A simplified purification procedure for galactose oxidase

Probing structural and catalytic characteristics of galactose oxidase confined in nanoscale chemical environments

Synopsis: structural and catalytic features of a complex enzyme galactose oxidase confined in nanoscale chemical environments were investigated to show the catalytic efficiency of the enzyme depending on both the degree of space confinement and immobilization method.

Galactose oxidase as a model for reactivity at a copper superoxide center

The mononuclear copper enzyme, galactose oxidase, has been investigated under steady-state conditions via O(2)-consumption assays using 1-O-methyl-alpha-D-galactopyranoside as the sugar substrate to produce an aldehyde at the C-6 position. The rate-determining step of the oxidative half-reaction was probed through the measurement of substrate and solvent deuterium and O-18 isotope effects on k(cat)/K(m)(O(2)). The reaction conforms to a ping-pong mechanism with the kinetic parameters for the reductive half, k(cat)/K(m)(S) = 8.3 x 10(3) M(-1) s(-1) at 10 degrees C and pH 7.0, comparing favorably to literature values. The oxidative half-reaction yielded a value of k(cat)/K(m)(O(2)) = 2.5 x 10(6) M(-1) s(-1). A substrate deuterium isotope effect of 32 was measured for the k(cat)/K(m)(S), while a smaller, but significant value of 1.6-1.9 was observed on k(cat)/K(m)(O(2)). O-18 isotope effects of 1.0185 with either protiated or deuterated sugar, together with the absence of any solvent isotope effect, lead to the conclusion that hydrogen atom transfer from reduced cofactor to a Cu(II)-superoxo intermediate is fully rate-determining for k(cat)/K(m)(O(2)). The measured O-18 isotope effects provide corroborative evidence for the reactive superoxo species in the dopamine beta-monooxygenase/peptidylglycine alpha-hydroxylating monooxygenase family, as well as providing a frame of reference for copper-superoxo reactivity. The combination of solvent and substrate deuterium isotope effects rules out solvent deuterium exchange into reduced enzyme as the origin of the relatively small substrate deuterium isotope effect on k(cat)/K(m)(O(2)). These data indicate fundamental differences in the hydrogen transfer step from the carbon of substrate vs the oxygen of reduced cofactor during the reductive and oxidative half-reactions of galactose oxidase.

Pulse radiolysis studies on galactose oxidase

Single-Cu-containing galactose oxidase in the GOase(semi) state (Cu(II), no Tyr(*) radical) reacts with pulse radiolysis generated formate radicals CO(2)(*-) to give an intermediate UV-vis spectrum assigned as RSSR(*-), peak at 450 nm (epsilon = 8100 M(-1) cm(-1)). From a detailed kinetic analysis at 450 nm, pH 7.0, the following steps have been identified. First the strongly reducing CO(2)(*-) (-1.9V) reduces GOase(semi) (k(0) > or = 6.5 x 10(8) M(-1) s(-1)) to a species GOase(semi)(*-). This is followed by biphasic reactions (i) GOase(semi)(*-) + GOase(semi) (k(1) = 1.6 x 10(7) M(-1) s(-1)) to give GOase(semi) + P(*-) and (ii) P(*-) + GOase(semi) (k(2) = 6.7 x 10(6) M(-1) s(-1)) to give GOase(semi)RSSR(*-). There are no significant absorbance changes for the formation of GOase(semi)(*-) and P(*-), which are Cu(I) (or related) species. However, GOase(semi)RSSR(*-) has an absorption spectrum which differs significantly from that of GOase(semi). The 450 nm peak is characteristic of an RSSR(*-) radical with two cysteines in close sequence proximity and is here assigned to Cys515-Cys518, which is at the GOase surface and 10.2 A from the Cu. On chemical modification of the RSSR group with HSPO(3)(2-) to give RSSPO(3)H(-) and RS(-), absorbance changes are approximately 50% of those previously observed. The decay of RSSR(*-) (0.17 s(-1)) results in the formation of GOase(red). No RSSR(*-) formation is observed in the reaction of GOase(semi) Tyr495Phe with CO(2)(*-), and a single process giving GOase(red)Tyr495Phe occurs. Similarly in the reaction of GOase(ox) with CO(2)(*-), a single-stage reaction gives GOase(semi).

