Unstable carbanions. General acid catalysis of the cleavage of 1-phenylcyclopropanol and 1-phenyl-2-arylcyclopropanol anions

Thibblin, Alf; Jencks, William P.

doi:10.1021/ja00511a028

Cited by 83 publications

(71 citation statements)

References 12 publications

(15 reference statements)

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“…Such a push-pull proton-assisted cyclopropane fragmentation has been demonstrated by Jenck's model studies on phenylcyclopropane cleavage (27). A similar quinone methide species 13 postulated as an intermediate formed during C-3 deoxygenation in the active-site of El is clearly a highly relevant precedent (9).…”

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confidence: 77%

Coenzyme B6 dependent novel bond cleavage reactions

Liu¹

1998

Pure and Applied Chemistry

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Coenzyme B6 is an important cofactor participating in many diverse biotransformations. W e the function of this coenzyme in enzyme catalysis has been well established, recent studies have led to two novel coenzyme B6-dependent reactions whose catalyses involve unique mechanistic variants of usual coenzyme B6 chemistry. These are a C -0 bond cleavage and a C-C bond cleavage reaction catalyzed by CDP-6-deoxy-L-threo-D-glycero-4-hexulose-3-dehydrase (El) and 1-aminocyclopropane-1-carboxylate deaminase (ACC deaminase), respectively. Enzyme El is the only known pyridoxamine 5'-phosphate (PMP) dependent ironsulfur containing dehydratase, and it is one of a few &dependent enzymes with a radical mechanism. ACC deaminase is a pyridoxal 5'-phosphate (PLP) dependent enzyme which acts on a cyclopropane substrate, and catalyzes the cleavage between the Ca-Cp bond. Current knowledge concerning the properties and mechanisms of these unusual enzymes is summarized in this paper.As a coenzyme, vitamin B6 phosphate exhibits a remarkably versatile array of catalytic roles, ranging from transaminations, racemization, p-and y-eliminatiodsubstitution, decarboxylation, etc (1). The achievement of these catalyses by B6-dependent enzymes involves a common recurring theme, relying on the electron-withdrawing capability of this cofactor to stabilize the anion generated at the a-C of the substratecoenzyme complex. The transiently delocalized electrons are later used for the cleavage and/or formation of covalent bonds. While extensive knowledge about the genetics, structure, function, and mechanism of this class of enzymes have been accumulated over the years, several recent studies have revealed a few new traits of coenzyme B6 that are beyond the scope previously established for its catalyzed reactions. For example, the traditional role of coenzyme B6 has been varied by lysine-2,3-aminomutase which utilizes PLP to catalyze a radical rearrangement of the a-amino group of lysine (2). A Bg-dependent aminotransferase was also implicated in the conversion of cytosylglucuronic acid to blasticidin S (3). The role of this as yet to be isolated enzyme in blasticidin biosynthesis has been hypothesized not only to act as a transaminase but also to catalyze the dideoxygenation at C-2' and C-3' of the glucuronate residue. In this paper, two other unusual cases of coenzyme B6 dependent reactions will be presented: one involved in C-3 deoxygenation of a 4-keto-6-deoxyhexose and the other in the ring opening of a cyclopropane amino acid, both of which have been the subjects of our research in recent years. The current information regarding their modes of action will be summarized herein. CDP-6-deoxy-L-threo-D-gZycero-4-hexulose-3-dehydrase (El)Enzyme El is a homo-dimeric protein (49 KDa per subunit) requiring a pyridoxamine 5'-phosphate (PMP) and an adrenodoxidputidaredoxin-type center per monomer for full activity (4). This enzyme, together with its reductase (CDP-6-deoxy-L-threo-D-glycero-4-hexulose-3-dehydrase reductase, E3). plays an essent...

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confidence: 77%

Coenzyme B6 dependent novel bond cleavage reactions

Liu¹

1998

Pure and Applied Chemistry

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“…The formation of the anion can readily be followed by trapping it with triiodine. 64 We compared the ionization of nitroethane catalyzed by NAO, as reflected in the k cat /K m value, with the reaction catalyzed by acetate. The results are summarized in Table 1.…”

Section: Alcohol Oxidasesmentioning

confidence: 99%

Insights into the mechanisms of flavoprotein oxidases from kinetic isotope effects

