title: Site-directed mutants of a soluble form of penicillin-binding protein 5 from Escherichia coli and their catalytic properties.
abstract: Soluble, truncated mutant and wild-type forms of penicillin-binding protein 5 (sPBP 5) from Escherichia coli were produced in large amounts by placing the dacA gene that encodes PBP 5 under the control of the trp-lac fusion promoter. The 3' end of the dacA gene used in this study contains a stop codon that results in the deletion of 15 amino acids from the carboxyl terminus and the production of a soluble protein. Using oligonucleotide-directed mutagenesis, the role of cysteine 115 in the mechanism of sPBP 5 was investigated. Alkylation of cysteine 115 with sulfhydryl reagents has previously been shown to inhibit severely the D-alanine carboxypeptidase activity of PBP 5. Alkylation also inhibits the hydrolysis of bound penicillin G, with only a slight effect on its binding. Cysteine 115 in sPBP 5 was changed to either a serine (sPBP 5C-S) or an alanine (sPBP 5C-A) residue. The wild-type and mutant sPBPs were purified in milligram amounts from induced cultures by ampicillin affinity chromatography. The mutant PBPs showed only a 2-fold increase in the half-life of the penicilloyl-PBP complex, and had a binding affinity for penicillin G identical to wild-type PBP 5. The Km for the release of D-alanine from the peptide L-Ala-D-gamma-Glu-L-Lys-D-Ala-D-Ala was 5.0, 3.5, and 7.8 mM for PBP 5, PBP 5C-S, and PBP 5C-A, respectively, while the values for Vmax were 2.5, 3.3, and 5.1 mumol/min/mg. From these data it was concluded that the cysteine residue does not directly participate in the enzymatic mechanism.
authors.affiliation: Department of Biochemistry and Molecular Biology, Harvard University, Cambridge, Massachusetts 02138.
authors.forename: R A
authors.initials: RA
authors.lastname: Nicholas
authors.forename: J L
authors.initials: JL
authors.lastname: Strominger
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins; D001483:Base Sequence; D002352:Carrier Proteins / Q000235:genetics*; D003001:Cloning, Molecular; D003545:Cysteine; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000378:metabolism*; D005798:Genes, Bacterial; D006207:Half-Life; D006602:Hexosyltransferases; D007544:Isopropyl Thiogalactoside; D002267:Muramoylpentapeptide Carboxypeptidase / Q000235:genetics*; D009154:Mutation; D046915:Penicillin-Binding Proteins; D010458:Peptidyl Transferases; D012995:Solubility; D013329:Structure-Activity Relationship
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; D046915:Penicillin-Binding Proteins; D007544:Isopropyl Thiogalactoside; D010458:Peptidyl Transferases; D006602:Hexosyltransferases; D002267:Muramoylpentapeptide Carboxypeptidase; D003545:Cysteine