Stabilization of quaternary structure of water-soluble quinoprotein glucose dehydrogenase

Water-soluble quinoprotein glucose dehydrogease (PQQGDH-B) is a dimeric enzyme whose application for glucose sensing is the focus of much attention. We attempted to increase the thermal stability of PQQGDH-B by introducing a disulfide bond at the dimer interface. The Ser residue at position 415 was selected for substitution with Cys, as structural information revealed that its side chains face each other at the dimer interface of PQQGDH-B. PQQGDH-B with Ser415Cys showed 30-fold greater thermal stability at 55°C than did the wild-type enzyme without any decrease in catalytic activity. After incubation at 70°C for 10 min, Ser415Cys retained 90% of the GDH activity of the wild-type enzyme. Disulfide bond formation between the mutant subunits was confirmed by analyses with sodium dodecylsulfate-polyacrylamide gel electrophoresis in the presence and absence of reductants. Our results indicate that the introduction of one Cys residue in each monomer of PQQGDH-B resulted in formation of a disulfide bond at the dimer interface and thus achieved a large increase in the thermal stability of the enzyme.     

Construction and characterization of heterodimeric soluble quinoprotein glucose dehydrogenase

In order to study in greater detail the subunit interaction of the homodimeric soluble quinoprotein glucose dehydrogenase (PQQGDH-B), we developed an effective method of creating heterodimeric PQQGDH-B. Two different homodimers are combined, one of which has a polyarginine tail (Arg-tail), and subjected to a protein dissociation/redimerization procedure. Separation of the mixture by cation exchange chromatography results in three peaks showing GDH activity, eluting at 133, 231 and 273 mM NaCl concentration. These peaks were determined to correspond to the Arg-tailless homodimer, heterodimer, and Arg-tailed homodimer, respectively. To test this approach, we constructed and characterized heterodimeric PQQGDH-B composed of native (wild-type) and inactive mutant (His168Gln) subunits. The heterodimeric wild-type-His168Gln showed slightly decreased GDH activity and almost identical substrate specificity profile to the wild-type enzyme. Moreover, the Hill coefficient of the heterodimer was calculated as 1.13, indicating positive cooperativity.     

Construction of Engineered Water-soluble PQQ Glucose Dehydrogenase with Improved Substrate Specificity

This was the first study that achieved a narrowing of the substrate specificity of water soluble glucose dehydrogenase harboring pyrroloquinoline quinone as their prosthetic group, PQQGDH-B. We conducted the introduction of amino acid substitutions into the loop 6BC region of the enzyme, which made up the active site cleft without directly interacting with the substrate, and constructed a series of site directed mutants. Among these mutants, Asn452Thr showed the least narrowed substrate specificity while retaining a similar catalytic efficiency, thermal stability and EDTA tolerance as the wild-type enzyme. The relative activities of mutant enzyme with lactose were lower than that of the wild-type enzyme. The altered substrate specificity profile of the mutant enzyme was found to be mainly due to increase in Km value for substrate than glucose. The predicted 3D structures of Asn452Thr and the wild-type enzyme indicated that the most significant impact of the amino acid substitution was observed in the interaction between the 6BC loop region with lactose.     

Construction of Engineered Water-soluble PQQ Glucose Dehydrogenase with Improved Substrate Specificity

This was the first study that achieved a narrowing of the substrate specificity of water soluble glucose dehydrogenase harboring pyrroloquinoline quinone as their prosthetic group, PQQGDH-B. We conducted the introduction of amino acid substitutions into the loop 6BC region of the enzyme, which made up the active site cleft without directly interacting with the substrate, and constructed a series of site directed mutants. Among these mutants, Asn452Thr showed the least narrowed substrate specificity while retaining a similar catalytic efficiency, thermal stability and EDTA tolerance as the wild-type enzyme. The relative activities of mutant enzyme with lactose were lower than that of the wild-type enzyme. The altered substrate specificity profile of the mutant enzyme was found to be mainly due to increase in Km value for substrate than glucose. The predicted 3D structures of Asn452Thr and the wild-type enzyme indicated that the most significant impact of the amino acid substitution was observed in the interaction between the 6BC loop region with lactose.     

