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J. Bacteriol. doi:10.1128/JB.01467-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Cyclohexa-1,5-diene-1-carbonyl-coenzyme A hydratases of Geobacter metallireducens and Syntrophus aciditrophicus: evidence for a common benzoyl-CoA degradation pathway in facultative and strict anaerobes

Institute for Biology II, Microbiology, University of Freiburg, 79104 Freiburg, Germany; Department of Botany and Microbiology, University of Oklahoma, Norman, Oklahoma 73019, USA; Insitute for Biochemistry, University of Leipzig, 04103 Leipzig, Germany

^* To whom correspondence should be addressed. Email: boll{at} uni-leipzig.de.

	Abstract

In the denitrifying bacterium Thauera aromatica the central intermediate of anaerobic aromatic metabolism, benzoyl-coenzyme A (CoA), is dearomatized by the ATP-dependent benzoyl-CoA reductase to cyclohexa-1,5-diene-1-carbonyl-CoA (dienoyl-CoA). The dienoyl-CoA is further metabolized by a series of -oxidation-like reactions of the so-called benzoyl-CoA pathway resulting in ring cleavage. Recently, evidence was obtained that obligately anaerobic bacteria that use aromatic growth substrates do not contain an ATP-dependent benzoyl-CoA reductase. In these bacteria, the reactions involved in dearomatization and cleavage of the aromatic ring have not been shown, so far. In this work, a characteristic enzymatic step of the benzoyl-CoA pathway in obligate anaerobes was demonstrated and characterized. Dienoyl-CoA hydratase activities were determined in extracts of cells from Geobacter metallireducens (iron-reducing), Syntrophus aciditrophicus (fermenting) and Desulfococcus multivorans (sulfate-reducing) grown with benzoate. The benzoate-induced genes putatively coding for the dienoyl-CoA hydratases in the benzoate degraders, G. metallireducens and S. aciditrophicus, were heterologously expressed and characterized. Both gene products specifically catalyzed the reversible hydration of dienoyl-CoA to 6-hydroxycyclohexenoyl-CoA (K_m: 80 and 35 µM, v_max = 350 and 550 µmol min^-1 mg^-1, respectively). Neither enzyme had significant activity with cyclohex-1-ene-1-carbonyl-CoA or crotonyl-CoA. The results suggest that benzoyl-CoA degradation proceeds via dienoyl-CoA and 6-hydroxycyclohexanoyl-CoA in strictly anaerobic bacteria. The steps involved in dienoyl-CoA metabolism appear identical in all non-photosynthetic anaerobic bacteria, although totally differing benzene ring-dearomatizing enzymes are present in facultative and obligate anaerobes.

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