title: Purification and detailed study of two clinically different human glucose 6-phosphate dehydrogenase variants, G6PD(Plymouth) and G6PD(Mahidol): Evidence for defective protein folding as the basis of disease.
abstract: In an attempt to investigate the molecular mechanism underlying human glucose-6-phosphate dehydrogenase (G6PD) deficiency caused by two mutations, G6PD(Plymouth) (G163D) and G6PD(Mahidol) (G163S), the two variants were constructed by site-directed mutagenesis and expressed in G6PD-deficient E. coli DF 213 cells. A first indication of impaired folding came from problems in expressing these clinical mutants, which were only overcome by lowering the growth temperature or co-expressing with molecular chaperones (GroEL and GroES). Both strategies significantly increased soluble expression of recombinant G6PD(Plymouth) and G6PD(Mahidol), judged by both G6PD activity in extracts and the amount of immunoreactive protein. Using a modified 3-step protocol, the two mutant enzymes were successfully purified for the first time. Steady-state kinetic parameters (K(m) for NADP(+), K(m) for G6P and k(cat)) of the two mutants are very similar to the wild-type values, indicating that the catalytic efficiency of the two mutants remains unchanged. The two mutants are, however, markedly less stable than wild-type G6PD in both thermostability and urea-induced inactivation tests. In a typical experiment at 37 degrees C and pH 7.2 after 24h G6PD WT, G6PD(Mahidol) and G6PD(Plymouth) retained 58.3%, 27.0% and 3.9%, respectively, of their corresponding initial activity. The stability of all three enzymes is enhanced by addition of NADP(+). According to unfolding and refolding experiments, the two mutants are impaired in their folding properties. Thus structural instability appears to be the molecular basis of the clinical phenotype in G6PD(Plymouth) and G6PD(Mahidol) and in particular of the differing clinical severity of the two mutations. The 3-D structure solved for G6PD(Canton) allows an interpretation of these effects in terms of steric hindrance.
authors.affiliation: Department of Biochemistry, The University of Hong Kong, Hong Kong SAR, China.
authors.forename: Yuxiang
authors.initials: Y
authors.lastname: Huang
authors.forename: Mei Yee
authors.initials: MY
authors.lastname: Choi
authors.forename: Shannon Wing Ngor
authors.initials: SW
authors.lastname: Au
authors.forename: Deborah Man Yee
authors.initials: DM
authors.lastname: Au
authors.forename: Veronica Min Sien
authors.initials: VM
authors.lastname: Lam
authors.forename: Paul C
authors.initials: PC
authors.lastname: Engel
mesh_terms: D018835:Chaperonin 10 / Q000235:genetics / Q000378:metabolism; D018834:Chaperonin 60 / Q000235:genetics / Q000378:metabolism; D018833:Chaperonins / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D005954:Glucosephosphate Dehydrogenase / Q000737:chemistry / Q000235:genetics* / Q000302:isolation & purification* / Q000378:metabolism; D005955:Glucosephosphate Dehydrogenase Deficiency / Q000235:genetics*; D006801:Humans; D008958:Models, Molecular; D050505:Mutant Proteins / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification* / Q000378:metabolism; D011489:Protein Denaturation; D017510:Protein Folding; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D013696:Temperature
chemical_list: D018835:Chaperonin 10; D018834:Chaperonin 60; D050505:Mutant Proteins; D011993:Recombinant Fusion Proteins; D005954:Glucosephosphate Dehydrogenase; D018833:Chaperonins