Einelektronen‐Redoxreaktionen von Coenzym‐A‐Estern in anaeroben Bakterien – ein Vorschlag für einen neuen Mechanismus

Keese, Reinhart

doi:10.1002/ange.19951071321

Cited by 28 publications

(31 citation statements)

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“…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).…”

mentioning

confidence: 65%

“…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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confidence: 65%

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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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“…It should be noted that the removal of a hydrogen atom (proton+electron) from an alcohol is indeed an oxidation. Ketyls, which hitherto have not been considered as reactive species in biochemistry, may also play important roles in other enzymatic mechanisms such as ribonucleotide reductase, anaerobic degradation of aromatic compounds or pyruvate ferredoxin reductase (see [23]). …”

Section: Discussionmentioning

confidence: 99%

“…This 'energy-rich' electron could be able to reduce the thiol ester of (R)-2-hydroxyglutaryl-CoA to a ketyl, which eliminates the adjacent hydroxyl group yielding an enoxy radical. The radical is deprotonated to the corresponding ketyl of glutaconyl-CoA followed by oxidation to the product [11,23]. Hence, the dehydration involves a one-electron cycle, which may last for many turnovers, explaining the catalytic rather than stoichiometric requirement for ATP.…”

Section: Dehydration Of 2-hydroxyacyl-coa Estersmentioning

confidence: 99%

“…Reactivation is achieved by brief exposure to air (few seconds) or by the more controlled oxidation with hexacyanoferrate(III) [25]. In a possible mechanism, adapted from [11,23], deprotonation at the c~-carbon yields the enolate which -in contrast to (R)-2-hydroxyglutaryl-CoA dehydratase-is oxidized by FAD to an enoxy radical. The now facilitated abstraction of a proton from the ]3-carbon gives a ketyl, from which the hydroxyl group is removed forming the dienoxy radical.…”

Section: -Hydroxybutyryl-coa Dehydratasementioning

confidence: 99%

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

Buckel

1996

FEBS Letters

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Dehydratases have been detected in anaerobic bacteria which use 2-, 4-or 5-hydroxyacyI-CoA as substrates and are involved in the removal of hydrogen atoms from the unactivated ~-or T-positions. In addition there are bacterial dehydratases acting on 1,2-diols which are substrates lacking any activating group. These enzymes contain either FAD, or flavins + ironsulfur clusters or coenzyme B12. It has been proposed that the overall dehydrations are actually reductions followed by oxidations or vice versa mediated by these prosthetic groups. Whereas the ~hydrogen of 5-hydroxyvaleryl-CoA is activated by a transient two-electron oq~-oxidation, the other substrates are proposed to require either a transient one-electron reduction or an oxidation to a ketyl (radical anion).

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4‐Hydroxybenzoyl‐CoenzymeAReductase

Boll

2004

Handbook of Metalloproteins

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The dehydroxylating 4‐hydroxybenzoyl‐CoA reductase (HBCR) plays an important role in the degradation of para ‐hydroxylated aromatic compounds in anaerobic bacteria. The enzyme catalyzes the removal of the phenolic hydroxyl group from 4‐hydroxybenzoyl‐CoA, yielding benzoyl‐CoA and water; a reduced ferredoxin serves as an electron donor. The enzyme belongs to the xanthine oxidase (XO) family of molybdenum cofactor containing enzymes and contains a molybdopterin‐cytosine dinucleotide cofactor, two [2Fe–2S] clusters, FAD and a [4Fe–4S] cluster per functional unit. HBCR is unique among XO family members in catalyzing the irreversible reduction of the substrate. A mechanism that differs from all other XO members via highly reactive radical intermediates has been suggested. The structural, electrochemical, spectroscopic, and kinetic properties of HBCR from Thauera aromatica are highlighted and compared with other members of the xanthine oxidase family.

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Einelektronen‐Redoxreaktionen von Coenzym‐A‐Estern in anaeroben Bakterien – ein Vorschlag für einen neuen Mechanismus

Cited by 28 publications

References 35 publications

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

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

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

4‐Hydroxybenzoyl‐CoenzymeAReductase

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