Human alpha1,3/4-fucosyltransferases. II. A single amino acid at the COOH terminus of FucT III and V alters their kinetic properties

Site-specific fucosylation of sialylated polylactosamines by alpha1,3/4-fucosyltransferases-V and -VI Is defined by amino acids near the N terminus of the catalytic domain

Fucose transfer from GDP-fucose to GlcNAc residues of the sialylated polylactosamine acceptor NeuAcalpha2-3Galbeta1-4Glc-NAcbeta1-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glcbeta1-ceramide leads to two isomeric monofucosyl antigens, VIM2 and sialyl-Le(x). Human alpha1,3/4-fucosyltransferase (FucT)-V catalyzes primarily the synthesis of VIM2, whereas human FucT-VI catalyzes primarily the synthesis of sialyl-Le(x). Thus, these two enzymes have distinct "site-specific fucosylation" properties. Amino acid sequence alignment of these enzymes showed that there are 24 amino acid differences in their catalytic domains. Studies were conducted to determine which of the amino acid differences are responsible for the site-specific fucosylation properties of each enzyme. Domain swapping (replacing a portion of the catalytic domain from one enzyme with an analogous portion from the other enzyme) demonstrated that site-specific fucosylation was defined within a 40-amino acid segment containing 8 amino acid differences between the two enzymes. Site-directed mutagenesis studies demonstrated that the site-specific fucosylation properties of these enzymes could be reversed by substituting 4 amino acids from one sequence with the other. These results were observed in both in vitro enzyme assays and flow cytometric analyses of Chinese hamster ovary cells transfected with plasmids containing the various enzyme constructs. Modeling studies of human FucT using a structure of a bacterial fucosyltransferase as a template demonstrated that the amino acids responsible for site-specific fucosylation map near the GDP-fucose-binding site. Additional enzyme studies demonstrated that FucT-VI has approximately 12-fold higher activity compared with FucT-V and that the Trp(124)/Arg(110) site in these enzymes is responsible primarily for this activity difference.

Non-isosteric C-glycosyl analogues of natural nucleotide diphosphate sugars as glycosyltransferase inhibitors

A series of C-glycosyl ethylphosphonophosphate analogues of UDP-Glc, UDP-Gal, UDP-GlcNAc and GDP-Fuc were synthesized from the corresponding C-glycosyl ethylphosphonic acids. Analogues were obtained as alpha-anomers through either diastereoselective photo-induced radical addition of glycosyl bromides (D-Glc, D-Gal and L-Fuc) to diethyl vinylphosphonate, or a multi-step sequence (D-GlcNAc), with subsequent coupling with morpholidate-activated nucleotide monophosphates. The in vitro inhibitory activity of UDP-Gal, GDP-Fuc and UDP-GlcNAc analogues towards glycosyltransferases (beta-1,4-GalT, FUT3 and LgtA) was evaluated through a competition fluorescence assay and IC(50) values of 40 microM, 2 mM and 3.5 mM were obtained, respectively.

Glycosyltransferases involved in type 1 chain and Lewis antigen biosynthesis exhibit glycan and core chain specificity

Sialyl Lewis A (SLe(a)), Lewis A (Le(a)), and Lewis B (Le(b)) have been studied in many different biological contexts, for example in microbial adhesion and cancer. Their biosynthesis is complex and involves beta1,3-galactosyltransferases (beta3Gal-Ts) and a combined action of alpha2- and/or alpha4-fucosyltransferases (Fuc-Ts). Further, O-glycans with different core structures have been identified, and the ability of beta3Gal-Ts and Fuc-Ts to use these as substrates has not been resolved. Therefore, to examine the in vivo specificity of enzymes involved in SLe(a), Le(a), and Le(b) synthesis, we have transiently transfected CHO-K1 cells with relevant human glycosyltransferases and, on secreted reporter proteins, detected the resulting Lewis antigens on N- and O-linked glycans using western blotting and Le-specific antibodies. beta3Gal-T1, -T2, and -T5 could synthesize type 1 chains on N-linked glycans, but only beta3Gal-T5 worked on O-linked glycans. The latter enzyme could use both core 2 and core 3 precursor structures. Furthermore, the specificity of FUT5 and FUT3 in Le(a) and Le(b) synthesis was different, with FUT5 fucosylating H type 1 only on core 2, but FUT3 fucosylating H type 1 much more efficient on core 3 than on core 2. Finally, FUT1 and FUT2 were both found to direct alpha2-fucosylation on type 1 chains on both N- and O-linked structures. This knowledge enables us to engineer recombinant glycoproteins with glycan- and core chain-specific Lewis antigen substitution. Such tools will be important for investigations on the fine carbohydrate specificity of Le(b)-binding lectins, such as Helicobacter pylori adhesins and DC-SIGN, and may also prove useful as therapeutics.