Increasing the thermal stability of the water-soluble pyrroloquinoline quinone glucose dehydrogenase by single amino acid replacement

Based on the characterization of a PCR mutation of water-soluble glucose dehydrogenase possessing pyrroloquinoline quinone (PQQ), PQQGDH-B, Ser231Cys, we have constructed a series of Ser231 variants. The replacement of Ser231 to Cys, Met, Leu, Asp, Asn, His, or Lys resulted in an increase in thermal stability. Among these variants, Ser231Lys showed the highest level of thermal stability and also showed high catalytic activity. Considering that Ser231Lys showed more than an 8-fold increase in its half-life during the thermal inactivation at 55 degrees C compared with the wild-type enzyme, and also retained catalytic activity similar to a wild-type enzyme, the application of this mutant enzyme as a glucose sensor constituent may develop into a stable glucose sensor construction.     

Guanidinium chloride- and urea-induced unfolding of the dimeric enzyme glucose oxidase

We have carried out a systematic study on the guanidinium chloride- and urea-induced unfolding of glucose oxidase from Aspergillus niger, an acidic dimeric enzyme, using various optical spectroscopic techniques, enzymatic activity measurements, glutaraldehyde cross-linking, and differential scanning calorimetry. The urea-induced unfolding of GOD was a two-state process with dissociation and unfolding of the native dimeric enzyme molecule occurring in a single step. On the contrary, the GdmCl-induced unfolding of GOD was a multiphasic process with stabilization of a conformation more compact than the native enzyme at low GdmCl concentrations and dissociation along with unfolding of enzyme at higher concentrations of GdmCl. The GdmCl-stabilized compact dimeric intermediate of GOD showed an enhanced stability against thermal and urea denaturation as compared to the native GOD dimer. Comparative studies on GOD using GdmCl and NaCl demonstrated that binding of the Gdm(+) cation to the enzyme results in stabilization of the compact dimeric intermediate of the enzyme at low GdmCl concentrations. An interesting observation was that a slight difference in the concentration of urea and GdmCl associated with the unfolding of GOD was observed, which is in violation of the 2-fold rule for urea and GdmCl denaturation of proteins. This is the first report where violation of the 2-fold rule has been observed for a multimeric protein.     

Dissociation and reconstitution of bovine seminal RNAase: Construction of a hyperactive hybrid dimer

The quaternary structure of bovine seminal ribonuclease, the only dimeric protein in the superfamily of ribonucleases, is maintained both by noncovalent forces and by two intersubunit disulfides. The available monomeric derivatives of the enzyme may not be reassembled into dimers. They are catalytically active, but do not retain certain properties of the dimeric enzyme, such as: (i) the ability to respond cooperatively to increasing substrate concentrations in the rate-limiting reaction step; and (ii) the antitumor and immunosuppressive actions. In this report we describe the preparation of stable monomers of seminal ribonuclease which can be reassociated into covalent dimers indistinguishable from the native protein. With this procedure a hybrid dimer was constructed, made up of a native subunit associated to a subunit catalytically inactivated by selective alkylation of the active site His-119. This dimer was found to have enzymic properties typical of monomeric ribonucleases, such as a hyperbolic saturation curve in the hydrolytic rate-limiting step of the reaction. However, the hybrid dimer was one order-of-magnitude more active than the dimeric enzyme.     

Modified substrate specificity of pyrroloquinoline quinone glucose dehydrogenase by biased mutation assembling with optimized amino acid substitution

A biased mutation-assembling method—that is, a directed evolution strategy to facilitate an optimal accumulation of multiple mutations on the basis of additivity principles, was applied to the directed evolution of water-soluble PQQ glucose dehydrogenase (PQQGDH-B) to reduce its maltose oxidation activity, which can lead to errors in blood glucose determination. Mutations appropriate for the reduction without fatal deterioration of its glucose oxidation activity were developed by an error-prone PCR method coupled with a saturation mutagenesis method. Moreover, two types of incorporation frequency based on their contribution were assigned to the mutations: high (80%) and evens (50%), in constructing a multiple mutant library. The best mutant created showed a marked reduction in maltose oxidation activity, corresponding to 4% of that of the wild-type enzyme, with 35% retention of glucose oxidation activity. In addition, this mutant showed a reduction in galactose oxidation activity corresponding to 5% of that of the wild-type enzyme. In conclusion, we succeeded in developing the PQQGDH-B mutants with improved substrate specificity and validated our method coupled with optimized mutations and their contribution-based incorporation frequencies by applying it to the development.Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

The production of soluble pyrroloquinoline quinone glucose dehydrogenase by Klebsiella pneumoniae, the alternative host of PQQ enzymes

Klebsiella pneumoniae, which produces PQQ and is available for use with a conventional expression vector system, was selected as the host strain for soluble PQQ glucose dehydrogenase (PQQGDH-B) production. The recombinant K. pneumoniaeexpressed PQQGDH-B in its holo-form at about 18000 U l^–1, equal to that achieved in recombinant Escherichia coli. The signal sequence of recombinant PQQGDH-B produced by K. pneumoniaewas correctly processed. K. pneumoniaecan become an alternative host microorganism not only for PQQGDH-B production but also for recombinant PQQ enzymes production.     

