NMR Spectroscopic Characterization of the Sialyltransferase CstII from Campylobacter jejuni: Histidine 188 Is the General Base

Genetics behind the Biosynthesis of Nonulosonic Acid-Containing Lipooligosaccharides in Campylobacter coli

Campylobacter jejuni and Campylobacter coli are the most common causes of bacterial gastroenteritis in the world. Ganglioside mimicry by C. jejuni lipooligosaccharide (LOS) is the triggering factor of Guillain-Barré syndrome (GBS), an acute polyneuropathy. Sialyltransferases from glycosyltransferase family 42 (GT-42) are essential for the expression of ganglioside mimics in C. jejuni Recently, two novel GT-42 genes, cstIV and cstV, have been identified in C. coli Despite being present in ∼11% of currently available C. coli genomes, the biological role of cstIV and cstV is unknown. In the present investigation, mutation studies with two strains expressing either cstIV or cstV were performed and mass spectrometry was used to investigate differences in the chemical composition of LOS. Attempts were made to identify donor and acceptor molecules using in vitro activity tests with recombinant GT-42 enzymes. Here we show that CstIV and CstV are involved in C. coli LOS biosynthesis. In particular, cstV is associated with LOS sialylation, while cstIV is linked to the addition of a diacetylated nonulosonic acid residue.IMPORTANCE Despite the fact that Campylobacter coli a major foodborne pathogen, its glycobiology has been largely neglected. The genetic makeup of the C. coli lipooligosaccharide biosynthesis locus was largely unknown until recently. C. coli harbors a large set of genes associated with lipooligosaccharide biosynthesis, including genes for several putative glycosyltransferases involved in the synthesis of sialylated lipooligosaccharide in Campylobacter jejuni In the present study, C. coli was found to express lipooligosaccharide structures containing sialic acid and other nonulosonate acids. These findings have a strong impact on our understanding of C. coli ecology, host-pathogen interaction, and pathogenesis.

Glycosyltransferases: mechanisms and applications in natural product development

Glycosylation reactions mainly catalyzed by glycosyltransferases (Gts) occur almost everywhere in the biosphere, and always play crucial roles in vital processes.

N-acetylglucosaminidases from CAZy family GH3 are really glycoside phosphorylases, thereby explaining their use of histidine as an acid/base catalyst in place of glutamic acid

CAZy glycoside hydrolase family GH3 consists primarily of stereochemistry-retaining β-glucosidases but also contains a subfamily of β-N-acetylglucosaminidases. Enzymes from this subfamily were recently shown to use a histidine residue within a His-Asp dyad contained in a signature sequence as their catalytic acid/base residue. Reasons for their use of His rather than the Glu or Asp found in other glycosidases were not apparent. Through studies on a representative member, the Nag3 β-N-acetylglucosaminidase from Cellulomonas fimi, we now show that these enzymes act preferentially as glycoside phosphorylases. Their need to accommodate an anionic nucleophile within the enzyme active site explains why histidine is used as an acid/base catalyst in place of the anionic glutamate seen in other GH3 family members. Kinetic and mechanistic studies reveal that these enzymes also employ a double-displacement mechanism involving a covalent glycosyl-enzyme intermediate, which was directly detected by mass spectrometry. Phosphate has no effect on the rates of formation of the glycosyl-enzyme intermediate, but it accelerates turnover of the N-acetylglucosaminyl-enzyme intermediate ∼3-fold, while accelerating turnover of the glucosyl-enzyme intermediate several hundredfold. These represent the first reported examples of retaining β-glycoside phosphorylases, and the first instance of free β-GlcNAc-1-phosphate in a biological context.