Galactosyl-biomimetic dye-ligands for the purification of Dactylium dendroides galactose oxidase

Two anthraquinone galactosyl-biomimetic dye-ligands comprising, as terminal biomimetic moiety, galactose analogues (1-amino-1-deoxy-beta-D-galactose and D(+)-galactosamine) were designed for the enzyme galactose oxidase (GAO), using molecular modelling, synthesized and characterized. The biomimetic ligands were immobilized on agarose beads and the affinity adsorbents, together with a non-biomimetic adsorbent bearing Cibacron Blue 3GA, were studied for their ability to purify GAO from Dactylium dendroides. Both biomimetic adsorbents showed higher purifying ability for GAO compared to the non-biomimetic adsorbent, thus demonstrating their superior effectiveness as affinity chromatography materials. In particular, the affinity adsorbent comprising, as terminal biomimetic moiety, 1-amino-1-deoxy-beta-D-galactose (BM1) exhibited the highest purifying ability for GAO. This affinity adsorbent did not bind galactose dehydrogenase, glucose dehydrogenase, alcohol dehydrogenase, or glucose oxidase. The dissociation constant (K(D)) of the immobilized BM1 ligand with GAO was found to be equal to 45.8 microM, whereas the binding capacity was equal to 709 U per ml adsorbent. Therefore, the BMI adsorbent was integrated in a facile two-step purification procedure for GAO. The purified enzyme showed a specific activity equal to 2038 U/mg, the highest reported so far, approximately 74% overall recovery and a single band after sodium dodecylsulfate-polyacrylamide gel electrophoresis analysis.

Interconversion of Cu(I) and Cu(II) forms of galactose oxidase: comparison of reduction potentials

A procedure for the preparation of the fully reduced Cu(I) form of galactose oxidase, GOase(red), involving reduction of GOase(semi) (or GOase(ox)) with non-coordinating [Ru(NH(3))(6)](2+) (51 mV vs. nhe) is described. Air-free conditions and a two-fold excess of [Ru(NH(3))(6)](2+) give a stable product with no further UV-Vis changes over >1.5 h. Rate constants for the reduction of GOase(semi) (k(f)=860 M(-1) s(-1)) give a first-order [H(+)]-dependence (pK(1a)=7.9), but the reverse process involving [Ru(NH(3))(6)](3+) oxidation of GOase(red) (k(b)=18.6 M(-1) s(-1)) is independent of pH (5.5 to 9.5). The reduction potential E(2)(o)' (vs. nhe) for the GOase(semi)/GOase(red) (i.e. Cu(II)/Cu(I)) couple is 149 mV at pH 7.5, which varies from 160 mV (pH 5.5) to 120 mV (pH 10.5), suggesting pK(1a) (GOase(semi)) and pK(2a) (GOase(red)) acid dissociation constants both involving Tyr-495. It is concluded that pK(2a) is for acid dissociation of uncoordinated H(+)Tyr-495. Consistent with this interpretation rate constants/M(-1) s(-1) for the GOase(semi) Tyr495 Phe variant, k(f)=1.59x10(3) and k(b)=16.1, respectively, are independent of pH and give a reduction potential of 169 mV. Comparisons are made of reduction potentials (E(1)(o)'/mV pH 7.5) for the GOase(ox)/GOase(semi) (i.e. Tyr(.)/Tyr) couple, and are for the Cys228Gly variant (630), for enzyme with N(3)(-) for H(2)O at the substrate binding exogenous site (393), and for apo-protein (570). These compare with previously reported values for the variants Trp290His (730) and Tyr495Phe (450), and together serve to quantify different contributions to the unusually small E(1)(o)' of 400 mV for the Tyr(.)/Tyr couple. At pH 7.5 the reduction potential for the two-equivalent GOase(ox)/GOase(red) couple is calculated to be 275 mV. The rate constant for the reaction of GOase(red) with GOase(ox) is 4.4x10(3) M(-1) s(-1) at pH 7.5.