Purification, Kinetic Characterization, and Mapping of the Minimal Catalytic Domain and the Key Polar Groups of Helicobacter pylori α-(1,3/1,4)-Fucosyltransferases

The minimal catalytic domain of alpha-(1,3/1,4)-fucosyltransferases (FucTs) from Helicobacter pylori strains NCTC11639 and UA948 was mapped by N- and C-terminal truncations. Only the C terminus could be truncated without significant loss of activity. 11639FucT and UA948FucT contain 10 and 8 heptad repeats, respectively, which connect the catalytic domain with the C-terminal putative amphipathic alpha-helices. Deletion of all heptad repeats almost completely abolished enzyme activity. Nevertheless, with only one heptad repeat 11639FucT is fully active, whereas UA948FucT is partially active. Removal of the two putative amphipathic alpha-helices dramatically increased protein expression and solubility, enabling purification with yields of milligrams/liter. Steady-state kinetic analysis of the purified FucTs showed that 11639FucTs possessed slightly tighter binding affinity for both Type II acceptor and GDP-fucose donor than UA948FucT, and its kcat of 2.3 s(-1) was double that of UA948FucT, which had a kcat value of 1.1 s(-1) for both Type II and Type I acceptors. UA948FucT strongly favors Type II over the Type I acceptor with a 20-fold difference in acceptor Km. Sixteen modified Type I and Type II series acceptors were employed to map the molecular determinants of acceptors required for recognition by H. pylori alpha-(1,3/1,4)-FucTs. Deoxygenation at 6-C of the galactose in Type II acceptor caused a 5000-fold decrease in alpha1,3 activity, whereas in Type I acceptor this completely abolished alpha1,4 activity, indicating that this hydroxyl group is a key polar group.

Dynamic Control of Oligosaccharide Modification in the Mammary Gland: Linking Recombinant Human Erythropoietin

We analyzed two transgenic mouse lines that secrete rhEPO in their milk to assess the dynamic control of N-linked oligosaccharides. Since pharmaceutically available epoetin alpha and beta are produced in CHO cells, we compared transgenic mammary gland-derived rhEPO to its CHO cell-derived counterpart. The major glycosyltransferases that determine the N-oligosaccharides patterns of rhEPO include N-acetylglycosaminyltransferase (GnT) and alpha1,3/4 fucosyltransferase (Fuc-TIV), GnT-III, -V and Fuc-TIV expression in the mouse mammary gland is significantly higher than that in Chinese hamster ovary (CHO)-derived cells, where the protein is not detectable. The data suggest that N-linked sugar chain patterns of recombinant glycoproteins, produced by the mammary gland differ, since GnT-III alters the sugar pattern extensively. In our experiments, rhEPO produced by the transgenic mice contains more tetra-acidic oligosaccharide structures than epoetin alpha derived from CHO cells, a rhEPO that is widely used therapeutically. Accordingly, we examined milk-derived rhEPO activity, both in vitro and in vivo. The rhEPO protein purified from the milk of mammary glands upregulates the EPO receptor-mediated expression of the STAT5 gene in MCF-7 cells in a dose-dependent manner, similar to the effects of epoetin alpha. Furthermore, direct injection of rhEPO into the mouse tail vein leads to an increase in the levels of blood components, such as red blood cells and platelets. In light of these findings, we suggest that the mammary glands of transgenic animals provide a sufficient environment to generate rhEPO with post-translational modifications for biopharmaceutical use.

