title: Chalcone Isomerase from Eubacterium ramulus Catalyzes the Ring Contraction of Flavanonols.
abstract: The enzyme catalyzing the ring-contracting conversion of the flavanonol taxifolin to the auronol alphitonin in the course of flavonoid degradation by the human intestinal anaerobe Eubacterium ramulus was purified and characterized. It stereospecifically catalyzed the isomerization of (+)-taxifolin but not that of (-)-taxifolin. The Kmfor (+)-taxifolin was 6.4 ± 0.8 μM, and the Vmaxwas 108 ± 4 μmol min-1(mg protein)-1The enzyme also isomerized (+)-dihydrokaempferol, another flavanonol, to maesopsin. Inspection of the encoding gene revealed its complete identity to that of the gene encoding chalcone isomerase (CHI) from E. ramulus Based on the reported X-ray crystal structure of CHI (M. Gall et al., Angew Chem Int Ed 53:1439-1442, 2014, http://dx.doi.org/10.1002/anie.201306952), docking experiments suggest the substrate binding mode of flavanonols and their stereospecific conversion. Mutation of the active-site histidine (His33) to alanine led to a complete loss of flavanonol isomerization by CHI, which indicates that His33 is also essential for this activity. His33 is proposed to mediate the stereospecific abstraction of a proton from the hydroxymethylene carbon of the flavanonol C-ring followed by ring opening and recyclization. A flavanonol-isomerizing enzyme was also identified in the flavonoid-converting bacterium Flavonifractor plautii based on its 50% sequence identity to the CHI from E. ramulus IMPORTANCE: Chalcone isomerase was known to be involved in flavone/flavanone conversion by the human intestinal bacterium E. ramulus Here we demonstrate that this enzyme moreover catalyzes a key step in the breakdown of flavonols/flavanonols. Thus, a single isomerase plays a dual role in the bacterial conversion of dietary bioactive flavonoids. The identification of a corresponding enzyme in the human intestinal bacterium F. plautii suggests a more widespread occurrence of this isomerase in flavonoid-degrading bacteria.
authors.affiliation: Department of Gastrointestinal Microbiology, German Institute of Human Nutrition Potsdam-Rehbruecke, Nuthetal, Germany braune@dife.de.
authors.forename: Annett
authors.initials: A
authors.lastname: Braune
authors.affiliation: Analytics Group, German Institute of Human Nutrition Potsdam-Rehbruecke, Nuthetal, Germany.
authors.forename: Wolfram
authors.initials: W
authors.lastname: Engst
authors.affiliation: Pharmaceutical Institute, Pharmaceutical Chemistry I, University of Bonn, Bonn, Germany.
authors.forename: Paul W
authors.initials: PW
authors.lastname: Elsinghorst
authors.affiliation: Pharmaceutical Institute, Pharmaceutical Chemistry I, University of Bonn, Bonn, Germany Bonn-Aachen International Center for Information Technology, University of Bonn, Bonn, Germany.
authors.forename: Norbert
authors.initials: N
authors.lastname: Furtmann
authors.affiliation: Bonn-Aachen International Center for Information Technology, University of Bonn, Bonn, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Bajorath
authors.affiliation: Pharmaceutical Institute, Pharmaceutical Chemistry I, University of Bonn, Bonn, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Gütschow
authors.affiliation: Department of Gastrointestinal Microbiology, German Institute of Human Nutrition Potsdam-Rehbruecke, Nuthetal, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Blaut
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002384:Catalysis; D020134:Catalytic Domain; D005051:Eubacterium / Q000737:chemistry / Q000201:enzymology*; D044950:Flavanones / Q000737:chemistry / Q000378:metabolism*; D019753:Intramolecular Lyases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D015394:Molecular Structure
chemical_list: D001426:Bacterial Proteins; D044950:Flavanones; D019753:Intramolecular Lyases; C019865:chalcone isomerase; C028610:flavanone