Purification and properties of galactokinase from Tetrahymena thermophila

Galactokinase and galactose metabolism in Leishmania spp

In Leishmania, the nucleotide-sugar UDP-galactose can be synthesized by a salvage pathway, the Isselbacher route, involving phosphorylation of galactose and the action of UDP-sugar pyrophosphorylase. The first enzyme of the pathway, galactokinase, has yet to be studied in this parasite. Here, we report a molecular and biochemical characterization of this enzyme in Leishmania mexicana. We showed that recombinant galactokinase (LmxGALK) phosphorylates galactose in the presence of ATP with Km values of 0.077 mM for galactose and 0.017 mM for ATP. We proved by immunodetection that GALK is expressed in promastigotes and amastigotes of L. mexicana, L. braziliensis and L. infantum. In agreement with the presence of a type 1 peroxisome-targeting signal sequence present at the C-terminus of LmxGALK, the protein is localized mostly within glycosomes as shown by selective membrane permeabilization with digitonin, differential centrifugation, and immunofluorescence. Indeed, LmxGALK enzymatic activity was measured in the fractions corresponding to the homogenate and glycosomes, proving that it is active in promastigotes. In addition, it was shown that galactose cannot serve as an important carbon source for sustaining parasite growth, as cultures of promastigotes from three Leishmania species in LIT medium containing either no sugar or supplemented with D-galactose (20 mM) grew to lower density compared to these cultured with D-glucose (20 mM). These results suggest that D-galactose is mainly used for UDP-galactose synthesis by the salvage route, functioning when glucose is depleted from the medium, similar to the conditions promastigotes experience in the gut of the insect vector during its life cycle.

A galactokinase-like protein from the liver fluke Fasciola hepatica

Galactokinase catalyses the ATP-dependent phosphorylation of galactose. A galactokinase-like sequence was identified in a Fasciola hepatica EST library. Recombinant expression of the corresponding protein in Escherichia coli resulted in a protein of approximately 50 kDa. The protein is monomeric, like galactokinases from higher animals, yeasts and some bacteria. The protein has no detectable enzymatic activity with galactose or N-acetylgalactosamine as a substrate. However, it does bind to ATP. Molecular modelling predicted that the protein adopts a similar fold to galactokinase and other GHMP kinases. However, a key loop in the active site was identified which may influence the lack of activity. Sequence analysis strongly suggested that this protein (and other proteins annotated as "galactokinase" in the trematodes Schistosoma mansoni and Clonorchis sinensis) are closer to N-acetylgalactosamine kinases. No other galactokinase-like sequences appear to be present in the genomes of these three species. This raises the intriguing possibility that these (and possibly other) trematodes are unable to catabolise galactose through the Leloir pathway due to the lack of a functional galactokinase.

Multiplexed Capillary Electrophoresis as Analytical Tool for Fast Optimization of Multi-Enzyme Cascade Reactions - Synthesis of Nucleotide Sugars: Dedicated to Prof. Dr. Vladimir Křen on the occasion of his 60th birthday

Nucleotide sugars are considered as bottleneck and expensive substrates for enzymatic glycan synthesis using Leloir-glycosyltransferases. Synthesis from cheap substrates such as monosaccharides is accomplished by multi-enzyme cascade reactions. Optimization of product yields in such enzyme modules is dependent on the interplay of multiple parameters of the individual enzymes and governed by a considerable time effort when convential analytic methods like capillary electrophoresis (CE) or HPLC are applied. We here demonstrate for the first time multiplexed CE (MP-CE) as fast analytical tool for the optimization of nucleotide sugar synthesis with multi-enzyme cascade reactions. We introduce a universal separation method for nucleotides and nucleotide sugars enabling us to analyze the composition of six different enzyme modules in a high-throughput format. Optimization of parameters (T, pH, inhibitors, kinetics, cofactors and enzyme amount) employing MP-CE analysis is demonstrated for enzyme modules for the synthesis of UDP-α-D-glucuronic acid (UDP-GlcA) and UDP-α-D-galactose (UDP-Gal). In this way we achieve high space-time-yields: 1.8 g/L⋆h for UDP-GlcA and 17 g/L⋆h for UDP-Gal. The presented MP-CE methodology has the impact to be used as general analytical tool for fast optimization of multi-enzyme cascade reactions.

Natural product glycorandomization

glycorandomization is a chemoenzymatic strategy that overcomes the limitations in natural product derivatization associated with both solely chemistry-based approaches or in vivo engineering. In this article we present the basic strategies for glycorandomization development as a next-generation tool in drug discovery.

Creation of the first anomeric D/L-sugar kinase by means of directed evolution

Chemoenzymatic routes toward complex glycoconjugates often depend on the availability of sugar-1-phosphates. Yet the chemical synthesis of these vital components is often tedious, whereas natural enzymes capable of anomeric phosphorylation are known to be specific for one or only a few monosaccharides. Herein we describe the application of directed evolution and a high-throughput multisugar colorimetric screen to enhance the catalytic capabilities of the Escherichia coli galactokinase GalK. From this approach, one particular GalK mutant carrying a single amino acid exchange (Y371H) displayed a surprisingly substantial degree of kinase activity toward sugars as diverse as d-galacturonic acid, d-talose, l-altrose, and l-glucose, all of which failed as wild-type GalK substrates. Furthermore, this mutant provides enhanced turnover of the small pool of sugars converted by the wild-type enzyme. Comparison of this mutation to the recently solved structure of Lactococcus lactis GalK begins to provide a blueprint for further engineering of this vital class of enzyme. In addition, the rapid access to such promiscuous sugar C-1 kinases will significantly enhance accessibility to natural and unnatural sugar-1-phosphates and thereby impact both in vitro and in vivo glycosylation methodologies, such as natural product glycorandomization.