Synthesis of unnatural sugar nucleotides and their evaluation as donor substrates in glycosyltransferase-catalyzed reactions

New unnatural sugar nucleotides, UDP-Fuc and CDP-Fuc were synthesized from fucose-beta-1-phosphate and nucleotide monophosphates activated as morpholidates. Furthermore, a nucleotide analogue was prepared by phosphorylation of 1-(beta-D-ribofuranosyl)cyanuric acid, itself obtained as a protected derivative by condensation of the persilylated derivative of cyanuric acid with 1-O-acetyl-2,3,5-tri-O-benzoyl-beta-D-ribofuranose in 74% yield. This phosphate activated according to the same procedure was condensed with fucose-beta-1-phosphate, affording a new sugar nucleotide conjugate (NDP-Fuc) which was evaluated together with UDP-Fuc, CDP-Fuc and ADP-Fuc, as fucose donors in alpha-(1-->4/3)-fucosyltransferase (FucT-III) catalyzed reaction. Fucose transfer could be observed with each of the donors and kinetic parameters were determined using a fluorescent acceptor substrate. Efficiency of the four analogues towards FucT-III was in the following order: UDP-Fuc=ADP-Fuc>NDP-Fuc>CDP-Fuc. According to the same strategy ADP-GlcNAc was prepared from AMP-morpholidate and N-acetylglucosamine-alpha-1-phosphate; tested as a glucosaminyl donor towards Neisseria meningitidis N-acetylglucosaminyl transferase (LgtA), ADP-GlcNAc was recognized with 0.1% efficiency as compared with UDP-GlcNAc, the natural donor substrate.

Highly conserved cysteines of mouse core 2 beta1,6-N-acetylglucosaminyltransferase I form a network of disulfide bonds and include a thiol that affects enzyme activity

Core 2 beta1,6-N-acetylglucosaminyltransferase I (C2GnT-I) plays a pivotal role in the biosynthesis of mucin-type O-glycans that serve as ligands in cell adhesion. To elucidate the three-dimensional structure of the enzyme for use in computer-aided design of therapeutically relevant enzyme inhibitors, we investigated the participation of cysteine residues in disulfide linkages in a purified murine recombinant enzyme. The pattern of free and disulfide-bonded Cys residues was determined by liquid chromatography/electrospray ionization tandem mass spectrometry in the absence and presence of dithiothreitol. Of nine highly conserved Cys residues, under both conditions, one (Cys217) is a free thiol, and eight are engaged in disulfide bonds, with pairs formed between Cys59-Cys413, Cys100-Cys172, Cys151-Cys199, and Cys372-Cys381. The only non-conserved residue within the beta1,6-N-acetylglucosaminyltransferase family, Cys235, is also a free thiol in the presence of dithiothreitol; however, in the absence of reductant, Cys235 forms an intermolecular disulfide linkage. Biochemical studies performed with thiolreactive agents demonstrated that at least one free cysteine affects enzyme activity and is proximal to the UDP-GlcNAc binding site. A Cys217 --> Ser mutant enzyme was insensitive to thiol reactants and displayed kinetic properties virtually identical to those of the wild-type enzyme, thereby showing that Cys217, although not required for activity per se, represents the only thiol that causes enzyme inactivation when modified. Based on the pattern of free and disulfide-linked Cys residues, and a method of fold recognition/threading and homology modeling, we have computed a three-dimensional model for this enzyme that was refined using the T4 bacteriophage beta-glucosyltransferase fold.

