title: Dehydration of (R)-2-hydroxyacyl-CoA to enoyl-CoA in the fermentation of alpha-amino acids by anaerobic bacteria.
abstract: Several clostridia and fusobacteria ferment alpha-amino acids via (R)-2-hydroxyacyl-CoA, which is dehydrated to enoyl-CoA by syn-elimination. This reaction is of great mechanistic interest, since the beta-hydrogen, to be eliminated as proton, is not activated (pK 40-50). A mechanism has been proposed, in which one high-energy electron acts as cofactor and transiently reduces the electrophilic thiol ester carbonyl to a nucleophilic ketyl radical anion. The 2-hydroxyacyl-CoA dehydratases are two-component systems composed of an extremely oxygen-sensitive component A, an activator, and component D, the actual dehydratase. Component A, a homodimer with one [4Fe-4S]cluster, transfers an electron to component D, a heterodimer with 1-2 [4Fe-4S]clusters and FMN, concomitant with hydrolysis of two ATP. From component D the electron is further transferred to the substrate, where it facilitates elimination of the hydroxyl group. In the resulting enoxyradical the beta-hydrogen is activated (pK14). After elimination the electron is handed-over to the next incoming substrate without further hydrolysis of ATP. The helix-cluster-helix architecture of component A forms an angle of 105 degrees, which probably opens to 180 degrees upon binding of ATP resembling an archer shooting arrows. Therefore we designated component A as 'Archerase'. Here, we describe 2-hydroxyglutaryl-CoA dehydratase from Acidaminococcus fermentans, Clostridium symbiosum and Fusobacterium nucleatum, 2-phenyllactate dehydratase from Clostridium sporogenes, 2-hydroxyisocaproyl-CoA dehydratase from Clostridium difficile, and lactyl-CoA dehydratase from Clostridium propionicum. A relative of the 2-hydroxyacyl-CoA dehydratases is benzoyl-CoA reductase from Thauera aromatica. Analogous but unrelated archerases are the iron proteins of nitrogenase and bacterial protochlorophyllide reductase. In anaerobic organisms, which do not oxidize 2-oxo acids, a second energy-driven electron transfer from NADH to ferredoxin, the electron donor of component A, has been established. The transfer is catalysed by a membrane-bound NADH-ferredoxin oxidoreductase driven by an electrochemical Na(+)-gradient. This enzyme is related to the Rnf proteins involved in Rhodobacter capsulatus nitrogen fixation.
authors.affiliation: Laboratorium für Mikrobiologie, Fachbereich Biologie, Philipps-Unversität, Karl-von-Frisch Strasse, D-35032 Marburg, Germany.
authors.forename: Jihoe
authors.initials: J
authors.lastname: Kim
authors.forename: Marc
authors.initials: M
authors.lastname: Hetzel
authors.forename: Clara Dana
authors.initials: CD
authors.lastname: Boiangiu
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Buckel
mesh_terms: D045850:Acidaminococcus / Q000201:enzymology / Q000378:metabolism; D000214:Acyl Coenzyme A / Q000378:metabolism*; D000596:Amino Acids / Q000378:metabolism*; D001421:Bacteria, Anaerobic / Q000201:enzymology / Q000378:metabolism*; D003013:Clostridium / Q000201:enzymology / Q000378:metabolism; D005285:Fermentation; D005287:Ferredoxin-NADP Reductase / Q000378:metabolism; D016967:Fusobacterium nucleatum / Q000201:enzymology / Q000378:metabolism; D006836:Hydro-Lyases / Q000378:metabolism; D009591:Nitrogenase / Q000378:metabolism; D016354:Rhodobacter capsulatus / Q000201:enzymology / Q000378:metabolism; D020628:Thauera / Q000201:enzymology / Q000378:metabolism
chemical_list: D000214:Acyl Coenzyme A; D000596:Amino Acids; C047009:lactoyl-coenzyme A; D005287:Ferredoxin-NADP Reductase; D009591:Nitrogenase; C054464:2-hydroxyglutaryl-CoA dehydratase; D006836:Hydro-Lyases; C411816:phenyllactate dehydratase