Autoredox Interconversion of Two Galactose Oxidase Forms GOaseox and GOasesemi with and without Dioxygen

Solutions of galactose oxidase stored in air give in 3-4 h a mix of GOase(ox)(Cu(II)-Tyr(*)) and GOase(semi)(Cu(II)-Tyr), as a result of processes involving the formation and decay of the Cu(II)-coordinated tyrosyl radical (Tyr(*)). In this work the two reactions have been studied by UV-vis spectrophotometry and separate rate laws defined. The first involves the "spontaneous" autoreduction of GOase(ox) to GOase(semi), which in air-free conditions is 100% complete. Rate constants (k(red)) are dependent on pH, and previously defined acid dissociation constants pK(1a) = 5.7 (exogenous H(2)O ligand), and pK(2a) = 8.0 (axial H(+)Tyr-495) apply. Values of k(red)(25 degrees C) range from 1.55 x 10(-4) s(-1) (pH 5.5) to 2.69 x 10(-4) s(-1) (pH 8.6), I = 0.100 M (NaCl). No reaction occurs with N(3)(-) or NCS(-) present in amounts sufficient to give >98% binding at the substrate binding (exogenous) site, while CH(3)CO(2)(-) and phosphate (less extensively bound) also inhibit the reaction. From such inhibition studies K(25 degrees C) is 161 M(-1) at pH 6.4 for acetate (previous value 140 M(-)(1)) and 46 M(-1) at pH 7.0 for phosphate. No reaction occurs when the disulfide Cys515-Cys518 (10.2 A from the Cu) is chemically modified with HSPO(3)(2-), and electron transfer via the disulfide and exogenous position is proposed (source of the electron not established). The conversion of GOase(semi) to GOase(ox) only occurs with O(2) present, when a first-order dependence on [O(2)] is observed, giving k(ox)(25 degrees C) = 0.021 M(-1) s(-1) at pH 7.5. This process is unaffected by NCS(-) or N(3)(-) bound at the exogenous site, and a mechanism involving outer-sphere reaction of O(2) to O(2)(-) followed by a fast step O(2) to H(2)O(2) is proposed. As GOase(ox) is formed, autoreduction back to GOase(semi) occurs, and at pH 7.5 with O(2) in large excess (1.13 mM) the maximum conversion to GOase(ox) is 69%. The k(ox) reaction proceeds to completion with >98% N(3)(-) bound at the exogenous site.

Thermodynamic, kinetic and pH studies on the reactions of NCS-, N3-, and CH3CO2- with Fusarium galactose oxidase

Thermodynamic and kinetic studies on the X- = NCS-, N3-, and CH3CO2- replacement of H2O/OH- at the CuII exogenous site of the tyrosyl-radical-containing enzyme galactose oxidase (GOaseox) from Fusarium (NRR 2903), have been studied by methods involving UV-vis spectrophotometry (25 degrees C), pH range 5.5-8.7, I = 0.100 M (NaCl). In the case of N3- and CH3CO2- previous X-ray structures have confirmed coordination at the exogenous H2O/OH- site. From the effect of pH on the UV-vis spectrum of GOaseox under buffer-free conditions, acid dissociation constants of 5.7 (pK1a; coordinated H2O) and 7.0 (pK2a; H+Tyr-495) have been determined. At pH 7.0 formation constants K(25 degrees C)/M-1 are NCS- (480), N3- (1.98 x 10(4)), and CH3CO2- (104), and from the variations in K with pH the same two pKa values are seen to apply. No pK1a is observed when X- is coordinated. From equilibration stopped-flow studies rate constants at pH 7.0 for the formation reaction kf(25 degrees C)/M-1 s-1 are NCS- (1.13 x 10(4)) and N3- (5.2 x 10(5)). Both K and kf decrease with increasing pH, consistent with the electrostatic effect of replacing H2O by OH-. In the case of the GOaseox Tyr495Phe variant pK1a is again 5.7, but no pK2a is observed, confirming the latter as acid dissociation of protonated Tyr-495. At pH 7.0, K for the reaction of four-coordinate GOaseox Tyr495Phe with NCS- (1.02 x 10(5) M-1) is more favorable than the value for GOaseox. Effects of H+Tyr-495 deprotonation on K are smaller than those for the H2O/OH- change. The pK1a for GOasesemi is very similar (5.6) to that for GOaseox (both at CuII), but pK2a is 8.0. At pH 7.0 values of K for GOasesemi are NCS- (270 M-1), N3- (4.9 x 10(3)), and CH3CO2- (107).