C-terminal amino acids of Helicobacter pylori alpha1,3/4 fucosyltransferases determine type I and type II transfer

The alpha1,3/4 fucosyltransferase (FucT) enzyme from Helicobacter pylori catalyzes fucose transfer from donor GDP-beta-l-fucose to the GlcNAc group of two series of acceptor substrates in H. pylori lipopolysaccharide: betaGal1,3betaGlcNAc (Type I) or betaGal1,4betaGlcNAc (Type II). Fucose is added either in alpha1,3 linkage of Type II acceptor to produce Lewis X or in alpha1,4 linkage of Type I acceptor to produce Lewis A, respectively. H. pylori FucTs from different strains have distinct Type I or Type II substrate specificities. FucT in H. pylori strain NCTC11639 has an exclusive alpha1,3 activity because it recognizes only Type II substrates, whereas FucT in H. pylori strain UA948 can utilize both Type II and Type I acceptors; thus it has both alpha1,3 and alpha1,4 activity, respectively. To identify elements conferring substrate specificity, 12 chimeric FucTs were constructed by domain swapping between 11639FucT and UA948FucT and characterized for their ability to transfer fucose to Type I and Type II acceptors. Our results indicate that the C-terminal region of H. pylori FucTs controls Type I and Type II acceptor specificity. In particular, the highly divergent C-terminal portion, seven amino acids DNPFIFC at positions 347-353 in 11639FucT, and the corresponding 10 amino acids CNDAHYSALH at positions 345-354 in UA948FucT, controls the Type I and Type II acceptor recognition. This is the opposite of mammalian FucTs where acceptor preference is determined primarily by the N-terminal residues in the hypervariable stem domain.

A highly conserved His-His motif present in alpha1-->3/4fucosyltransferases is required for optimal activity and functions in acceptor binding

Alpha1-->3/4fucosyltransferases (FucTs) from several species contain a highly conserved His-His motif adjacent to an enzyme region correlating with the ability to catalyze fucose transfer to type 1 chain acceptors. Site-directed mutagenesis has been employed to analyze structure-function relationships of this His-His motif in human FucT-IV. The results indicate that most changes of His(113) and His(114) and nearby residues of FucT-IV reduced the specific activity of the enzymes. Analysis of acceptor properties demonstrated close similarity of most mutants with wild-type FucT-IV, whereas an apparent preference for the H-type II acceptor was observed for the His(114) mutants. Kinetic studies demonstrated that mutants of His(114) had a substantially increased K(m) for acceptor compared to other enzymes tested. The dramatic increase in acceptor K(m) for the His(114) mutants, particularly for the nonfucosylated acceptor, suggests that this His-His motif is involved in acceptor binding and perhaps interacts with GlcNAc residues of type 2 acceptors. The presence of fucose in acceptor substrates may promote more efficient substrate binding and presumably partially overcomes the weaker interaction with GlcNAc caused by the mutation.

Chemical modifications of alpha1,6-fucosyltransferase define amino acid residues of catalytic importance

alpha1,6-Fucosyltransferase (alpha6FucT) of human platelets was subjected to the action of phenylglyoxal (PLG), pyridoxal-5'-phosphate/NaBH(4) (PLP), and diethyl pyrocarbonate (DEPC) the reagents that selectively modify the structure of amino acids arginine, lysine and histidine, respectively, as well as to N-ethylmaleimide (NEM), mersalyl, p-chloromercuribenzoate (pCMB), iodoacetate, iodoacetamide, and methyl iodide that react with sulfhydryl group of cysteine. In addition, we treated the enzyme with beta-mercaptoethanol, a reagent that disrupts disulfide bonds. All reagents except NEM significantly inactivated alpha6FucT. Protection against the action of PLG, PLP and sulfhydryl modifying reagents was offered by GDP-fucose, GDP, and the acceptor substrate, a transferrin-derived biantennary glycopeptide with terminal GlcNAc residues. Neither donor nor acceptor substrate offered, however, any protection against inactivation by DEPC or beta-mercaptoethanol. We conclude that arginine, cysteine and probably lysine residues are present in, or closely by, the donor and acceptor substrate binding domains of the enzyme, whereas histidine may be a part of its catalytic domain. However, the primary structure of alpha6FucT does not show cysteine residues in proximity to the postulated GDP-fucose-binding site and acceptor substrate binding site of the enzyme that contains two neighboring arginine residues and one lysine residue (Glycobiol. 10 (2000) 503). To rationalize our results we postulate that platelet alpha6FucT is folded through disulfide bonds that bring together donor/acceptor-binding- and cysteine- and lysine-rich, presumably acceptor substrate binding sites, thus creating a catalytic center of the enzyme.

