title: Substitution of the conserved tryptophan 31 in Escherichia coli thioredoxin by site-directed mutagenesis and structure-function analysis.
abstract: All prokaryotic and eukaryotic thioredoxins contain a conserved tryptophan residue, exposed at the active site disulfide/dithiol. The role of this W31 in Escherichia coli thioredoxin (Trx) was studied by site-directed mutagenesis. Four mutant Trx with W31Y, W31F, W31H, and W31A replacements were characterized. Very low tryptophan fluorescence emission from the remaining W28 was observed in all mutant Trx; reduction resulted in large, but variable increases (up to 11-fold) of fluorescence, to levels higher than in native or denatured wild-type Trx, demonstrating a previously postulated change involving W28. All W31 mutant Trx were good substrates for E. coli thioredoxin reductase. Compared with wild type, the apparent Km values were increased less than 2-fold for the W31A, W31H, and W31F Trx and the W31Y Trx showed even slightly higher catalytic efficiency (kcat/Km value). Functions of reduced Trx with ribonucleotide reductase and in reduction of insulin disulfides were more strongly influenced by the W31 replacements, in particular at low pH for A and H residues. T7 DNA polymerase activity generated by T7 gene 5 protein and reduced Trx was lowered by large factors for W31Y, W31A, or W31H compared with W31F or the wild-type protein. The in vivo function of Trx was studied by using pUC118-trxA expression in an E. coli trxA- background. The trxA genes with W31Y and W31F substitutions restored, fully and partly, the methionine sulfoxide utilization of a trxA- metE- test strain; W31A and W31H mutations resulted in no growth. Propagation of M13 was moderately impeded by W31Y and W31F or severely by W31A and W31H replacements. Growth of a phage T3/7 hybrid was possible only with the W31Y and W31F substitutions reflecting the in vitro results for T7 DNA polymerase.
authors.affiliation: Department of Physiological Chemistry, Karolinska Institutet, Stockholm, Sweden.
authors.forename: G
authors.initials: G
authors.lastname: Krause
authors.forename: A
authors.initials: A
authors.lastname: Holmgren
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D003001:Cloning, Molecular; D004252:DNA Mutational Analysis; D004259:DNA-Directed DNA Polymerase / Q000378:metabolism; D004220:Disulfides / Q000737:chemistry; D004926:Escherichia coli / Q000201:enzymology*; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009841:Oligonucleotides / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013329:Structure-Activity Relationship; D013604:T-Phages / Q000201:enzymology; D013879:Thioredoxins / Q000737:chemistry* / Q000235:genetics; D014364:Tryptophan / Q000737:chemistry
chemical_list: D004220:Disulfides; D009841:Oligonucleotides; D013879:Thioredoxins; D014364:Tryptophan; D004259:DNA-Directed DNA Polymerase