Unusual DAhydrations in anaerobic bacteria: considering ketyls (radical anions) as reactive intermediates in enzymatic reactions

Buckel, Wolfgañg

doi:10.1016/0014-5793(96)00530-3

Cited by 59 publications

(59 citation statements)

References 41 publications

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“…Note that the nonphysiologic substrate hydroxylamine becomes ATP dependently reduced by BCR (2). [8][9][10][11][12][13][14] C]ATP from unlabeled ATP in the absence of an electron-accepting substrate was followed (Fig. 3).…”

Section: Resultsmentioning

confidence: 99%

“…Furthermore, the subunits of BCR show 38 -52% similarities to the twocomponent enzyme system activase (Hgd C-homodimer)͞2-hydroxyglutaryl-CoA dehydratase (Hgd A͞B-heterodimer) of Acidaminococcus fermentans (11,13). The monodimeric activase of this enzyme system catalyzes a nonstoichiometric ATPdependent activation of electrons that are essential for the initiation of water elimination from the ␣-position of 2-hydroxyglutaryl-CoA (7,14). Based on conserved amino acid motifs (11,13) and on the recently solved structure of activase from A. fermentans (15), BCR is postulated to contain two functionally distinct modules ( Fig.…”

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confidence: 99%

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Mechanism of ATP-driven electron transfer catalyzed by the benzene ring-reducing enzyme benzoyl- CoA reductase

Unciuleac

Boll

2001

Proc. Natl. Acad. Sci. U.S.A.

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Benzoyl-CoA reductase (BCR) from the bacterium Thauera aromatica catalyzes the two-electron reduction of benzoyl-CoA (BCoA) to a nonaromatic cyclic diene. In a process analogous to enzymatic nitrogen reduction, BCR couples the electron transfer to the aromatic ring to a stoichiometric hydrolysis of 2 ATP͞2e ؊ . Reduced but not oxidized BCR hydrolyzes ATP to ADP. In this work, purified BCR was shown to catalyze an isotope exchange from [ 14 C]ADP to [ 14 C]ATP, which was Ϸ15% of the ATPase activity in the presence of equimolar amounts of ADP and ATP. In accordance, BCR (␣␤␥␦-composition) autophosphorylated its ␥-subunit when incubated with [␥-32 P]ATP. Formation of the enzyme-phosphate was independent of the redox state, whereas only dithionitereduced BCR catalyzed a dephosphorylation associated with the ATPase activity. This finding suggests that the ATPase-and autophosphatase-partial activities of BCR exhibit identical redox dependencies. BCoA or the nonphysiological electron-accepting substrate hydroxylamine stimulated the redox-dependent effects; the rates of both the overall ATPase and the autophosphatase activities of reduced BCR were increased 6-fold. In contrast, BCoA and hydroxylamine had no effect on oxidized and phosphorylated BCR. The reactivity of the phosphoamino acid fits best with a phosphoamidate or acylphosphate linkage. The results obtained suggest a mechanism of ATP hydrolysis-driven electron transfer, which differs from that of nitrogenase by the transient formation of a phosphorylated enzyme.B enzoyl-CoA (BCoA) is a central intermediate in the metabolism of many aromatic compounds in anaerobic bacteria. This compound becomes reduced by benzoyl-CoA reductase (BCR), a key enzyme in anaerobic aromatic metabolism (1, 2). It catalyzes the reductive dearomatization of BCoA to cyclohexa-1,5-diene-1-carbonyl-CoA ( Fig. 1; ref. 3). In this reaction, the hydrolysis of two ATP to ADP and Pi is stoichiometrically coupled to the transfer of two single electrons from reduced ferredoxin to the aromatic substrate. Notably, BCR also catalyzes the ATP-dependent reduction of the nonphysiological substrate hydroxylamine (2). The stoichiometric coupling of ATP hydrolysis to an electron transfer process has long been considered as a unique feature of nitrogenases (4, 5). However, there are no amino acid sequence similarities between BCR and nitrogenases.The reduction of the aromatic ring is a mechanistically and energetically difficult process that in organic synthesis requires solvated electrons, which are generated by dissolving alkali metals in liquid ammonia. In a so-called Birch mechanism, the reduction of the aromatic ring proceeds in single electron and proton transfer steps by means of radical intermediates (6). A similar mechanism has been proposed for enzymatic benzene ring reduction (7). The crucial step is the first electron transfer to the aromatic ring yielding a nonaromatic radical anion, which requires a redox potential of Ϫ1.9 V (3). BCR overcomes this high redox barrier by coupling a stoichiomet...

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Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

Mechanism of ATP-driven electron transfer catalyzed by the benzene ring-reducing enzyme benzoyl- CoA reductase

Unciuleac

Boll

2001

Proc. Natl. Acad. Sci. U.S.A.

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“…45), thus facilitating the formation of the C-H bond by the intermediate 19 to give the labile diol 20 (bond dissociation energy for H-CH 2 R ϳ98 kcal mol Ϫ1 ) (42). A similar addition (with NuH ϭ H 2 O) has been suggested in the case of the diol dehydratase reaction mechanism (18,36,37,39). It remains to be elucidated whether this reaction mechanism also underlies anaerobic cleavage of PEG and its derivatives.…”

Section: Discussionmentioning

confidence: 76%

“…6B), based on the intermediacy of the resonance stabilized (␣-carbonyl 7 enoxy) radical 18 (33), is supported by the propensity of ketyls (radical anions) (e.g. 17) to eliminate adjacent leaving groups as a result of their electron-rich character (38,39). The cleavage of the ␤-C,O-bond can also be facilitated by stereoelectronic effects in the appropriate conformation of the radical anion 17 (40).…”

Section: Discussionmentioning

confidence: 99%

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Mechanism of Anaerobic Ether Cleavage

Speranza

Mueller

Orlandi

et al. 2002

Journal of Biological Chemistry

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During the course of the biotransformation process, the molecular integrity of the glycolic unit was completely retained, no loss of the migrating deuterium occurred by exchange with the medium, and the 1,2-deuterium shift was intramolecular. A diol dehydrataselike mechanism could explain the enzymatic cleavage of the ether bond of 2-phenoxyethanol, provided that an intramolecular H/OC 6 H 5 exchange is assumed, giving rise to the hemiacetal precursor of acetaldehyde. However, an alternative mechanism is proposed that is supported by the well recognized propensity of ␣-hydroxyradical and of its conjugate base (ketyl anion) to eliminate a ␤-positioned leaving group.

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