Fucosyltransferases: structure/function studies

Alpha3-fucosyltransferases (alpha3-FucTs) catalyze the final step in the synthesis of a range of important glycoconjugates that function in cell adhesion and lymphocyte recirculation. Six members of this family of enzymes have been cloned from the human genome, and their expression pattern has been shown to be highly regulated. Each enzyme has a unique acceptor substrate binding pattern, and each generates a unique range of fucosylated products. Results from a range of studies have provided information on amino acids in the FucT sequence that contribute to the differential acceptor specificity for the FucTs, and to the binding of the nucleotide sugar donor GDP-fucose. These results, in conjunction with results obtained from the analysis of the disulfide bond pattern, have provided useful clues about the spatial distribution of amino acids that influence or directly contribute to substrate binding. This information is reviewed here, and a molecular fold prediction is presented which has been constructed based on the available information and current modeling methodology.

Neighboring cysteine residues in human fucosyltransferase VII are engaged in disulfide bridges, forming small loop structures

Among alpha 3-fucosyltransferases (alpha3-FucTs) from most species, four cysteine residues appear to be highly conserved. Two of these cysteines are located at the N-terminus and two at the C-terminus of the catalytic domain. FucT VII possesses two additional cysteines in close proximity to each other located in the middle of the catalytic domain. We identified the disulfide bridges in a recombinant, soluble form of human FucT VII. Potential free cysteines were modified with a biotinylated alkylating reagent, disulfide bonds were reduced and alkylated with iodoacetamide, and the protein was digested with either trypsin or chymotrypsin, before characterization by high-performance liquid chromatography/electrospray ionization mass spectrometry. More than 98% of the amino acid sequence for the truncated enzyme (beginning at amino acid 53) was verified. Mass spectrometry analysis also demonstrated that both potential N-linked sites are occupied. All six cysteines in the FucT VII sequence were shown to be disulfide-linked. The pairing of the cysteines was determined by proteolytic cleavage of nonreduced protein and subsequent analysis by mass spectrometry. The results demonstrated that Cys(68)-Cys(76), Cys(211)-Cys(214), and Cys(318)-Cys(321) are disulfide-linked. We have used this information, together with a method of fold recognition and homology modeling, using the (alpha/beta)(8)-barrel fold of Escherichia coli dihydrodipicolinate synthase as a template to propose a model for FucT VII.

Ancestral exonic organization of FUT8, the gene encoding the alpha6-fucosyltransferase, reveals successive peptide domains which suggest a particular three-dimensional core structure for the alpha6-fucosyltransferase family

Based on PCR strategies and expression studies, we define the genomic organization of the FUT8b gene. This gene encodes the only known mammalian enzyme transferring fucose in an alpha1-->6 linkage on the asparagine-branched GlcNAc residue of the chitobiose unit of complex N:-glycans. The intron/exon organization of the bovine coding sequence determines five successive functional domains. The first exon encodes a domain homologous to cytoskeleton proteins, the second presents a proline-rich region including a motif XPXPPYXP similar to the peptide ligand of the SH3-domain proteins, the third encodes a gyrase-like domain (an enzyme which can bind nucleotides), and the fourth encodes a peptide sequence homologous to the catalytic domain of proteins transferring sugars. Finally, the last exon encodes a domain homologous to the SH3 conserved motif of the SH2-SH3 protein family. This organization suggests that intramolecular interactions might give a tulip-shaped scaffolding, including the catalytic pocket of the enzyme in the Golgi lumen. Deduced from the published sequence of chromosome 14 (AL109847), the human gene organization of FUT8 seems to be similar to that of bovine FUT8b, although the exon partition is more pronounced (bovine exons 1 and 2 correspond to human exons 1-6). The mosaicism and phylogenetic positions of the alpha6-fucosyltransferase genes are compared with those of other fucosyltransferase genes.