Creation of the first anomeric D/L-sugar kinase by means of directed evolution

Chemoenzymatic routes toward complex glycoconjugates often depend on the availability of sugar-1-phosphates. Yet the chemical synthesis of these vital components is often tedious, whereas natural enzymes capable of anomeric phosphorylation are known to be specific for one or only a few monosaccharides. Herein we describe the application of directed evolution and a high-throughput multisugar colorimetric screen to enhance the catalytic capabilities of the Escherichia coli galactokinase GalK. From this approach, one particular GalK mutant carrying a single amino acid exchange (Y371H) displayed a surprisingly substantial degree of kinase activity toward sugars as diverse as d-galacturonic acid, d-talose, l-altrose, and l-glucose, all of which failed as wild-type GalK substrates. Furthermore, this mutant provides enhanced turnover of the small pool of sugars converted by the wild-type enzyme. Comparison of this mutation to the recently solved structure of Lactococcus lactis GalK begins to provide a blueprint for further engineering of this vital class of enzyme. In addition, the rapid access to such promiscuous sugar C-1 kinases will significantly enhance accessibility to natural and unnatural sugar-1-phosphates and thereby impact both in vitro and in vivo glycosylation methodologies, such as natural product glycorandomization.

Studies on the substrate specificity of Escherichia coli galactokinase

In vitro glycorandomization (IVG) technology is dependent upon the ability to rapidly synthesize sugar phosphates. Compared with chemical synthesis, enzymatic (kinase) routes to sugar phosphates would be attractive for this application. This work focuses upon the development of a high-throughput colorimetric galactokinase (GalK) assay and its application toward probing the substrate specificity and kinetic parameters of Escherichia coli GalK. The demonstrated dinitrosalicylic assay should also be generally applicable to a variety of sugar-processing enzymes. [reaction: see text]

Biochemical adaptations of two sugar kinases from the hyperthermophilic archaeon Pyrococcus furiosus

The hyperthermophilic archaeon Pyrococcus furiosus possesses a modified Embden-Meyerhof pathway, including an unusual ADP-dependent glucokinase (ADP-GLK) and an ADP-dependent phosphofructokinase. In the present study, we report the characterization of a P. furiosus galactokinase (GALK) and its comparison with the P. furiosus ADP-GLK. The pyrococcal genes encoding the ADP-GLK and GALK were functionally expressed in Escherichia coli, and the proteins were subsequently purified to homogeneity. Both enzymes are specific kinases with an optimal activity at approx. 90 degrees C. Biochemical characterization of these enzymes confirmed that the ADP-GLK is unable to use ATP as the phosphoryl group donor, but revealed that GALK is ATP-dependent and has an extremely high affinity for ATP. There is a discussion about whether the unusual features of these two classes of kinases might reflect adaptations to a relatively low intracellular ATP concentration in the hyperthermophilic archaeon P. furiosus.

Overexpression and purification of the galactose operon enzymes from Escherichia coli

A convenient new procedure for the purification of galactokinase, galactose-1-phosphate uridyltransferase, and UDP-galactose 4-epimerase overexpressed in Escherichia coli is presented. The procedure is shorter than any other described in the literature and facilitates the purification of the three recombinant enzymes in considerable amounts and at high purity and specific activity. The purified gal operon enzymes were biochemically characterized by gel-filtration column chromatography and isoelectric focusing, and the Km values for their substrates were determined.

Galactokinase of Vicia faba seeds

Galactokinase (EC 2.7.1.6) from the dormant seeds of Vicia faba was purified approximately 1300-fold with an 18% recovery through an eight-step procedure. The preparation showed the presence of only minor contaminations as judged by disc-gel electrophoresis. The native enzyme displayed a molecular weight of approximately 60 000 (determined by Sephadex G-100 gel-filtration) and the subunit value was 30 000. The isoelectric point of the enzyme was 5.3 and the amino acid analysis showed high percentage of acidic amino acids. The pH optimum of the enzyme was 7.3 at 25 degrees C. The relative activity for phosphorylating various monosaccharides followed the order, D-galactose greater than 2-deoxy-D-galactose greater than D-galactosamine; D-fucose, L-arabinose, L-galactose and D-glucose were not phosphorylated. Whereas ATP acted as an efficient phosphate donor, ADP, GTP and UTP were unable to act in this capacity. The Km and the V values of the substrates were determined. The metal ion requirement for the enzymic activity followed the order, Mg2+ greater than Co2+ greater than Mn2+ greater than Ni2+ greater than Ca2+. The enzymic reaction was inhibited by heavy metal ions and sulphydryl reagents indicating the participation of -SH group(s) in enzymic catalysis. Product inhibition was observed; galactose 1-phosphate and ADP were competitive and non-competitive inhibitors, respectively. Seed germination showed an increase in galactokinase level up to 24 h followed by a rapid decrease. The level of raffinose and stachyose decreased continually. The galactokinase level was found to be sufficiently high to phosphorylate the liberated galactose. No free galactose was observed at any stage of germination.