Fitzpatrick

2007

Labelled Comp Radiopharmac

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Deuterium, solvent, and (15)N kinetic isotope effects have been used to probe the mechanisms by which flavoproteins oxidize carbon-oxygen and carbon-nitrogen bonds in amines, hydroxy acids, and alcohols. For the amine oxidases d-amino acid oxidase, N-methyltryptophan oxidase, and tryptophan monooxygenase, d-serine, sarcosine, and alanine are slow substrates for which CH bond cleavage is fully rate limiting. Inverse isotope effects for each of 0.992-0.996 are consistent with a common mechanism involving hydride transfer from the uncharged amine. Computational analyses of possible mechanisms support this conclusion. Deuterium and solvent isotope effects with wild-type and mutant variants of the lactate dehydrogenase flavocytochrome b(2) show that OH and CH bond cleavage are not concerted, but become so in the Y254F enzyme. This is consistent with a highly asynchronous reaction in which OH bond cleavage precedes hydride transfer. The results of Hammett analyses and solvent and deuterium isotope effects support a similar mechanism for alcohol oxidase.

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“…The higher the pK, the better able Glu376 would be expected to abstract an R-proton from substrate (23)(24)(25)(26). Desolvation…”

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confidence: 99%

Protonic Equilibria in the Reductive Half-Reaction of the Medium-Chain Acyl-CoA Dehydrogenase

1998

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Oxidation of thioester substrates in the medium-chain acyl-CoA dehydrogenase involves R-proton abstraction by the catalytic base, Glu376, with transfer of a -hydride equivalent to the flavin prosthetic group. Polarization of bound acyl-CoA derivatives by the recombinant human liver enzyme has been studied with 4-thia-trans-2-enoyl-CoA analogues. Polarization is maximal at low pH, with an apparent pK of 9.2 for complexes with the C8 analogue, and progressively lower pK values as the length of the chain increases. This pH effect reflects ionization of the catalytic base, since polarization of a variety of enoyl-CoA analogues by the Glu376Gln mutant is pH independent. Binding of these ligands is accompanied by uptake of about 1 proton with the wild-type enzyme, but only about 0.1 proton with the Glu376Gln mutant. Rapid reaction studies show that proton uptake with the wild-type enzyme occurs at the same rate as polarization of the enoyl-CoA thioester, but is much slower than the initial ligand binding step. Studies with 6-OH-FAD-substituted enzyme show that this isomerization reaction also influences the flavin prosthetic group inducing deprotonation to the green anionic form. The failure of the bound thioether analogue, octyl-SCoA, to elicit pK shifts to flavin and Glu376 shows the importance of the thioester carbonyl oxygen in modulating key properties of the medium-chain enzyme. The role of thioester-mediated desolvation within the active site of the acyl-CoA dehydrogenases is discussed.The reductive half-reaction in the acyl-CoA dehydrogenases involves abstraction of the pro-R-R-proton of a bound acyl-CoA thioester with elimination of the pro-R--hydrogen as a hydride equivalent to the N-5 position of the flavin. The reaction appears concerted with normal (Scheme 1) substrates (1-5). The transition state for the dehydrogenation reaction is likely to have appreciable enolate character as negative charge migrates from the carboxylate base through the thioester to the flavin prosthetic group (6-10) (Scheme 2). Substitution of the C-3 methylene group in substrate analogues such as compounds 1 and 2 in Chart 1 precludes hydride transfer, but proton abstraction leads to the generation of strongly absorbing enolate to flavin chargetransfer complexes (7,11,12). These studies illustrate the profound stabilization of the enolates of these weakly acidic substrate analogues (with pK values lowered by 8-12 units; 7, 11, 12) on binding to the enzyme. The acidification of normal thioester substrates has been suggested to be even greater (12). The crystal structure of the medium-chain acylCoA dehydrogenase complexed with acyl-CoA substrates (13) suggests that the developing enolate might be stabilized in part by two hydrogen bonds: one from the 2′-OH of the ribityl side chain of FAD and one from a peptide N-H group to the substrate carbonyl oxygen (7,10,13; heavy dashed lines in Scheme 2). Thus, removal of one of these interactions by reconstitution of the enzyme with 2′-deoxy-FAD leads to a profound (g10 6 -fold) sl...

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Unstable carbanions. General acid catalysis of the cleavage of 1-phenylcyclopropanol and 1-phenyl-2-arylcyclopropanol anions

Cited by 83 publications

References 12 publications

Coenzyme B6 dependent novel bond cleavage reactions

Coenzyme B6 dependent novel bond cleavage reactions

Insights into the mechanisms of flavoprotein oxidases from kinetic isotope effects

Protonic Equilibria in the Reductive Half-Reaction of the Medium-Chain Acyl-CoA Dehydrogenase

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