Human alpha 1,3/4 fucosyltransferases. Characterization of highly conserved cysteine residues and N-linked glycosylation sites

Human alpha1,3 fucosyltransferases (FucTs) contain four highly conserved cysteine (Cys) residues, in addition to a free Cys residue that lies near the binding site for GDP-fucose (Holmes, E. H., Xu, Z. , Sherwood, A. L., and Macher, B. A. (1995) J. Biol. Chem. 270, 8145-8151). The participation of the highly conserved Cys residues in disulfide bonds and their functional significance were characterized by mass spectrometry (MS) analyses and site-directed mutagenesis, respectively. Among the human FucTs is a subset of enzymes (FucT III, V, and VI) having highly homologous sequences, especially in the catalytic domain, and Cys residues in FucT III and V were characterized. The amino acid sequence of FucT III was characterized. Peptides containing the four conserved Cys residues were detected after reduction and alkylation, and found to be involved in disulfide bonds. The disulfide bond pattern was characterized by multiple stage MS analysis and the use of Glu-C protease and MS/MS analysis. Disulfide bonds in FucT III occur between Cys residues (Cys(81) to Cys(338) and Cys(91) to Cys(341)) at the N and C termini of the catalytic domain, bringing these ends close together in space. Mutagenesis of highly conserved Cys residues to Ser in FucT V resulted in proteins lacking enzymatic activity. Three of the four mutants have molecular weights similar to wild type enzyme and maintained an ability to bind GDP, whereas the other (Cys(104)) produced a series of lower molecular weight bands when characterized by Western blot analysis, and did not bind GDP. FucTs have highly conserved, potential N-linked sites, and our mass spectrometry analyses demonstrated that both N-linked sites are modified with oligosaccharides.

Chemo-enzymatic synthesis of fluorinated sugar nucleotide: useful mechanistic probes for glycosyltransferases

An effective procedure for the synthesis of 2-deoxy-2-fluoro-sugar nucleotides via Select fluor-mediated electrophilic fluorination of glycals with concurrent nucleophilic addition or chemo-enzymatic transformation has been developed, and the fluorinated sugar nucleotides have been used as probes for glycosyltransferases, including fucosyltransferase III, V, VI, and VII, and sialyl transferases. In general, these fluorinated sugar nucleotides act as competitive inhibitors versus sugar nucleotide substrates and form a tight complex with the glycosyltransferase.

A GDP-fucose-protected, pyridoxal-5'-Phosphate/NaBH(4)-sensitive lys residue common to human alpha1-->3Fucosyltransferases corresponds to Lys(300) in FucT-IV

Human alpha1-->3/4fucosyltransferases (FucTs) contain a common essential pyridoxal-5'-phosphate(PLP)/NaBH(4) reactive, GDP-fucose-protectable Lys. For identification, site-directed mutants at lysines of FucT-IV and -VII were prepared and tested. Non conserved lysine mutants K119Y and K394Q were similar to wild-type FucT-IV. However, mutants of conserved lysines K228R and K300R were distinct. The specific activity of K228R was 2- to 3-fold lower but retained K(m) values for donor and acceptor substrates as wild-type FucT-IV. The specific activity of K300R was reduced over 400-fold with an apparent K(m) for GDP-fucose over 200 microM. FucT-VII mutants K169R and K240R (equivalent to K228R and K300R for FucT-IV, respectively) were inactive. No change in PLP/NaBH(4) sensitivity occurred with K119Y, K228R, and K394Q compared to wild-type FucT-IV. These and previous results (A. L. Sherwood, A. T. Nguyen, J. M. Whitaker, B. A. Macher, M. R. Stroud, and E. H. Holmes, J. Biol. Chem. 273, 25256-25260, 1998) demonstrate that of three conserved lysines in FucT-IV, two (Lys(228) and Lys(283)) are not involved in substrate binding but perhaps in catalysis. The third site, Lys(300), is involved in GDP-fucose binding and PLP/NaBH(4) inactivation.