Crossroads between Bacterial and Mammalian Glycosyltransferases

Bacterial glycosyltransferases (GT) often synthesize the same glycan linkages as mammalian GT; yet, they usually have very little sequence identity. Nevertheless, enzymatic properties, folding, substrate specificities, and catalytic mechanisms of these enzyme proteins may have significant similarity. Thus, bacterial GT can be utilized for the enzymatic synthesis of both bacterial and mammalian types of complex glycan structures. A comparison is made here between mammalian and bacterial enzymes that synthesize epitopes found in mammalian glycoproteins, and those found in the O antigens of Gram-negative bacteria. These epitopes include Thomsen–Friedenreich (TF or T) antigen, blood group O, A, and B, type 1 and 2 chains, Lewis antigens, sialylated and fucosylated structures, and polysialic acids. Many different approaches can be taken to investigate the substrate binding and catalytic mechanisms of GT, including crystal structure analyses, mutations, comparison of amino acid sequences, NMR, and mass spectrometry. Knowledge of the protein structures and functions helps to design GT for specific glycan synthesis and to develop inhibitors. The goals are to develop new strategies to reduce bacterial virulence and to synthesize vaccines and other biologically active glycan structures.

Structure-based mutagenic analysis of mechanism and substrate specificity in mammalian glycosyltransferases: porcine ST3Gal-I

Sialyltransferases (STs) play essential roles in signaling and in the cellular recognition processes of mammalian cells by selectively installing cell-surface sialic acids in an appropriate manner both temporally and organ-specifically. The availability of the first three-dimensional structure of a mammalian (GT29) sialyltransferase has, for the first time, allowed quantitative structure/function analyses to be performed, thereby providing reliable insights into the roles of key active site amino acids. Kinetic analyses of mutants of ST3Gal-I, in conjunction with structural studies, have confirmed the mechanistic roles of His302 and His319 as general acid and base catalysts, respectively, and have quantitated other interactions with the cytosine monophosphate-N-acetyl β-neuraminic acid donor substrate. The contributions of side chains that provide key interactions with the acceptor substrate, defining its specificity, have also been quantitated. Particularly important transition-state interactions of 2.5 and 2.7 kcal mol(-1) are found between the acceptor axial 4-hydroxyl and the conserved side chains of Gln108 and Tyr269, respectively. These results provide a basis for the engineering of mammalian STs to accommodate non-natural substrate analogs that should prove valuable as chemical biological probes of sialyltransferase function.

Nuclear magnetic resonance spectral assignments of α-1,4-galactosyltransferase LgtC from Neisseria meningitidis: substrate binding and multiple conformational states

Lipopolysaccharide α-1,4-galactosyltransferase C (LgtC) from Neisseria meningitidis is responsible for a key step in lipooligosaccharide biosynthesis involving the transfer of α-galactose from the sugar donor UDP-galactose to a terminal acceptor lactose. Crystal structures of the complexes of LgtC with Mn(2+) and the sugar donor analogue UDP-2-deoxy-2-fluorogalactose in the absence and presence of the sugar acceptor analogue 4'-deoxylactose provided key insights into the galactosyl-transfer mechanism. Combined with kinetic analyses, the enzymatic mechanism of LgtC appears to involve a "front-side attack" S(N)i-like mechanism with a short-lived oxocarbenium-phosphate ion pair intermediate. As a prerequisite for investigating the required roles of structural dynamics in this catalytic mechanism by nuclear magnetic resonance techniques, the transverse relaxation-optimized amide (15)N heteronuclear single-quantum correlation and methyl (13)C heteronuclear multiple-quantum correlation spectra of LgtC in its apo, substrate analogue, and product complexes were partially assigned. This was accomplished using a suite of complementary spectroscopic approaches, combined with selective isotopic labeling and mutagenesis of all the isoleucine residues in the protein. Only ~70% of the amide signals could be detected, whereas more than the expected number of methyl signals were observed, indicating that LgtC adopts multiple interconverting conformational states. Chemical shift perturbation mapping provided insights into substrate and product binding, including the demonstration that the sugar donor analogue (UDP-2FGal) associates with LgtC only in the presence of a metal ion (Mg(2+)). These spectral assignments provide the foundation for detailed studies of the conformational dynamics of LgtC.

