title: Enzyme catalyzed radical dehydrations of hydroxy acids.
abstract: BACKGROUND
  The steadily increasing field of radical biochemistry is dominated by the large family of S-adenosylmethionine dependent enzymes, the so-called radical SAM enzymes, of which several new members are discovered every year. Here we report on 2- and 4-hydroxyacyl-CoA dehydratases which apply a very different method of radical generation. In these enzymes ketyl radicals are formed by one-electron reduction or oxidation and are recycled after each turnover without further energy input. Earlier reviews on 2-hydroxyacyl-CoA dehydratases were published in 2004 [J. Kim, M. Hetzel, C.D. Boiangiu, W. Buckel, FEMS Microbiol. Rev. 28 (2004) 455-468. W. Buckel, M. Hetzel, J. Kim, Curr. Opin. Chem. Biol. 8 (2004) 462-467.]
  
  
  METHODS
  The review focuses on four types of 2-hydroxyacyl-CoA dehydratases that are involved in the fermentation of amino acids by anaerobic bacteria, especially clostridia. These enzymes require activation by one-electron transfer from an iron-sulfur protein driven by hydrolysis of ATP. The review further describes the proposed mechanism that is highlighted by the identification of the allylic ketyl radical intermediate and the elucidation of the crystal structure of 2-hydroxyisocapryloyl-CoA dehydratase. With 4-hydroxybutyryl-CoA dehydratase the crystal structure, the complete stereochemistry and the function of several conserved residues around the active site could be identified. Finally potential biotechnological applications of the radical dehydratases are presented.
  
  
  CONCLUSIONS
  The action of the activator as an 'Archerase' shooting electrons into difficultly reducible acceptors becomes an emerging principle in anaerobic metabolism. The dehydratases may provide useful tools in biotechnology. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.
authors.affiliation: Max-Planck-Institut für terrestrische Mikrobiologie, 35043 Marburg, Germany. buckel@staff.uni-marburg.de
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Buckel
authors.forename: Jin
authors.initials: J
authors.lastname: Zhang
authors.forename: Peter
authors.initials: P
authors.lastname: Friedrich
authors.forename: Anutthaman
authors.initials: A
authors.lastname: Parthasarathy
authors.forename: Huan
authors.initials: H
authors.lastname: Li
authors.forename: Ivana
authors.initials: I
authors.lastname: Djurdjevic
authors.forename: Holger
authors.initials: H
authors.lastname: Dobbek
authors.forename: Berta M
authors.initials: BM
authors.lastname: Martins
mesh_terms: D000255:Adenosine Triphosphate / Q000737:chemistry / Q000378:metabolism; D000693:Anaerobiosis; D001421:Bacteria, Anaerobic / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D055162:Biocatalysis; D020134:Catalytic Domain; D005609:Free Radicals / Q000737:chemistry / Q000378:metabolism; D006836:Hydro-Lyases / Q000737:chemistry / Q000378:metabolism*; D007506:Iron-Sulfur Proteins / Q000737:chemistry / Q000378:metabolism*; D007527:Isoenzymes / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D010084:Oxidation-Reduction; D012436:S-Adenosylmethionine / Q000737:chemistry / Q000378:metabolism*; D013237:Stereoisomerism
chemical_list: D001426:Bacterial Proteins; D005609:Free Radicals; D007506:Iron-Sulfur Proteins; D007527:Isoenzymes; D012436:S-Adenosylmethionine; D000255:Adenosine Triphosphate; D006836:Hydro-Lyases