Monovalent cation-induced conformational change in glucose oxidase leading to stabilization of the enzyme

Glucose oxidase (GOD) from Aspergillus niger is an acidic dimeric enzyme having a high degree of localization of negative charges on the enzyme surface and dimer interface. We have studied the effect of monovalent cations on the structure and stability of GOD using various optical spectroscopic techniques, limited proteolysis, size exclusion chromatography, differential scanning calorimetry, and enzymic activity measurements. The monovalent cations were found to influence the enzymic activity and tertiary structure of GOD, but no effect on the secondary structure of the enzyme was observed. The monovalent cation-stabilized GOD was found to have a more compact dimeric structure but lower enzymic activity than the native enzyme. The enzyme's K(m) for D-glucose was found to be slightly enhanced for the monovalent cation-stabilized enzyme (maximum enhancement of about 35% for LiCl) as compared to native GOD. Comparative denaturation studies on the native and monovalent cation-stabilized enzyme demonstrated a significant resistance of cation-stabilized GOD to urea (about 50% residual activity at 6.5 M urea) and thermal denaturation (Delta T(m) maximum of 10 degrees C compared to native enzyme). However, pH-induced denaturation showed a destabilization of monovalent cation-stabilized GOD as compared to the native enzyme. The effectiveness of monovalent cations in stabilizing GOD structure against urea and thermal denaturation was found to follow the Hofmeister series: K(+) > Na(+) > Li(+).     

Biochemical and genetic characterization of the structure of yeast ornithine decarboxylase

The ornithine decarboxylase gene of S. cerevisiae encodes a predicted protein of approximately 53 kD highly homologous with the ornithine decarboxylase of other species. However, the native enzyme has been reported as an 86 kD protein. Our molecular sieve analysis indicated a Mr = 110,000 for the native enzyme. SDS-PAGE analysis of [H3]-alpha-difluoromethylornithine labelled enzyme demonstrated a subunit Mr of approximately 50 kD and suggested the native enzyme is a dimer. Genetic analyses support this conclusion. The complementary, ornithine decarboxylase deficient mutations spe 1A and spe 1B were mapped to the enzyme structural gene by linkage analysis and gene conversion mapping. This demonstrated that the mutations exhibit intragenic complementation which suggests protein-protein interactions and an oligomeric structure for the yeast enzyme. We conclude that yeast ornithine decarboxylase is a dimeric enzyme of 53 kD subunits.     

Cloning and sequencing of the leucine dehydrogenase gene from Bacillus sphaericus IFO 3525 and importance of the C-terminal region for the enzyme activity

The structural gene (leudh) coding for leucine dehydrogenase from Bacillus sphaericus IFO 3525 was cloned into Escherichia coli cells and sequenced. The open reading frame coded for a protein of 39.8 kDa. The deduced amino acid sequence of the leucine dehydrogenase from B. sphaericus showed 76–79% identity with those of leucine dehydrogenases from other sources. About 16% of the amino acid residues of the deduced amino acid sequence were different from the sequence obtained by X-ray analysis of the B. sphaericus enzyme. The recombinant enzyme was purified to homogeneity with a 79% yield. The enzyme was a homooctamer (340 kDa) and showed the activity of 71.7 μmol·min⁻¹·mg⁻¹) of protein. The mutant enzymes, in which more than six amino acid residues were deleted from the C-terminal of the enzyme, showed no activity. The mutant enzyme with deletion of four amino acid residues from the C-terminal of the enzyme was a dimer and showed 4.5% of the activity of the native enzyme. The dimeric enzyme was more unstable than the native enzyme, and the K_m values for -leucine and NAD⁺ increased. These results suggest that the Asn-Ile-Leu-Asn residues of the C-terminal region of the enzyme play an important role in the subunit interaction of the enzyme.     