Novel dextranase catalyzing cycloisomaltooligosaccharide formation and identification of catalytic amino acids and their functions using chemical rescue approach

A novel endodextranase from Paenibacillus sp. (Paenibacillus sp. dextranase; PsDex) was found to mainly produce isomaltotetraose and small amounts of cycloisomaltooligosaccharides (CIs) with a degree of polymerization of 7-14 from dextran. The 1,696-amino acid sequence belonging to the glycosyl hydrolase family 66 (GH-66) has a long insertion (632 residues; Thr(451)-Val(1082)), a portion of which shares identity (35% at Ala(39)-Ser(1304) of PsDex) with Pro(32)-Ala(755) of CI glucanotransferase (CITase), a GH-66 enzyme that catalyzes the formation of CIs from dextran. This homologous sequence (Val(837)-Met(932) for PsDex and Tyr(404)-Tyr(492) for CITase), similar to carbohydrate-binding module 35, was not found in other endodextranases (Dexs) devoid of CITase activity. These results support the classification of GH-66 enzymes into three types: (i) Dex showing only dextranolytic activity, (ii) Dex catalyzing hydrolysis with low cyclization activity, and (iii) CITase showing CI-forming activity with low dextranolytic activity. The fact that a C-terminal truncated enzyme (having Ala(39)-Ser(1304)) has 50% wild-type PsDex activity indicates that the C-terminal 392 residues are not involved in hydrolysis. GH-66 enzymes possess four conserved acidic residues (Asp(189), Asp(340), Glu(412), and Asp(1254) of PsDex) of catalytic candidates. Their amide mutants decreased activity (1⁄1,500 to 1⁄40,000 times), and D1254N had 36% activity. A chemical rescue approach was applied to D189A, D340G, and E412Q using α-isomaltotetraosyl fluoride with NaN(3). D340G or E412Q formed a β- or α-isomaltotetraosyl azide, respectively, strongly indicating Asp(340) and Glu(412) as a nucleophile and acid/base catalyst, respectively. Interestingly, D189A synthesized small sized dextran from α-isomaltotetraosyl fluoride in the presence of NaN(3).

Characterization of α2,3- and α2,6-sialyltransferases from Helicobacter acinonychis

Genome sequence data were used to clone and express two sialyltransferase enzymes of the GT-42 family from Helicobacter acinonychis ATCC 51104, a gastric disease isolate from Cheetahs. The deposited genome sequence for these genes contains a large number of tandem repeat sequences in each of them: HAC1267 (RQKELE)(15) and HAC1268 (EEKLLEFKNI)(13). We obtained two clones with different numbers of repeat sequences for the HAC1267 gene homolog and a single clone for the HAC1268 gene homolog. Both genes could be expressed in Escherichia coli and sialyltransferase activity was measured using synthetic acceptor substrates containing a variety of terminal sugars. Both enzymes were shown to have a preference for N-acetyllactosamine, and they each made a product with a different linkage to the terminal galactose. HAC1267 is a mono-functional α2,3-sialyltransferase, whereas HAC1268 is a mono-functional α2,6-sialyltransferase and is the first member of GT-42 to show α2,6-sialyltransferase activity.

Sialic acid metabolism and sialyltransferases: natural functions and applications

Sialic acids are a family of negatively charged monosaccharides which are commonly presented as the terminal residues in glycans of the glycoconjugates on eukaryotic cell surface or as components of capsular polysaccharides or lipooligosaccharides of some pathogenic bacteria. Due to their important biological and pathological functions, the biosynthesis, activation, transfer, breaking down, and recycle of sialic acids are attracting increasing attention. The understanding of the sialic acid metabolism in eukaryotes and bacteria leads to the development of metabolic engineering approaches for elucidating the important functions of sialic acid in mammalian systems and for large-scale production of sialosides using engineered bacterial cells. As the key enzymes in biosynthesis of sialylated structures, sialyltransferases have been continuously identified from various sources and characterized. Protein crystal structures of seven sialyltransferases have been reported. Wild-type sialyltransferases and their mutants have been applied with or without other sialoside biosynthetic enzymes for producing complex sialic acid-containing oligosaccharides and glycoconjugates. This mini-review focuses on current understanding and applications of sialic acid metabolism and sialyltransferases.