Kinetic Studies on the Redox Interconversion of GOase(semi) and GOase(ox) Forms of Galactose Oxidase with Inorganic Complexes as Redox Partners

Redox interconversions between the GOase(semi) (Cu(II), Tyr) and tyrosyl radical containing GOase(ox) (Cu(II), Tyr(*)) oxidation states of the Cu-containing enzyme galactose oxidase (GOase) from Fusarium NRRL 2903 have been studied. The inorganic complexes [Fe(CN)(6)](3)(-) (410 mV), [Co(phen)(3)](3+) (370 mV), [W(CN)(8)](3)(-) (530 mV), and [Co(dipic)(2)](-) (362 mV) (E degrees ' values vs NHE; dipic = 2,6-dicarboxylatopyridine) were used as oxidants for GOase(semi), and [Fe(CN)(6)](4)(-) and [Co(phen)(3)](2+) as reductants for GOase(ox). On oxidation of GOase(semi) a radical is generated at the coordinated phenolate of Tyr-272 to give GOase(ox). The one-electron reduction potential E degrees ' (25 degrees C) for the GOase(ox)/GOase(semi) couple varies with pH and is 400 mV vs NHE at pH 7.5, the smallest value so far observed for a tyrosyl radical. The reactions are very sensitive to pH, or more precisely to pK(a) values of GOase(semi) and GOase(ox), and the charge on the inorganic reagent. For example, with [Fe(CN)(6)](3)(-) as oxidant, the rate constant (25 degrees C)/M(-)(1) s(-)(1) of 0.16 x 10(3) (pH approximately 9.5) increases to 4.3 x 10(3) (pH approximately 5.5), while for [Co(phen)(3)](3+) a value of 4.9 x 10(3) (pH approximately 9.5) decreases to 0.04 x 10(3) (pH approximately 5.5), I = 0.100 M (NaCl). From the kinetics a single GOase(semi) acid dissociation process, pK(a) = 8.0 (average), has been confirmed by UV-vis spectrophotometric studies (7.9). The corresponding value for GOase(ox) is 6.7. No comparable kinetic or spectrophotometric pH dependences are observed with the Tyr495Phe variant, indicating the axial Tyr-495 as the site of protonation. Neutral CH(3)CO(2)H and HN(3) species bind at the substrate binding site of GOase(semi), thus mimicking the behavior of primary alcohols RCH(2)OH, the natural substrate of GOase. On coordination, loss of a proton occurs, and inhibition of the oxidation with [Fe(CN)(6)](3)(-) is observed.

Chemoenzymic Approaches to the Preparation of 5-C-(Hydroxymethyl)hexoses

Synthesis of 5-C-(hydroxymethyl)hexoses, carbohydrates resistant to metabolism, is described. These compounds are obtained in the reaction of hexose 6-aldehydes with formaldehyde. 5-C-(Hydroxymethyl)-L-arabino-hexopyranoses can be efficiently obtained from D-galactosides by a two-step chemoenzymic synthesis using galactose oxidase for the preparation of required hexose 6-aldehydes. This method is an example of carbohydrate synthesis without use of protecting groups. Other 5-C-(hydroxymethyl)hexoses are prepared by a typical chemical methodology requiring specific protection of the hexose hydroxyl groups.

Role of carbohydrate content on the properties of galactose oxidase from Dactylium dendroides

The stability of intracellular, extracellular, and deglycosylated forms of galactose oxidase was compared with respect to the denaturing effects of heat, pH, and guanidine hydrochloride. The highly glycosylated forms were found to be more stable to pH and thermal inactivation. All forms were reversibly denaturated by guanidine hydrochoride, but the extent was dependent on the carbohydrate content. Deglycosylation did not affect the affinity of the enzyme for dihydroxyacetone and galactose. Exposure of different forms of galactose oxidase to proteases like pronase and trypsin resulted in a rapid degradation of the glycoenzymes with the formation of stable products. After pronase digestion of intra- and extracellular forms of galactose oxidase catalytic species were isolated by gel filtration. The species (61 and 42 kDa) isolated from pronase-digested extracellular enzyme lost their ability to oxidize primary alcohols. Species (67 and 46 kDa) obtained from the intracellular enzyme kept the specificity of the original enzyme. Active pronase-derived peptides (42 and 46 kDa, respectively) had a higher carbohydrate content than the inactive ones.

