ABSTRACT
Hydrophobization of alpha-helices is one of the general approaches used for improving the thermal stability of enzymes. A total of 11 serine residues located in alpha-helices have been found based on multiple alignments of the amino acid sequences of D-amino acid oxidases from different organisms and the analysis of the 3D-structure of D-amino acid oxidase from yeast Trigonopsis variabilis (TvDAAO, EC 1.4.3.3). As a result of further structural analysis, eight Ser residues in 67, 77, 78, 105, 270, 277, 335, and 336 positions have been selected to be substituted with Ala. S78A and S270A substitutions have resulted in dramatic destabilization of the enzyme. Mutant enzymes were inactivated during isolation from cells. Another six mutant TvDAAOs have been highly purified and their properties have been characterized. The amino acid substitutions S277A and S336A destabilized the protein globule. The thermal stabilities of TvDAAO S77A and TvDAAO S335A mutants were close to that of the wild-type enzyme, while S67A and S105A substitutions resulted in approximately 1.5- and 2.0-fold increases in the TvDAAO mutant thermal stability, respectively. Furthermore, the TvDAAO S105A mutant showed on average a 1.2- to 3.0-fold higher catalytic efficiency with D-Asn, D-Tyr, D-Phe, and D-Leu as compared to the wild-type enzyme.
ABSTRACT
Based on Escherichia coli, highly sensitive specific lux-biosensors for the detection of tetracycline and beta-lactam antibiotics, quinolones, and aminoglycosides have been obtained. To make biosensors, bacteria were used that contained fungal plasmids pTetA'::lux, pAmpC'::lux, pColD'::lux, and plbpA'::lux, in which transcription of the reporter Photorhabdus luminescens luxCDABE genes occurred from the inducible promoters of the tetA, ampC, cda, and ibpA genes, respectively. The main parameters (threshold sensitivity and response time) of lux-biosensors were measured. The high specificity of biosensors responding only to antibiotics of a certain type was demonstrated.
