title: The role of cysteine residues in structure and enzyme activity of a maize beta-glucosidase.
abstract: The maize Zm-p60.1 gene encodes a beta-glucosidase that can release active cytokinins from their storage forms, cytokinin-O-glucosides. Mature catalytically active Zm-p60.1 is a homodimer containing five cysteine residues per a subunit. Their role was studied by mutating them to alanine (A), serine (S), arginine (R) or aspartic acid (D) using site-directed mutagenesis, and subsequent heterologous expression in Escherichia coli. All substitutions of C205 and C211 resulted in decreased formation and/or stability of the homodimer, manifested as accumulation of high levels of monomer in the bacterial expression system. Examination of urea- and glutathione-induced dissociation patterns of the homodimer to the monomers, HPLC profiles of hydrolytic fragments of reduced and oxidized forms, and a homology-based three-dimensional structural model revealed that an intramolecular disulfide bridge formed between C205 and C211 within the subunits stabilized the quaternary structure of the enzyme. Mutating C52 to R produced a monomeric enzyme protein, too. No detectable effects on homodimer formation were apparent in C170 and C479 mutants. Given the Km values for C170A/S mutants were equal to that for the wild-type enzyme, C170 cannot participate in enzyme-substrate interactions. Possible indirect effects of C170A/S mutations on catalytic activity of the enzyme were inferred from slight decreases in the apparent catalytic activity, k'cat. C170 is located on a hydrophobic side of an alpha-helix packed against hydrophobic amino-acid residues of beta-strand 4, indicating participation of C170 in stabilization of a (beta/alpha)8 barrel structure in the enzyme. In C479A/D/R/S mutants, Km and k'cat were influenced more significantly suggesting a role for C479 in enzyme catalytic action.
authors.affiliation: Institute of Biophysics, Academy of Sciences of the Czech Republic, Brno, Czech Republic.
authors.forename: V
authors.initials: V
authors.lastname: Rotrekl
authors.forename: E
authors.initials: E
authors.lastname: Nejedlá
authors.forename: I
authors.initials: I
authors.lastname: Kucera
authors.forename: F
authors.initials: F
authors.lastname: Abdallah
authors.forename: K
authors.initials: K
authors.lastname: Palme
authors.forename: B
authors.initials: B
authors.lastname: Brzobohatý
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002851:Chromatography, High Pressure Liquid; D003545:Cysteine / Q000737:chemistry; D004926:Escherichia coli / Q000235:genetics; D017343:Genes, Plant; D005978:Glutathione / Q000494:pharmacology; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010446:Peptide Fragments / Q000302:isolation & purification; D011487:Protein Conformation; D011489:Protein Denaturation / Q000187:drug effects; D020836:Protein Structure, Quaternary; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D014508:Urea / Q000494:pharmacology; D003313:Zea mays / Q000201:enzymology* / Q000235:genetics; D001617:beta-Glucosidase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
chemical_list: D010446:Peptide Fragments; D011994:Recombinant Proteins; D014508:Urea; D001617:beta-Glucosidase; D005978:Glutathione; D003545:Cysteine