title: Directed evolution of (βα)(8)-barrel enzymes: establishing phosphoribosylanthranilate isomerisation activity on the scaffold of the tryptophan synthase α-subunit.
abstract: Phosphoribosylanthranilate (PRA) isomerase (TrpF) and tryptophan synthase α-subunit (TrpA) are (βα)(8)-barrel enzymes that are involved in the biosynthesis of tryptophan. They contain a conserved phosphate binding site, which indicates a common evolutionary origin. In order to experimentally back this hypothesis, we have established TrpF activity on the scaffold of TrpA from Salmonella typhimurium using protein engineering. Based on the superposition of crystal structures with bound ligands, two residues in the active site of TrpA were replaced with catalytic residues from TrpF using site-directed mutagenesis. This TrpA variant as well as wild-type TrpA were each subjected to random mutagenesis using error-prone PCR. The two resulting trpA gene libraries were used to transform an auxotrophic Escherichia coli trpF deletion strain, and TrpA variants with PRA isomerisation activity were isolated by in vivo complementation. The amino acid substitutions of the selected TrpA variants were recombined by DNA shuffling, again followed by complementation in vivo. Several TrpA variants were produced in E. coli and purified, and their catalytic TrpF activities were determined in vitro by steady-state enzyme kinetics. Our results support that TrpA and TrpF have evolved by gene duplication and diversification from each other or a common predecessor, and provide insights into the minimum requirements for the catalysis of PRA isomerisation.
authors.affiliation: Biochemistry Department, Faculty of Science,  Ege University, 35100 Bornova-Izmir,  Turkey.
authors.forename: Serap
authors.initials: S
authors.lastname: Evran
authors.forename: Azmi
authors.initials: A
authors.lastname: Telefoncu
authors.forename: Reinhard
authors.initials: R
authors.lastname: Sterner
mesh_terms: D019747:Aldose-Ketose Isomerases / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D019943:Amino Acid Substitution; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D020134:Catalytic Domain; D032621:DNA Shuffling; D019020:Directed Molecular Evolution / Q000379:methods*; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D015723:Gene Library; D005816:Genetic Complementation Test; D007700:Kinetics; D008958:Models, Molecular; D015202:Protein Engineering / Q000379:methods*; D021122:Protein Subunits; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012486:Salmonella typhimurium / Q000201:enzymology / Q000235:genetics; D014364:Tryptophan / Q000378:metabolism; D014367:Tryptophan Synthase / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D021122:Protein Subunits; D011994:Recombinant Proteins; D014364:Tryptophan; D014367:Tryptophan Synthase; D019747:Aldose-Ketose Isomerases; C029865:phosphoribosylanthranilate isomerase