title: Evidence for direct interaction between cysteine 138 and the flavin in thioredoxin reductase. A study using flavin analogs.
abstract: The flavoenzyme thioredoxin reductase from Escherichia coli contains an oxidation-reduction active disulfide made up of Cys135 and Cys138. Mutations changing each Cys residue to a Ser residue have been effected (Prongay, A. J., engelke, D. R., and Williams, C. H., Jr. (1989) J. Biol. Chem. 264, 2656-2664). The FAD prosthetic group of each altered thioredoxin reductase has been replaced with 1-deaza-FAD (a flavin analog with carbon substituted for nitrogen at position 1), 4-thio-FAD (a flavin analog with sulfur substituted for oxygen at position 4), and 6-thiocyanato-FAD. 1-Deaza-FAD-TRR(Cys135,Ser138) has absorbance and fluorescence spectral properties similar to the oxidized form of wild type apothioredoxin reductase reconstituted with 1-deaza-FAD. The absorbance spectrum of 1-deaza-FAD-TRR(Ser135,Cys138) is similar to the spectrum of the two-electron reduced form of wild type apothioredoxin reductase reconstituted with 1-deaza-FAD, indicating that it is a mixture of two species (O'Donnell, M. E., and Williams, C. H., Jr. (1984) J. Biol. Chem. 259, 2243-2251). The spectrum of one of these species of 1-deaza-FAD-TRR(Ser135,Cys138) resembles the spectrum of oxidized 1-deaza-FAD bound to wild type apothioredoxin reductase. The other species has an absorbance spectrum with a single peak at 400 nm (epsilon 400 = 11,100 M-1 cm-1) and resembles the spectrum of a thiolate adduct at the C4a position of the 1-deaza-FAD. The equilibrium between these species is pH-dependent, with a maximum of 50% C4a-adduct formation at low pH, and is linked to pK alpha values at 8.2 and 9.3. The absorbance spectrum of 4-thio-FAD-TRR(Cys135,Ser138) resembles the spectrum of the unbound 4-thio-FAD, whereas 4-thio-FAD-TRR(Ser135,Cys138) has a spectrum indicative of a mixture of 4-thio-FAD and FAD, suggesting a reaction between the 4-position of the flavin and Cys138. The binding of 6-thiocyanato-FAD to the apoprotein of the mutated enzymes showed no evidence for a reaction between the thiols and the group at the 6-position of the flavin.
authors.affiliation: Department of Biological Chemistry, University of Michigan, Ann Arbor 48105.
authors.forename: A J
authors.initials: AJ
authors.lastname: Prongay
authors.forename: C H
authors.initials: CH
authors.lastname: Williams
mesh_terms: D001665:Binding Sites; D003545:Cysteine; D004926:Escherichia coli / Q000201:enzymology*; D005182:Flavin-Adenine Dinucleotide / Q000031:analogs & derivatives* / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D013053:Spectrophotometry; D013880:Thioredoxin-Disulfide Reductase / Q000235:genetics / Q000378:metabolism*
chemical_list: D005182:Flavin-Adenine Dinucleotide; D013880:Thioredoxin-Disulfide Reductase; D003545:Cysteine