title: A trade-off between catalytic power and substrate inhibition in TCHQ dehalogenase.
abstract: Tetrachlorohydroquinone (TCHQ) dehalogenase is profoundly inhibited by its aromatic substrates, TCHQ and trichlorohydroquinone (TriCHQ). Surprisingly, mutations that change Ile12 to either Ser or Ala give an enzyme that shows no substrate inhibition. We have previously shown that TriCHQ is a noncompetitive inhibitor of the thiol-disulfide exchange reaction between glutathione and ESSG, a covalent adduct between Cys13 and glutathione formed during dehalogenation of the substrate. Substrate inhibition of the thiol-disulfide exchange reaction is less severe in the I12S and I12A mutant enzymes, primarily due to weaker binding of TriCHQ to ESSG. These mutations also result in a decrease in the rate of dehalogenation. Because the rate-limiting step in the I12S and I12A enzymes is dehalogenation, rather than the thiol-disulfide exchange reaction, the relatively modest inhibition of the thiol-disulfide exchange reaction does not affect the overall rate of turnover.
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology and Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado 80309, USA.
authors.forename: Joseph R
authors.initials: JR
authors.lastname: Warner
authors.forename: Linda S
authors.initials: LS
authors.lastname: Behlen
authors.forename: Shelley D
authors.initials: SD
authors.lastname: Copley
mesh_terms: D000595:Amino Acid Sequence; D002384:Catalysis; D004220:Disulfides / Q000737:chemistry / Q000378:metabolism; D006867:Hydrolases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D006873:Hydroquinones / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D008956:Models, Chemical; D008969:Molecular Sequence Data; D015394:Molecular Structure; D009154:Mutation; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D013438:Sulfhydryl Compounds / Q000737:chemistry / Q000378:metabolism
chemical_list: D004220:Disulfides; D006873:Hydroquinones; D013438:Sulfhydryl Compounds; D006867:Hydrolases; C078610:2,3,5,6-tetrachloro-4-hydroquinone reductive dehalogenase