Purification of galactose oxidase from Dactylium dendroides by affinity chromatography on melibiose-polyacrylamide

Galactose oxidase is a fungal enzyme which is known to oxidize the C-6 hydroxymethyl of galactose and galactosamine to an aldehyde group. It has been widely used in glycoconjugate research, for example in the labeling of asialoglycoproteins. We have developed a simple affinity purification for galactose oxidase using melibiose-polyacrylamide. This affinity procedure was used to purify the enzyme from ammonium sulfate precipitates of culture filtrates of Dactylium dendroides. The material containing proteases and other contaminants is eluted in the buffer wash. The galactose oxidase is then specifically eluted from the column with buffer containing 0.1 M D-fucose or D-galactose. Using this procedure, the enzyme was also purified from commercial samples of galactose oxidase which contain high proteolytic activity.

Purification and characterization of intracellular galactose oxidase from Dactylium dendroides

The intracellular galactose oxidase from Dactylium dendroides was purified to homogeneity with a 64% yield. The enzyme is a glycoprotein (7.7% neutral sugars, 1.7% aminosugars) with 72,000 Da of molecular mass. The enzyme showed nonlinear double reciprocal plots with O2 and D-galactose, suggesting cooperative binding for both substrates. The intracellular galactose oxidase catalyzes the oxidation of galactose derivatives and dihydroxyacetone but not of glycerol, glycolaldehyde, beta-hydroxipyruvate, and allyl alcohol which are substrates for the extracellular enzyme. Compared with the extracellular galactose oxidase, the intracellular enzyme showed higher carbohydrate content and sensitivity to diethyldithiocarbamate.

Oxidation rates of some desialylated glycoproteins by galactose oxidase

Patterns of oxidation of dilute solutions of desialylated fetuin and submaxillary mucin by galactose oxidase have been examined. A significant portion (20-40%) of the terminal galactosyls exposed on the glycoproteins, which theoretically were expected to be accessible to the enzyme, was not oxidized. In comparison, galactosyls in oligosaccharides released from completely desialylated glycoproteins were oxidized more effectively with an apparently lower degree of crypticity to the enzyme. Partial desialylation usually resulted in a reduction of both the rate and the final level of substrate oxidation. A second cycle of oxidation of a desialylated substrate earlier oxidized by galactose oxidase and then reduced by NaB3H4 revealed a selectivity in the pattern of galactosyl oxidation. The same galactosyl residues oxidized in the first cycle were again the most susceptible to oxidation in the second cycle, leaving unmodified the same fraction of galactosyls throughout both cycles. The relevance of these results to the application of the galactose oxidase-NaBH4 procedure for detecting and measuring desialylated glycoconjugates in solution and in biological membranes is discussed.

Labeling of the glycoprotein subunit of (Na,K)ATPase with fluorescent probes

Sodium plus potassium activated adenosinetriphosphatase [(Na,K)ATPase] is composed of a catalytic subunit (alpha) and a glycoprotein subunit (beta) of unknown function. A method has been developed to label the beta subunit of purified dog kidney (Na,K)ATPase with fluorescent probes. The method consists of oxidation of beta-subunit oligosaccharides, reaction of the resulting aldehydes with fluorescent hydrazides, and reduction of the hydrazones and unreacted aldehydes with NaBH4. Two oxidation methods were compared. Simultaneous treatment with neuraminidase and galactose oxidase did not inhibit significantly (Na,K)ATPase activity and allowed insertion of up to 11 mol of probe per mol of beta. In contrast, oxidation of (Na,K)ATPase oligosaccharides with periodate resulted in 50-80% inhibition of the (Na,K)ATPase activity with low or undetectable labeling. Eleven commercial probes and two novel hydrazides were tested for labeling of (Na,K)ATPase treated with galactose oxidase and neuraminidase. Eight probes did not label (Na,-K)ATPase but labeled red cell ghosts oxidized with periodate. Four probes labeled beta specifically but either adsorbed to the membrane tightly, or cross-linked the beta subunits, or formed unstable adducts. Lucifer yellow CH labeled beta specifically without membrane adsorption. Labeling stoichiometries from 1 to 11 mol of lucifer yellow CH per mol of beta were obtained without inhibition of (Na,K)ATPase activity and without significant alteration of the anthroylouabain binding capacity or its association and dissociation kinetics. Anthroylouabain specifically bound to the lucifer-labeled (Na,K)ATPase had a decreased quantum yield, probably due to resonance energy transfer. This suggests that the sites of lucifer attachment on beta are within energy transfer distance from the cardiac glycoside site on alpha.(ABSTRACT TRUNCATED AT 250 WORDS)