Genetic and biochemical studies on glutamate-pyruvate transaminase from Drosophila melanogaster

We have used electrophoretic variants of glutamate-pyruvate transaminase (GPT, E.C. 2.6.1.2) in Drosophila melanogaster to genetically map the structural gene to position 42.6 on the X chromosome. By pseudodominance tests over several deficiencies we have localized it cytogenetically to the interval 11Fl-2 to 12Al-2. The sedimentation constant (s _20,w) of the native enzyme was determined in sucrose density gradients to be 5.9 and the native molecular weight approximately 87,000. The similarity in physical properties to mammalian enzymes suggests that the enzyme may also be dimeric in D. melanogaster.     

Construction of a synthetic gene for an R-plasmid-encoded dihydrofolate reductase and studies on the role of the N-terminus in the protein

R67 dihydrofolate reductase (DHFR) is a novel protein that provides clinical resistance to the antibacterial drug trimethoprim. The crystal structure of a dimeric form of R67 DHFR indicates the first 16 amino acids are disordered [Matthews et al. (1986) Biochemistry 25, 4194-4204]. To investigate whether these amino acids are necessary for protein function, the first 16 N-terminal residues have been cleaved off by chymotrypsin. The truncated protein is fully active with kcat = 1.3 s-1, Km(NADPH) = 3.0 microM, and Km(dihydrofolate) = 5.8 microM. This result suggests the functional core of the protein resides in the beta-barrel structure defined by residues 27-78. To study this protein further, synthetic genes coding for full-length and truncated R67 DHFRs were constructed. Surprisingly, the gene coding for truncated R67 DHFR does not produce protein in vivo or confer trimethoprim resistance upon Escherichia coli. Therefore, the relative stabilities of native and truncated R67 DHFR were investigated by equilibrium unfolding studies. Unfolding of dimeric native R67 DHFR is protein concentration dependent and can be described by a two-state model involving native dimer and unfolded monomer. Using absorbance, fluorescence, and circular dichroism techniques, an average delta GH2O of 13.9 kcal mol-1 is found for native R67 DHFR. In contrast, an average delta GH2O of 11.3 kcal mol-1 is observed for truncated R67 DHFR. These results indicate native R67 DHFR is 2.6 kcal mol-1 more stable than truncated protein. This stability difference may be part of the reason why protein from the truncated gene is not found in vivo in E. coli.     

Characterization of a bacterial xylanase resistant to repression by glucose and xylose

Summary Bacillus subtilis CD4, when grown in nutrient broth or minimal medium in presence of xylan, produced extracellular xylanase that hydrolyzed xylan optimally at pH 5. The enzyme was induced by xylan, xylose and glucose. Addition of xylose or glucose in xylan containing medium did not affect enzyme production. The structural gene encoding xylanase was cloned and expressed in E. coli. The recombinant enzyme exhibited similar properties like that of native enzyme including resistance to repression by xylose and glucose.     

Active Dimeric Form of Inorganic Pyrophosphatase from Escherichia coli

A dimeric form can be obtained from native hexameric Escherichia coli inorganic pyrophosphatase (E-PPase) by destroying the hydrophobic intersubunit contacts, and it has been shown earlier to consist of the subunits of different trimers. The present paper is devoted to the kinetic characterization of such a "double-decked" dimer obtained by the dissociation of either the native enzyme or the mutant variant Glu145Gln. The dimeric form of the native inorganic pyrophosphatase was shown to retain high catalytic efficiency that is in sharp contrast to the dimers obtained as a result of the mutations at the intertrimeric interface. The dimeric enzymes described in the present paper, however, have lost the regulatory properties, in contrast to the hexameric and trimeric forms of the enzyme.     

