Aliphatic hydroxylation via oxygen rebound. Oxygen transfer catalyzed by iron

239

232

Oxo-iron(IV) species are implicated as key intermediates in the catalytic cycles of heme and nonheme oxygen activating iron enzymes that selectively functionalize aliphatic C-H bonds. Ferryl complexes can exist in either quintet or triplet ground states. Density functional theory calculations predict that the quintet oxo-iron(IV) species is more reactive toward C-H bond activation than its corresponding triplet partner, however; the available experimental data on model complexes suggests that both spin multiplicities display comparable reactivities. To clarify this ambiguity, a detailed electronic structure analysis of alkane hydroxylation by an oxo-iron(IV) species on different spin-state potential energy surfaces is performed. According to our results, the lengthening of the Fe-oxo bond in ferryl reactants, which is the part of the reaction coordinate for H-atom abstraction, leads to the formation of oxyl-iron(III) species that then perform actual C-H bond activation. The differential reactivity stems from the fact that the two spin states have different requirements for the optimal angle at which the substrate should approach the ðFeOÞ 2þ core because distinct electron acceptor orbitals are employed on the two surfaces. The H-atom abstraction on the quintet surface favors the "σ-pathway" that requires an essentially linear attack; by contrast a "π-channel" is operative on the triplet surface that leads to an ideal attack angle near 90°. However, the latter is not possible due to steric crowding; thus, the attenuated orbital interaction and the unavoidably increased Pauli repulsion result in the lower reactivity of the triplet oxo-iron(IV) complexes. density functional calculation | nonheme iron | reaction mechanism O xo-iron(IV) intermediates have attracted much interest in bioinorganic chemistry because they are implicated as key intermediates in the catalytic cycles of heme and nonheme oxygen activating iron enzymes that selectively functionalize unactivated C-H bonds (1). Detailed experimental and theoretical studies on the hydroxylation of saturated hydrocarbons by ferryl species in heme systems, foremost cytochrome P450, have been performed (2). On the other hand a number of nonheme enzymes are able to activate molecular dioxygen to modify alkane or arene substrates as well. So far, nonheme ferryl species have been spectroscopically characterized in four mononuclear iron enzymes and were found to feature high-spin (HS) (S ¼ 2) electronic ground state configurations (3). In parallel, a wide range of synthetic FeðIVÞ¼O complexes were synthesized and characterized (4). In almost all cases they contain intermediate-spin (IS) (S ¼ 1) rather than HS ferryl centers (4). The only exceptions are ½FeðIVÞðOÞðH 2 OÞ 5 2þ (5) and the recently reported model complex ½FeðIVÞðOÞðTMG 3 trenÞ 2þ (TMG 3 tren ¼ N½CH 2 CH 2 N ¼ CðNMe 2 Þ 2 ) (6). Presumably because the ðFeOÞ 2þ core is sheltered by the sterically bulky supporting ligand in ½FeðIVÞðOÞ ðTMG 3 trenÞ 2þ , its reactivity toward C-H bond cleavage is only comparabl...

Section: Resultsmentioning

confidence: 99%

Nonheme oxo-iron(IV) intermediates form an oxyl radical upon approaching the C–H bond activation transition state

Neese

2011

239

232

“…The archetypal P450 hydroxylation involves C−H bond abstraction by P450-I and ensuing rapid oxygen insertion through a recombination process termed "oxygen rebound" (11,12). The hydrogen abstraction/rebound mechanism describes the strategy used for most P450 transformations, and is thought to describe catalysis by numerous metal-dependent oxygenases and synthetic bio-inspired metal−oxo complexes (e.g., refs.…”

