title: Replacement of active-site lysine-239 of thermostable aspartate aminotransferase by S-(2-aminoethyl)cysteine: properties of the mutant enzyme.
abstract: The active-site lysine residue of thermostable aspartate aminotransferase, Lys-239, to which the cofactor, pyridoxal 5'-phosphate (PLP), is bound, has been converted to Cys by site-directed mutagenesis. The thiol group of Cys-239 was chemically aminoethylated with ethylenimine. Amino acid analysis of the modified enzyme showed that it contained about 1 mol of S-(2-aminoethyl)cysteine (SAEC) per mol subunit. The activity of the mutant enzyme (K239SAEC) was about 14% of that of the wild-type enzyme. No significant difference in thermostability was found between the wild-type and K239SAEC enzymes. The UV-visible spectrum of K239SAEC showed a peak (lambda max 380 nm), due to absorption by the cofactor, at a 20 nm longer wavelength than that of the wild-type enzyme. The circular dichroism band due to the bound cofactor of K239SAEC also shifted toward a 20 nm longer wavelength. We determined kinetic parameters (rate constants, kmax, and dissociation constants, Kd, for the substrates) for each half transamination catalyzed by the wild-type and K239SAEC mutant enzymes by the stopped-flow method. The kmax values for the mutant enzyme reactions were 2.6-24 times lower than those for the wild-type enzyme ones. The two enzymes showed similar Kd values for the same substrates except glutamate; the mutant enzyme showed higher affinity for glutamate than the wild-type enzyme.
authors.affiliation: Institute for Chemical Research, Kyoto University.
authors.forename: Y
authors.initials: Y
authors.lastname: Matsushima
authors.forename: D W
authors.initials: DW
authors.lastname: Kim
authors.forename: T
authors.initials: T
authors.lastname: Yoshimura
authors.forename: S
authors.initials: S
authors.lastname: Kuramitsu
authors.forename: H
authors.initials: H
authors.lastname: Kagamiyama
authors.forename: N
authors.initials: N
authors.lastname: Esaki
authors.forename: K
authors.initials: K
authors.lastname: Soda
mesh_terms: D001219:Aspartate Aminotransferases / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D003545:Cysteine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D004795:Enzyme Stability; D004926:Escherichia coli / Q000235:genetics; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008239:Lysine / Q000737:chemistry* / Q000378:metabolism; D008956:Models, Chemical; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D011732:Pyridoxal Phosphate / Q000378:metabolism; D013696:Temperature; D013816:Thermodynamics
chemical_list: C000204:S-2-aminoethyl cysteine; D011732:Pyridoxal Phosphate; D001219:Aspartate Aminotransferases; D008239:Lysine; D003545:Cysteine