to detail the challenges that we and others face in this research, particularly with respect to some long-standing questions about the system, as well as approaches that might be used to solve them.
| S-EPMC3079780 | biostudies-literature
Project description:A triptycene-based bis(benzimidazole) ester ligand, L3, was designed to enhance the electron donating ability of the heterocyclic nitrogen atoms relative to those of the first generation bis(benzoxazole) analogs, L1 and L2. A convergent synthesis of L3 was designed and executed. Three-component titration experiments using UV-visible spectroscopy revealed that the desired diiron(II) complex could be obtained with a 1:2:1 ratio of L3:Fe(OTf)2(MeCN)2:external carboxylate reactants. X-ray crystallographic studies of two diiron complexes derived in this manner from L3 revealed their formulas to be [Fe2L3(?-OH)(?-O2CR)(OTf)2], where R = 2,6-bis(p-tolyl)benzoate (7) or triphenylacetate (8). The structures are similar to that of a diiron complex derived from L1, [Fe2L1(?-OH)(?-O2CArTol)(OTf)2] (9) with a notable difference being that, in 7 and 8, the geometry at iron more closely resembles square-pyramidal than trigonal-bipyramidal. Mössbauer spectroscopic analyses of 7 and 8 indicate the presence of high-spin diiron(II) cores. These results demonstrate the importance of substituting benzimidazole for benzoxazole for assembling biomimetic diiron complexes with syn disposition of two N-donor ligands, as found in O2-activating carboxylate-bridged diiron centers in biology.
| S-EPMC3625018 | biostudies-literature
Project description:The hydroxylation or epoxidation of hydrocarbons by bacterial multicomponent monooxygenases (BMMs) requires the interplay of three or four protein components. How component protein interactions control catalysis, however, is not well understood. In particular, the binding sites of the reductase components on the surface of their cognate hydroxylases and the role(s) that the regulatory proteins play during intermolecular electron transfer leading to the hydroxylase reduction have been enigmatic. Here we determine the reductase binding site on the hydroxylase of a BMM enzyme, soluble methane monooxygenase (sMMO) from Methylococcus capsulatus (Bath). We present evidence that the ferredoxin domain of the reductase binds to the canyon region of the hydroxylase, previously determined to be the regulatory protein binding site as well. The latter thus inhibits reductase binding to the hydroxylase and, consequently, intermolecular electron transfer from the reductase to the hydroxylase diiron active site. The binding competition between the regulatory protein and the reductase may serve as a control mechanism for regulating electron transfer, and other BMM enzymes are likely to adopt the same mechanism.
| S-EPMC4105053 | biostudies-literature
Project description:The soluble methane monooxygenase (sMMO; EC 1.14.13.25) from the pseudothermophile Methylococcus capsulatus (Bath) is a three-component enzyme system that catalyzes the selective oxidation of methane to methanol. We have used NMR spectroscopy to produce a highly refined structure of MMOB, the 16-kDa regulatory protein of this system. This structure has a unique and intricate fold containing seven beta-strands forming two beta-sheets oriented perpendicular to each other and bridged by three alpha-helices. The rate and efficiency of the methane hydroxylation by sMMO depend on dynamic binding interactions of the hydroxylase with the reductase and regulatory protein components during catalysis. We have monitored by NMR the binding of MMOB to the hydroxylase in the presence and absence of the reductase. The results of these studies provide structural insight into how the regulatory protein interacts with the hydroxylase.
| S-EPMC22155 | biostudies-literature
Project description:The complex [FeIII2(μ-O2)(L3)4(S)2]4+ (L3 = 2-(4-thiazolyl)benzimidazole, S = solvent) forms upon reaction of [FeII(L3)2] with H2O2 and is a functional model of peroxo-diiron intermediates invoked during the catalytic cycle of oxidoreductases. The spectroscopic properties of the complex are in line with those of complexes formed with N-donor ligands. [FeIII2(μ-O2)(L3)4(S)2]4+ shows both nucleophilic (aldehydes) and electrophilic (phenol, N,N-dimethylanilines) oxidative reactivity and unusually also electron transfer oxidation.
| S-EPMC8168641 | biostudies-literature
Project description:Particulate methane monooxygenase (pMMO) is an integral membrane metalloenzyme that oxidizes methane to methanol in methanotrophic bacteria, organisms that live on methane gas as their sole carbon source. Understanding pMMO function has important implications for bioremediation applications and for the development of new, environmentally friendly catalysts for the direct conversion of methane to methanol. Crystal structures of pMMOs from three different methanotrophs reveal a trimeric architecture, consisting of three copies each of the pmoB, pmoA, and pmoC subunits. There are three distinct metal centers in each protomer of the trimer, mononuclear and dinuclear copper sites in the periplasmic regions of pmoB and a mononuclear site within the membrane that can be occupied by copper or zinc. Various models for the pMMO active site have been proposed within these structural constraints, including dicopper, tricopper, and diiron centers. Biochemical and spectroscopic data on pMMO and recombinant soluble fragments, denoted spmoB proteins, indicate that the active site involves copper and is located at the site of the dicopper center in the pmoB subunit. Initial spectroscopic evidence for O(2) binding at this site has been obtained. Despite these findings, questions remain about the active site identity and nuclearity and will be the focus of future studies.
| S-EPMC3474877 | biostudies-literature
Project description:During a single turnover of the hydroxylase component (MMOH) of soluble methane monooxygenase from Methylosinus trichosporium OB3b, several discrete intermediates are formed. The diiron cluster of MMOH is first reduced to the Fe(II)Fe(II) state (H(red)). O? binds rapidly at a site away from the cluster to form the Fe(II)Fe(II) intermediate O, which converts to an Fe(III)Fe(III)-peroxo intermediate P and finally to the Fe(IV)Fe(IV) intermediate Q. Q binds and reacts with methane to yield methanol and water. The rate constants for these steps are increased by a regulatory protein, MMOB. Previously reported transient kinetic studies have suggested that an intermediate P* forms between O and P in which the g = 16 EPR signal characteristic of the reduced diiron cluster of H(red) and O is lost. This was interpreted as signaling oxidation of the cluster, but a low level of accumulation of P* prevented further characterization. In this study, three methods for directly detecting and trapping P* are applied together to allow its spectroscopic and kinetic characterization. First, the MMOB mutant His33Ala is used to specifically slow the decay of P* without affecting its formation rate, leading to its nearly quantitative accumulation. Second, spectra-kinetic data collection is used to provide a sensitive measure of the formation and decay rate constants of intermediates as well as their optical spectra. Finally, the substrate furan is included to react with Q and quench its strong chromophore. The optical spectrum of P* closely mimics those of H(red) and O, but it is distinctly different from that of P. The reaction cycle rate constants allowed prediction of the times for maximal accumulation of the intermediates. Mössbauer spectra of rapid freeze-quench samples at these times show that the intermediates are formed at almost exactly the predicted levels. The Mössbauer spectra show that the diiron cluster of P*, quite unexpectedly, is in the Fe(II)Fe(II) state. Thus, the loss of the g = 16 EPR signal results from a change in the electronic structure of the Fe(II)Fe(II) center rather than oxidation. The similarity of the optical and Mössbauer spectra of H(red), O, and P* suggests that only subtle changes occur in the electronic and physical structure of the diiron cluster as P* forms. Nevertheless, the changes that do occur are necessary for O? to be activated for hydrocarbon oxidation.
| S-EPMC3712755 | biostudies-literature