Properties of a cGMP-dependent monomeric protein kinase from bovine aorta

A form of cGMP-dependent protein kinase (cGK) that was different from previously described cGK was purified from bovine aorta smooth muscle. The partial amino-terminal sequencing of this enzyme indicated that it was derived by endogenous proteolysis of the type I beta isozyme of cGK. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, this form migrated as a smaller protein (Mr = 70,000) than the parent cGK (Mr = 80,000), and since the calculated nondenatured Mr was approximately 89,000 compared to Mr = 170,000 for the dimeric native enzyme, it represented a monomeric form of cGK. The monomer bound approximately 2 mol of [3H]cGMP per mol of monomer, although it had only one rapid component in [3H]cGMP dissociation assays as compared to one rapid and one slow component for the native cGK. The specific catalytic activity of the kinase was similar to that of the native enzyme, suggesting that the catalytic domain was essentially intact. The monomeric cGK incorporated significant 32P when incubated with Mg2+ and [gamma-32P]ATP in the presence of cGMP, although the phosphorylation proceeded at a slower rate than that obtained with native cGK. In contrast to previous reports of monomeric forms of cGK, this monomer was highly cGMP-dependent, although it had a slightly higher Ka (0.8 microM) for cGMP than that of the native enzyme (0.4 microM) and a low Hill coefficient of 1.0 (1.6 for the native enzyme). The cGMP dependence of the monomer did not decrease with dilution, implying that the cGMP dependence was not due to monomer-monomer interactions in the assay. The results indicated that the catalytic domain, cGMP binding domain(s), and inhibitory domain of cGK interact primarily within the same subunit rather than between subunits of the dimer as previously hypothesized for dimeric cGK.     

Expression of double Vitreoscilla hemoglobin enhances growth and alters ribosome and tRNA levels in Escherichia coli

In several organisms, expression of a gene encoding dimeric hemoglobin (VHb) from the obligate aerobic bacterium Vitreoscilla stercoraria has been shown to increase microaerobic cell growth and enhance oxygen-dependent cell metabolism. In an attempt to further improve these effects of VHb, a gene encoding two vhb genes connected by a short linker of six base pairs was constructed and expressed in Escherichia coli(double VHb). Escherichia coli cells expressing double VHb reached a cell density 19% higher than that of cells expressing native VHb. The protein production per cell remained constant since the increase in cell growth was accompanied by an increase in protein content by 16%. Investigation of ribosome and tRNA content revealed that cells expressing double VHb reached their maximal capacity of protein synthesis later during cultivation than cells expressing native VHb, and furthermore they reached considerably higher levels of ribosome and tRNA compared to that of the VHb-expressing cells.     

Divalent cation induced changes in structural properties of the dimeric enzyme glucose oxidase: dual effect of dimer stabilization and dissociation with loss of cooperative interactions in enzyme monomer

Glucose oxidase (GOD) from Aspergillus niger is a dimeric enzyme having high localization of negative charges on the enzyme surface and at the dimer interface. The monovalent cations induce compaction of the native conformation of GOD and enhance stability against thermal and urea denaturation [Ahmad et al. (2001) Biochemistry 40, 1947-1955]. In this paper we report the effect of the divalent cations Ca2+ and Mg2+ on the structural and stability properties of GOD. A divalent cation concentration dependent change in native conformation and subunit assembly of GOD was observed. Low concentration (up to 1 M) of CaCl2 or MgCl2 induced compaction of the native conformation of GOD, and the enzyme showed higher stability as compared to the native enzyme against urea denaturation. However, higher concentration (> or =2.0 M) of CaCl2 or MgCl2 induced dissociation of the native dimeric enzyme, resulting in stabilization of the enzyme monomer. An interesting observation was that the 3 M CaCl2-stabilized monomer of GOD retained about 70% secondary structure present in the native GOD dimer; however, there was a complete loss of cooperative interactions between these secondary structural elements present in the enzyme. Regarding the mechanism of divalent cation induced structural changes in GOD, the studies suggest that organization of water molecules by divalent cation results in stabilization of enzyme at low divalent cation concentration, whereas direct binding of these cations to the enzyme, at higher divalent cation concentration, results in dissociation and partial unfolding of the dimeric enzyme molecule.     

Preparation of immobilized enzymes by N-vinylpyrrolidone and the general properties of the glucoamylase gel

Immobilized glucoamylase, invertase, and β-galactosidase were prepared by using N-vinylpyrrolidone monomer (VP) under γ-ray irradiation. The enzyme-VP solutions were gelled by irradiation with 2.9 Mrad and the added enzymes were almost completely entrapped. Activity losses on entrapping were 55% for the VP-glucoamylase gel, and more than 90% in the case of VP-invertase and VP-β-galactosidase gels. No leakage of enzyme from these gels could be detected within 1 hr. The VP-glucoamylase gel was capable of hydrolyzing dextrin (mol wt 10,400) to glucose and the glucose equivalent was equal to that obtain able with native enzyme. The optimum temperature, heat stability, pH activity curve, and pH stability of VP-glucoamylase gel were slightly inferior to those of native enzyme, while Km was a little larger than that of native enzyme.