title: Mössbauer study of 4-hydroxybutyryl-CoA dehydratase--probing the role of an iron-sulfur cluster in an overall non-redox reaction.
abstract: 4-Hydroxybutyryl-CoA dehydratase from Clostridium aminobutyricum catalyzes the dehydration of 4-hydroxybutyryl-CoA to crotonyl-CoA. Although dehydration is an overall non-redox reaction, the enzyme contains FAD and Fe-S clusters. Previous work has shown that the Fe-S clusters are difficult to reduce and therefore unlikely to be redox-active in catalysis. Here, Mössbauer spectroscopy has been used to characterise the Fe-S clusters in active as well as in air-inactivated enzyme. In zero magnetic field at 80 K and 4.2 K, the spectra of active dehydratase consisted mainly of one species (95%) with quadrupole splitting, deltaE(Q) = 1.00 mm s(-1) and isomer shift, delta = 0.43 mm s(-1). Magnetically perturbed Mössbauer spectra indicated a spin of zero. In the presence of 6 mM crotonyl-CoA, the spectra remained unchanged. Taken together, the data show that there are [4Fe-4S]2+ in the enzyme, most probably two clusters/homotetramer, that the four iron atoms in each cluster are coordinated in an identical fashion, and that there is no direct interaction with substrates. We therefore infer that the Fe-S clusters serve a structural rather than a catalytic role in 4-hydroxybutyryl-CoA dehydratase. In air-inactivated enzyme (10% residual activity), a new doublet appeared (58%) with deltaE(Q) = 0.72 mm s(-1), delta = 0.32 mm s(-1) and S = 0. The assignment of this subspectrum to [3Fe-4S]+ clusters, based on the typical Mössbauer parameters, is contradicted by the finding of spin zero for the species. One possible explanation could be spin-coupling of two [3Fe-4S]+ clusters in close proximity.
authors.affiliation: Laboratorium für Mikrobiologie am Fachbereich der Philipps Universität Marburg, Germany.
authors.forename: U
authors.initials: U
authors.lastname: Müh
authors.forename: W
authors.initials: W
authors.lastname: Buckel
authors.forename: E
authors.initials: E
authors.lastname: Bill
mesh_terms: D000388:Air; D004789:Enzyme Activation; D006836:Hydro-Lyases / Q000737:chemistry* / Q000378:metabolism; D007501:Iron / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D015204:Spectroscopy, Mossbauer; D013455:Sulfur / Q000737:chemistry / Q000378:metabolism
chemical_list: D013455:Sulfur; D007501:Iron; C066370:4-hydroxybutyryl-CoA dehydratase; D006836:Hydro-Lyases