A transition from ionic to free-radical mechanisms in chemistry and enzymology (original) (raw)
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Mechanistic studies on aromatase and related CC bond cleaving P-450 enzymes
The Journal of Steroid Biochemistry and Molecular Biology, 1993
Some P-450 systems, notably aromatase and 14~-demethylase catalyse not only the hydroxylate reaction but also the oxidation of an alcohol into a carbonyl compound as well as a CC bond cleavage process. All these reactions occur at the same active site. A somewhat analogous situation is noted with 17~-hydroxylase-17,20-1yase that participates in hydroxylation as well as CC bond cleavage process. The CC bond cleavage reactions catalysed by the above enzymes conform to the general equation:
The Journal of Organic Chemistry, 1997
The syntheses of racemic and enantiomerically enriched trans-1-methyl-2-(4-(trifluoromethyl)phenyl)cyclopropane (3) and the possible oxidation products from enzyme-catalyzed hydroxylation of 3 at the methyl group are reported. The important intermediate in the production of 3 was the Weinreb amide of the 2-arylcyclopropanecarboxylic acid which could be prepared in diastereomerically pure form and which also served as an intermediate for production of the cyclic oxidation products of 3. Hydroxylation of 3 by the cytochrome P450 isozyme CYP2B1 gave cyclic and ringopened products. The product ratios support an insertion mechanism for the enzyme-catalyzed hydroxylation reaction in which minor amounts of rearranged products are produced by radical fragmentation within the transition structure of the insertion and by a competing reaction involving a cationic species. Formation of cationic rearrangement products by a heterolytic fragmentation reaction of a first-formed protonated alcohol product is suggested on the basis of the apparent amounts of cationic products formed in the hydroxylation of 3. This pathway for cation production appears to require that the activated enzyme complex (equivalent to enzyme-substrate-H 2 O 2 ) oxidizes substrate before water dissociates to give an iron-oxo species.
The Origins of Enzyme Catalysis and Reactivity: Further Assessments
Asian Journal of Chemical Sciences, 2021
Alternatives to conventional mechanisms of enzyme catalyzed reactions, although within the ambit of transition state theory, are explored herein. This is driven by reports of a growing number of enzymes forming covalently linked enzyme-substrate intermediates, which clearly deviate from the conventional Michaelis-complex mechanism. It is argued that the formation of the covalent intermediates can be accommodated within the framework of transition state theory and the original Pauling hypothesis. This also obviates the need to invoke intramolecular reactivity to explain enzymic accelerations. Thus, the covalent binding of a substrate distorted towards the transition state, with the binding being fully manifested in the ensuing transition state, would conform to the traditional endergonic pre-equilibrium mechanism. Intriguingly, an alternative exergonic formation of the covalent intermediate would also lead to catalysis: in this case, any of the three steps–covalent binding, turnover ...
Radical reactions of thiamin pyrophosphate in 2-oxoacid oxidoreductases
Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics, 2012
Thiamin pyrophosphate (TPP) is essential in carbohydrate metabolism in all forms of life. TPP-dependent decarboxylation reactions of 2-oxo-acid substrates result in enamine adducts between the thiazolium moiety of the coenzyme and decarboxylated substrate. These central enamine intermediates experience different fates from protonation in pyruvate decarboxylase to oxidation by the 2-oxoacid dehydrogenase complexes, the pyruvate oxidases, and 2-oxoacid oxidoreductases. Virtually all of the TPP-dependent enzymes, including pyruvate decarboxylase, can be assayed by 1-electron redox reactions linked to ferricyanide. Oxidation of the enamines is thought to occur via a 2-electron process in the 2-oxoacid dehydrogenase complexes, wherein acyl group transfer is associated with reduction of the disulfide of the lipoamide moiety. However, discrete 1-electron steps occur in the oxidoreductases, where one or more [4Fe-4S] clusters mediate the electron transfer reactions to external electron acceptors. These radical intermediates can be detected in the absence of the acyl-group acceptor, coenzyme A (CoASH). The π-electron system of the thiazolium ring stabilizes the radical. The extensively delocalized character of the radical is evidenced by quantitative analysis of nuclear hyperfine splitting tensors as detected by electron paramagnetic resonance (EPR) spectroscopy and by electronic structure calculations. The second electron transfer step is markedly accelerated by the presence of CoASH. While details of the second electron transfer step and its facilitation by CoASH remain elusive, expected redox properties of potential intermediates limit possible scenarios. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.