A sequence motif involved in the donor substrate binding by alpha1,6-fucosyltransferase: the role of the conserved arginine residues

Alpha1,6-fucosyltransferase catalyzes the transfer of fucose to the innermost GlcNAc residue of an N-linked oligosaccharide. In order to identify the amino acid residue(s) which are associated with the enzyme activity and to investigate their function, we prepared a series of mutant human alpha1,6-fucosyltransferases in which the conserved residues in the region homologous to alpha1,2-fucosyltransferase had been replaced. These proteins were then characterized by kinetic analyses. The wild-type and mutant alpha1,6-fucosyltransferases were expressed using a baculovirus-insect cell system. The activity assay showed that replacement of Arg-365 by Ala or Lys led to a complete loss of activity while substitution of Ala or Lys for the neighboring Arg-366 decreased the activity to about 3% that of the wild type. Kinetic analyses revealed that the replacements of Arg-366 lead to an increase in the apparent K (m) value for both GDP-fucose and the acceptor oligosaccharide but did not markedly affect the apparent V (max). When these mutants were inhibited by GDP in a competitive manner with respect to the donor substrate, the K (i) values were found to be 50-100 times higher than the value in the wild type. On the other hand, in the inhibition by GMP, the K (i) values for the mutants were very similar to that of the wild type. These findings suggest that Arg-366 contributes to the binding of GDP-fucose via an interaction with the beta-phosphoryl group of the GDP moiety of the donor, and that Arg-365 may also play an essential role in substrate binding. The results suggest that the motif common to alpha1,2- and alpha1,6-fucosyltransferases is critical for binding of the donor substrate, GDP-fucose.

Fucose in N-glycans: from plant to man

Fucosylated oligosaccharides occur throughout nature and many of them play a variety of roles in biology, especially in a number of recognition processes. As reviewed here, much of the recent emphasis in the study of the oligosaccharides in mammals has been on their potential medical importance, particularly in inflammation and cancer. Indeed, changes in fucosylation patterns due to different levels of expression of various fucosyltransferases can be used for diagnoses of some diseases and monitoring the success of therapies. In contrast, there are generally at present only limited data on fucosylation in non-mammalian organisms. Here, the state of current knowledge on the fucosylation abilities of plants, insects, snails, lower eukaryotes and prokaryotes will be summarised.

Structure/function studies of glycosyltransferases

Glycosyltransferases are the enzymes that synthesize oligosaccharides, polysaccharides and glycoconjugates. The analysis of the wealth of sequences that are now available in databases allowed the determination of conserved peptide motifs for each class of enzyme. Recent experimental data demonstrated their importance in donor and acceptor substrate binding and in catalysis. Fold-recognition studies provided the first models of the catalytic domains of some of these enzymes, while the first successes in glycosyltransferase crystallography are opening new routes in structural glycobiology.

A single amino acid in the hypervariable stem domain of vertebrate alpha1,3/1,4-fucosyltransferases determines the type 1/type 2 transfer. Characterization of acceptor substrate specificity of the lewis enzyme by site-directed mutagenesis