mentioning

confidence: 99%

Catalytic strategy for carbon−carbon bond scission by the cytochrome P450 OleT

Grant

Mitchell

Makris

2016

102

OleT is a cytochrome P450 that catalyzes the hydrogen peroxidedependent metabolism of C n chain-length fatty acids to synthesize C n-1 1-alkenes. The decarboxylation reaction provides a route for the production of drop-in hydrocarbon fuels from a renewable and abundant natural resource. This transformation is highly unusual for a P450, which typically uses an Fe 4+ −oxo intermediate known as compound I for the insertion of oxygen into organic substrates. OleT, previously shown to form compound I, catalyzes a different reaction. A large substrate kinetic isotope effect (≥8) for OleT compound I decay confirms that, like monooxygenation, alkene formation is initiated by substrate C−H bond abstraction. Rather than finalizing the reaction through rapid oxygen rebound, alkene synthesis proceeds through the formation of a reaction cycle intermediate with kinetics, optical properties, and reactivity indicative of an Fe 4+ −OH species, compound II. The direct observation of this intermediate, normally fleeting in hydroxylases, provides a rationale for the carbon−carbon scission reaction catalyzed by OleT.C ytochrome P450 (CYP) enzymes catalyze an extraordinary breadth of physiologically important oxidations for xenobiotic detoxification and specialized biosynthetic pathways (1, 2). The metabolic diversity of CYP enzymes originates from a sophisticated interplay of substrate molecular recognition with precise tuning of metal oxygen species formed at the enzyme active site. CYPs use a thiolate-ligated heme iron cofactor to activate molecular oxygen and produce short-lived ferric superoxo (3-5), ferric (hydro)peroxo (6-8), and ferryl (9, 10) intermediates. Coordinated efforts over several decades have resulted in isolation of each intermediate, including recent characterization of the principal oxidant thought to be responsible for the vast majority of P450 oxidations, the Fe 4+ −oxo pi−cation radical species commonly referred to as compound I (or P450-I) (10).The archetypal P450 hydroxylation involves C−H bond abstraction by P450-I and ensuing rapid oxygen insertion through a recombination process termed "oxygen rebound" (11, 12). The hydrogen abstraction/rebound mechanism describes the strategy used for most P450 transformations, and is thought to describe catalysis by numerous metal-dependent oxygenases and synthetic bio-inspired metal−oxo complexes (e.g., refs. 13-17). Despite the ubiquity of this mechanism, in some metalloenzymes, a metal−oxo species is formed that is not ultimately destined for incorporation into a substrate. Some characterized examples include the nonheme dinuclear iron ribonucleotide reductase (RNR R2) (18,19) and mononuclear iron halogenase SyrB2 (20-22). The mechanisms for RNR and SyrB2 highlight the importance of quaternary structural elements, an extensive 35-Å proton-coupled electron transfer pathway in RNR (23), and extremely subtle (subangstrom) substrate positional tuning (SyrB2) (21,22), to enable efficient circumvention from the monooxygenation reaction coordinate.P450-I has also been link...

“…This is followed by fast rebound, in which the −OCH 2 radical species forms a carbonoxygen bond with the iron-hydroxo species (35,36). The resulting ferric complex of the hemiacetal derivative of the substrate dissociates from the enzyme and is hydrolyzed to form formaldehyde and a phenolic product.…”

mentioning

confidence: 99%

Understanding the determinants of selectivity in drug metabolism through modeling of dextromethorphan oxidation by cytochrome P450

Oláh

Mulholland

Harvey

2011

Cytochrome P450 enzymes play key roles in the metabolism of the majority of drugs. Improved models for prediction of likely metabolites will contribute to drug development. In this work, two possible metabolic routes (aromatic carbon oxidation and O-demethylation) of dextromethorphan are compared using molecular dynamics (MD) simulations and density functional theory (DFT). The DFT results on a small active site model suggest that both reactions might occur competitively. Docking and MD studies of dextromethorphan in the active site of P450 2D6 show that the dextromethorphan is located close to heme oxygen in a geometry apparently consistent with competitive metabolism. In contrast, calculations of the reaction path in a large protein model [using a hybrid quantum mechanical-molecular mechanics (QM/MM) method] show a very strong preference for O-demethylation, in accordance with experimental results. The aromatic carbon oxidation reaction is predicted to have a high activation energy, due to the active site preventing formation of a favorable transition-state structure. Hence, the QM/MM calculations demonstrate a crucial role of many active site residues in determining reactivity of dextromethorphan in P450 2D6. Beyond substrate binding orientation and reactivity of Compound I, successful metabolite predictions must take into account the detailed mechanism of oxidation in the protein. These results demonstrate the potential of QM/MM methods to investigate specificity in drug metabolism.C ytochrome P450 enzymes (P450s) form one of the most powerful defense mechanisms of living organisms: They protect against xenobiotics via an oxidative pathway, which in most instances leads to a less harmful and more soluble product with faster excretion. The same mechanism is involved in the metabolism of most drugs and alters the pharmacological activity of many drugs. As a consequence, P450-mediated transformations of drug candidates are of crucial importance in the pharmaceutical industry. The roles of P450s are manifold. Oxidation by P450s can lead to toxic products, but, on the other hand, local activation of, e.g., anticancer prodrugs by P450s to lethal intracellular toxins at the site of the tumor is an important strategy (1). Drug compounds can induce P450 expression but can also inhibit them in various ways (2-4). The metabolic clearance of most drugs depends on P450s, and they have been implicated in a large number of drug-drug interactions (5, 6), which can result in fatalities (7,8). Interactions of drug candidates with P450s must be taken into account during the drug discovery process if the expensive and time-consuming development of active compounds with hidden toxic effects is to be avoided.It is increasingly recognized that methods capable of predicting P450 oxidative activity for a given substrate, and also the predominant site(s) of metabolism, could contribute significantly to drug development. There are many aspects to this problem, including isoform selectivity (i.e., which P450 isoform will contribute mos...