A Putative Monooxygenase Mimic Which Functions via Well-Disguised Free Radical Chemistry1
Journal of the American Chemical Society, 1997
The hydroxylation of cycloalkanes at 25°C by the syringe pump addition of tert-alkyl hydroperoxides (10 and 1 equiv based on catalyst) to deoxygenated acetonitrile containing cycloalkanes (0.64 M) and 0.61 mM of the catalyst, [Fe III 2 O(TPA) 2 (H 2 O) 2 ] 4+ , is demonstrated to be a reaction which involves freely diffusing cycloalkyl radicals, i.e., free alkyl radicals. In recent years there have been many attempts to mimic the chemistry of monooxygenases such as cytochrome P450 and methane monooxygenase which can oxidize saturated hydrocarbons by processes which do not involve free (i.e., freely diffusing) radicals. 3 Following the lead provided by these enzymes, (ferric) iron has generally been chosen as the catalytically active metal and two-electron-reduced oxygen, in the form of H 2 O 2 or tert-butyl hydroperoxide (TBHP), has been utilized (to avoid the requirement for a sacrificial reductant). However, Fe III /TBHP systems may not undergo the desired heterolysis to give a high-valent iron-oxo species (formally Fe V dO) as is believed to occur when monooxygenases react with hydroperoxides. Instead, a homolysis may occur to form free tert-butoxyl radicals which then dominate the subsequent chemistry (see Scheme 1). The oxidation of an alkane to a mixture of alcohol, ketone, and the mixed peroxide (shown in bold face in the scheme) is a very clear indication that free-radical chemistry has occurred. Unfortunately, this signature has all too frequently been ignored. 2-Methyl-1-phenyl-2-propyl hydroperoxide (MPPH) is a probe capable of distinguishing between free alkoxyl radical chemistry and radical-free (enzyme mimetic) chemistry in iron/ tert-alkyl hydroperoxide/hydrocarbon oxidation systems. 4 This probe relies on the fact that if the corresponding tert-alkoxyl radical were formed and diffused from its site of formation into the bulk solution it would undergo far too rapid a-scission (k ∼ 2 × 10 8 s-1) for it to abstract a hydrogen atom from a saturated hydrocarbon, i.e., the equivalent of reaction 8 cannot occur. MPPH has been employed at the NRC in Ottawa to demonstrate that cycloalkane oxidations using TBHP and two tris(2-pyridinylmethyl)amine (TPA) complexes, 5 [Fe III Cl 2-(TPA)] + and [Fe III 2 O(OAc)(TPA) 2 ] 3+ , and a Fe III picolinate/ pyridine complex 6 all occurred via straighforward free radical † SIMS, NRC.
The generation of 5′-deoxyadenosyl radicals by adenosylmethionine-dependent radical enzymes
Current Opinion in Chemical Biology, 2003
Adenosylmethionine-dependent radical enzymes provide a novel mechanism for generating the highly oxidizing 5 0 -deoxyadenosyl radical in an anaerobic reducing environment. Recent studies suggest a unique covalent interaction between adenosylmethionine and a catalytic iron-sulfur cluster that may promote inner-sphere electron transfer to the sulfonium, resulting in the reductive cleavage of a C-S bond and the generation of a 5 0 -deoxyadenosyl radical. The utilization of this radical as a catalytic and stoichiometric oxidant in many different enzyme reactions reflects the broad diversity of radical enzymes throughout biology.
Enzyme Catalysis: Beyond Classical Paradigms
Accounts of Chemical Research, 1998
Amnon Kohen received his B.Sc. degree in chemistry from the Hebrew University in Jerusalem in 1989, and his D.Sc. degree from the Technion-Israel Institute of Technology in 1994 on mechanistic studies of the reaction catalyzed by the enzyme Kdo8P synthase. He is currently a postdoctoral fellow in J. P. Klinman's group at the University of California at Berkeley, studying hydrogen tunneling and other features contributing to enzyme catalysis.