Spectroscopic characterization of recombinant pyridoxal kinase mutants

xiii, 248 leaves : ill. ; 30 cm.PolyU Library Call No.: [THS] LG51 .H577M ABCT 2003 NgThis project is divided into two parts; the first part is the overexpression and biochemical characterization of recombinant porcine and human pyridoxal kinase. The second part is to investigate the structure-function relationships of porcine pyridoxal kinase by studying the site-directed mutants. In the first part of the project, the recombinant porcine pyridoxal kinase (PPK) and His-tagged human pyridoxal kinase (His-HPK) were overexpressed in E. coli and B. subtilis, respectively. They were purified to homogeneity (18-fold for PPK and 57.5-fold for His-HPK). Both purified enzymes exhibited a molecular mass of about 35 kDa when examined by 12% SDS-PAGE. The results of kinetic analysis showed that the Km values for pyridoxal were 506 ± 49 and 883 ± 69 μM for PPK and His-HPK, respectively, whereas, Km values for ATP were 48 ± 4 and 27± 2 μM. Of the monovalent and divalent cations studied, Zn²⁺ was the most effective divalent cation in catalyzing the formation of pyridoxal-5-phosphate from ATP and pyridoxal, whereas the enzymes had very little activity with Na⁺. The pH dependence of stability and activity were also examined. The stability of both enzymes was maximal in the 5.5 -7.5 pH range. The human enzyme had maximum catalytic activity in the narrow pH range of 5.5-6.0, while the activity of the porcine enzyme was maximal at pH 5.5. The physical interactions between PK and PLP-dependent enzymes (GAD and ALT) were demonstrated by fluorescence polarization. In addition, the temperature- and denaturant-induced denaturation of the two enzymes was studied by means of circular dichroism and fluorescence spectroscopy. Both enzymes showed an intermediate denaturation temperature (Tm): 48℃ for PPK and 46.4℃ for His-HPK. They also showed a markedly resistance against urea; the midpoint transition was 4.3 M for PPK and 3.45 M for His-HPK. On the contrary, both enzymes showed a weak resistance against GuHCl; the midpoint transition was 0.96 M for PPK and 1 M for His-HPK. In the second part of the project, five site-directed mutants were studied so as to examine the structure-function relationships of PK. The mutant enzymes including Y135A, Y135F, G240A, G242A, and H186A were expressed and purified to homogeneity. The H186A mutant had -70% residual activity, whereas the Y135F mutant exhibited less than 10% residual activity. The mutations in Y135A, G240A, and G242A completely abolished enzymatic activity. All mutants, except G240A, displayed similar CD spectra and fluorescence quenching result in comparison to the wild-type. The results of substrate binding analysis showed that mutation of tyrosine 135 to alanine (Y135A) or phenylalanine (Y135F), and mutation of glycine 242 and histidine 186 to alanine (G242A and H186A) affected pyridoxal binding, while the G240A mutant did not show any interaction with both pyridoxal and ATP. Taken together, these results suggested that Tyr135, Gly242, and His186 played a significant role in pyridoxal binding, and Gly240 seemed to be involved in maintaining the proper conformation of the protein rather than direct involvement in catalysis. In addition, the GuHCl-induced unfolding processes of the site-directed mutants were studied by circular dichroism and fluorescence spectroscopy. No dramatic change in the midpoint transition was observed, implying that the residues substituted were not essential for maintaining the protein conformational stability.Department of Applied Biology and Chemical TechnologyM.Phil., Dept. of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, 200