Alignment of 15 vertebrate alpha1,3-fucosyltransferases revealed one arginine conserved in all the enzymes employing exclusively type 2 acceptor substrates. At the equivalent position, a tryptophan was found in FUT3-encoded Lewis alpha1,3/1,4-fucosyltransferase (Fuc-TIII) and FUT5-encoded alpha1,3/1,4-fucosyltransferase, the only fucosyltransferases that can also transfer fucose in alpha1, 4-linkage. The single amino acid substitution Trp111 --> Arg in Fuc-TIII was sufficient to change the specificity of fucose transfer from H-type 1 to H-type 2 acceptors. The additional mutation of Asp112 --> Glu increased the type 2 activity of the double mutant Fuc-TIII enzyme, but the single substitution of the acidic residue Asp112 in Fuc-TIII by Glu decreased the activity of the enzyme and did not interfere with H-type 1/H-type 2 specificity. In contrast, substitution of Arg115 in bovine futb-encoded alpha1, 3-fucosyltransferase (Fuc-Tb) by Trp generated a protein unable to transfer fucose either on H-type 1 or H-type 2 acceptors. However, the double mutation Arg115 --> Trp/Glu116 --> Asp of Fuc-Tb slightly increased H-type 1 activity. The acidic residue adjacent to the candidate amino acid Trp/Arg seems to modulate the relative type 1/type 2 acceptor specificity, and its presence is necessary for enzyme activity since its substitution by the corresponding amide inactivated both Fuc-TIII and Fuc-Tb enzymes.

Human alpha1,3/4-fucosyltransferases. III. A Lys/Arg residue located within the alpha1,3-FucT motif is required for activity but not substrate binding

Amino acid sequence alignment of human alpha1, 3/4-fucosyltransferases (FucTs) demonstrates that three highly conserved Lys residues are present in the catalytic domain of FucTs III, IV, V, and VI. Two of these sites are conserved in FucT VII, with the third located within the alpha1,3-FucT motif as a conservative change to Arg at position 223. Site-directed mutagenesis experiments were conducted to change Lys255 of FucT V (equivalent to Arg223 of FucT VII) to either Arg255 or Ala255. Enzyme assays demonstrate that the FucT V K255R mutant has a 34-fold lower specific activity than native FucT V and that the K255A mutant is inactive. Site-directed mutagenesis of FucT VII was also conducted to change Arg223 to Lys223 for analysis of the effect on enzyme kinetic parameters. No differences in acceptor specificities or Km values for either substrate were observed between native FucT VII and the R223K mutant; however, the purified R223K mutant enzyme had a 2-fold increased specific activity compared with purified native FucT VII. No change in GDP-fucose-protectable pyridoxal-P/NaBH4 inactivation was observed for native or mutant FucT V or VII, further supporting the absence of involvement of this residue in sugar nucleotide binding. The results indicate that a basic residue in this position is required for enzyme activity, with a Lys residue providing higher intrinsic activity. The lack of influence of this site on substrate binding parameters and its location within the alpha1,3-FucT motif suggest that at least some of the residues within this motif are involved in catalysis rather than substrate binding.

Human alpha1,3/4-fucosyltransferases. I. Identification of amino acids involved in acceptor substrate binding by site-directed mutagenesis

In a previous study (Xu, Z., Vo, L., and Macher, B. A. (1996) J. Biol. Chem. 271, 8818-8823), a domain swapping approach demonstrated that a region of amino acids found in human alpha1, 3/4-fucosyltransferase III (FucT III) conferred a significant increase in alpha1,4-FucT acceptor substrate specificity into alpha1, 3-fucosyltransferase V (FucT V), which, under the same assay conditions, has extremely low alpha1,4-FucT acceptor substrate specificity. In the current study, site-directed mutagenesis was utilized to identify which of the eight amino acids, associated with alpha1,4-FucT acceptor substrate specificity, is/are responsible for conferring this new property. The results demonstrate that increased alpha1,4-FucT activity with both disaccharide and glycolipid acceptors can be conferred on FucT V by modifying as few as two (Asn86 to His and Thr87 to Ile) of the eight amino acids originally swapped from FucT III into the FucT V sequence. Neither single amino acid mutant had increased alpha1,4-FucT activity relative to that of FucT V. Kinetic analyses of FucT V mutants demonstrated a reduced Km for Galbeta1,3GlcNAc (type 1) acceptor substrates compared with native FucT V. However, this was about 20-fold higher than that found for native FucT III, suggesting that other amino acids in FucT III must contribute to its overall binding site for type 1 substrates. These results demonstrate that amino acid residues near the amino terminus of the catalytic domain of FucT III contribute to its acceptor substrate specificity.