Evidence for Basic Ferryls in Cytochromes P450

Behan, Rachel K.; Hoffart, Lee M.; Stone, Kari L.; Krebs, Carsten; Green, Michael T.

doi:10.1021/ja062428p

Cited by 93 publications

(119 citation statements)

References 33 publications

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“…Given the difference in pK a between the phosphate and the substrate (Δ pK a ~ 20) and the short excited-state lifetime of the catalyst (2.3 μs in MeCN at RT [216]), it is unlikely activation proceeds through stepwise PT/ET. Lastly, independent quenching experiments with N-protiated and N-deuterated acetanilide demonstrated a KIE of 1.15; such small KIEs have been demonstrated to be consistent with proton-involvement in multisite PCET processes [46,[217][218][219][220][221] . Taken together, our mechanistic data are most consistent with amide activation proceeding through concerted PCET.…”

Section: Amidesmentioning

confidence: 66%

“…The basicity implies this species is protonated at physiological pH; these authors credit remarkable unusually high basicity to the binding of an axial thiolate ligand. In a later study, Green concluded that the ferryl forms of P540 BM3 and P450 cam were protonated at physiological pH using a combination of Mössbauer spectroscopy and density functional calculations [46]. Where iron-heme complexes such as Horseradish peroxidase C (and its associated Compounds I and II) are poorly oxidizing (approximately 1.08 V at pH = 6.5) [47], these enzymes are among a small set of enzymes capable of cleaving strong C-H bonds [48].…”

Section: Metal-oxo Complexes For C-h Bond Oxidationmentioning

confidence: 99%

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Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Miller

Tarantino

Knowles

2016

Topics in Current Chemistry Collections

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Proton-coupled electron transfers (PCETs) are unconventional redox processes in which both protons and electrons are exchanged, often in a concerted elementary step. While PCET is now recognized to play a central a role in biological redox catalysis and inorganic energy conversion technologies, its applications in organic synthesis are only beginning to be explored. In this chapter we aim to highlight the origins, development and evolution of PCET processes most relevant to applications in organic synthesis. Particular emphasis is given to the ability of PCET to serve as a non-classical mechanism for homolytic bond activation that is complimentary to more traditional hydrogen atom transfer processes, enabling the direct generation of valuable organic radical intermediates directly from their native functional group precursors under comparatively mild catalytic conditions. The synthetically advantageous features of PCET reactivity are described in detail, along with examples from the literature describing the PCET activation of common organic functional groups.

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Section: Amidesmentioning

confidence: 66%

Section: Metal-oxo Complexes For C-h Bond Oxidationmentioning

confidence: 99%

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Miller

Tarantino

Knowles

2016

Topics in Current Chemistry Collections

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“…Recent experimental studies and density functional theory calculations suggest that the ⌬E Q value might correlate with the protonation state of some heme-based ferryl species (62,63). The parameter range for protonated Fe(IV)-OH species is 2.00 -2.5 mm/s (64), whereas the range for unprotonated Fe(IV)ϭO is 1.0 -1.6 mm/s (62)(63)(64)(65)(66).…”

Section: Discussionmentioning

confidence: 99%

Enzyme Reactivation by Hydrogen Peroxide in Heme-based Tryptophan Dioxygenase

Gupta

Geng

et al. 2011

Journal of Biological Chemistry

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An intriguing mystery about tryptophan 2,3-dioxygenase is its hydrogen peroxide-triggered enzyme reactivation from the resting ferric oxidation state to the catalytically active ferrous form. In this study, we found that such an odd Fe(III) reduction by an oxidant depends on the presence of L-Trp, which ultimately serves as the reductant for the enzyme. In the peroxide reaction with tryptophan 2,3-dioxygenase, a previously unknown catalase-like activity was detected. A ferryl species (␦ ‫؍‬ 0.055 mm/s and ⌬E Q ‫؍‬ 1.755 mm/s) and a protein-based free radical (g ‫؍‬ 2.0028 and 1.72 millitesla linewidth) were characterized by Mössbauer and EPR spectroscopy, respectively. This is the first compound ES-type of ferryl intermediate from a heme-based dioxygenase characterized by EPR and Mössbauer spectroscopy. Density functional theory calculations revealed the contribution of secondary ligand sphere to the spectroscopic properties of the ferryl species. In the presence of L-Trp, the reactivation was demonstrated by enzyme assays and by various spectroscopic techniques. A Trp-Trp dimer and a monooxygenated L-Trp were both observed as the enzyme reactivation byproducts by mass spectrometry. Together, these results lead to the unraveling of an over 60-year old mystery of peroxide reactivation mechanism. These results may shed light on how a metalloenzyme maintains its catalytic activity in an oxidizing environment.Hemoproteins perform a wide range of biological functions, including oxygen transport, storage, electron transfer, monooxygenation, and reduction of dioxygen. However, they rarely express dioxygenase activity as their native biological function. Tryptophan 2,3-dioxygenase (TDO) 3 is the first described exception (1-3). This enzyme employs a b-type ferrous heme prosthetic group to catalyze the oxidative cleavage of the indole ring of L-Trp, converting it to N-formylkynurenine (NFK) (Scheme 1). This is the first and rate-limiting step of the kynurenine pathway of L-Trp metabolism, which oxidizes over 99% of L-Trp in mammalian intracellular and extracellular pools (2, 4 -9). The kynurenine pathway constitutes the major steps in biosynthesis of NAD, an essential redox cofactor in all living systems (5).TDO is a hepatic enzyme first discovered in rat liver extracts in 1936 (1). An analogous enzyme, indoleamine 2,3-dioxygenase (IDO), was isolated 31 years later from tissues other than the liver (10). Although both enzymes catalyze the same reaction, TDO is highly substrate-specific with L-Trp, whereas IDO presents a more relaxed specificity. TDO is a homotetramer with a total mass of ϳ134 kDa, whereas IDO is a monomeric protein. The two enzymes share only 14% sequence identity but conserve similar active site architectures (11-13). In addition to humans, TDO has also been found in other mammals, such as rats and mice, as well as in mosquitoes and bacteria (2, 5, 14 -18). Recently, a potential heme-dependent dioxygenase enzyme superfamily has been proposed (19). Moreover, several other heme-based proteins, such as my...

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“…In particular, the role of the axial cysteinate ligand of cytochrome P450 has received much attention because of the enzyme's ability to hydroxylate unactivated COH bonds (7)(8)(9)(10). In recent reports, Ogliaro et al (10) and Green and coworkers (11)(12)(13)(14) demonstrated that the thiolate ligand in closely related chloroperoxidase increases the basicity of the iron(IV)-oxo moiety such that the Fe-O unit becomes protonated at neutral pH upon reduction by one electron to the Cpd II state. Green speculated that this increased basicity is a strategy that allows hydrogen atom abstraction by Cpd I to occur at a lower redox potential so that the surrounding enzyme environment can be protected from oxidative destruction.…”

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confidence: 99%