PmST3 from Pasteurella multocida encoded by Pm1174 gene is a monofunctional α2-3-sialyltransferase

Pasteurella multocida (Pm) strain Pm70 has three putative sialyltransferase genes including Pm0188, Pm0508, and Pm1174. A Pm0188 gene homolog in Pm strain P-1059 encodes a multifunctional α2-3-sialyltransferase, PmST1, that prefers oligosaccharide acceptors. A Pm0508 gene homolog in the same strain encodes a monofunctional sialyltransferase PmST2 that prefers glycolipid acceptors. Here, we report that the third sialyltransferase from Pm (PmST3) encoded by gene Pm1174 in strain Pm70 is a monofunctional α2-3-sialyltransferase that can use both oligosaccharides and glycolipids as efficient acceptors. Despite the existence of both Pm0188 and Pm0508 gene homologs encoding PmST1 and PmST2, respectively, in Pm strain P-1059, a Pm1174 gene homolog appears to be absent from Pm strains P-1059 and P-934. PmST3 was successfully obtained by cloning and expression using a synthetic gene of Pm1174 with codons optimized for Escherichia coli expression system as the DNA template for polymer chain reactions. Truncation of 35 amino acid residues from the carboxyl terminus was shown to improve the expression of a soluble and active enzyme in E. coli as a C-His(6)-tagged fusion protein. This sialidase-free monofunctional α2-3-sialyltransferase is a useful tool for synthesizing sialylated oligosaccharides and glycolipids.

Structural and kinetic analysis of substrate binding to the sialyltransferase Cst-II from Campylobacter jejuni

Sialic acids play important roles in various biological processes and typically terminate the oligosaccharide chains on the cell surfaces of a wide range of organisms, including mammals and bacteria. Their attachment is catalyzed by a set of sialyltransferases with defined specificities both for their acceptor sugars and the position of attachment. However, little is known of how this specificity is encoded. The structure of the bifunctional sialyltransferase Cst-II of the human pathogen Campylobacter jejuni in complex with CMP and the terminal trisaccharide of its natural acceptor (Neu5Ac-α-2,3-Gal-β-1,3-GalNAc) has been solved at 1.95 Å resolution, and its kinetic mechanism was shown to be iso-ordered Bi Bi, consistent with its dual acceptor substrate specificity. The trisaccharide acceptor is seen to bind to the active site of Cst-II through interactions primarily mediated by Asn-51, Tyr-81, and Arg-129. Kinetic and structural analyses of mutants modified at these positions indicate that these residues are critical for acceptor binding and catalysis, thereby providing significant new insight into the kinetic and catalytic mechanism, and acceptor specificity of this pathogen-encoded bifunctional GT-42 sialyltransferase.

Current trends in the structure-activity relationships of sialyltransferases

Sialyltransferases (STs) represent an important group of enzymes that transfer N-acetylneuraminic acid (Neu5Ac) from cytidine monophosphate-Neu5Ac to various acceptor substrates. In higher animals, sialylated oligosaccharide structures play crucial roles in many biological processes but also in diseases, notably in microbial infection and cancer. Cell surface sialic acids have also been found in a few microorganisms, mainly pathogenic bacteria, and their presence is often associated with virulence. STs are distributed into five different families in the CAZy database (http://www.cazy.org/). On the basis of crystallographic data available for three ST families and fold recognition analysis for the two other families, STs can be grouped into two structural superfamilies that represent variations of the canonical glycosyltransferase (GT-A and GT-B) folds. These two superfamilies differ in the nature of their active site residues, notably the catalytic base (a histidine or an aspartate residue). The observed structural and functional differences strongly suggest that these two structural superfamilies have evolved independently.

Structural and kinetic analysis of Bacillus subtilis N-acetylglucosaminidase reveals a unique Asp-His dyad mechanism

Three-dimensional structures of NagZ of Bacillus subtilis, the first structures of a two-domain β-N-acetylglucosaminidase of family 3 of glycosidases, were determined with and without the transition state mimicking inhibitor PUGNAc bound to the active site, at 1.84- and 1.40-Å resolution, respectively. The structures together with kinetic analyses of mutants revealed an Asp-His dyad involved in catalysis: His(234) of BsNagZ acts as general acid/base catalyst and is hydrogen bonded by Asp(232) for proper function. Replacement of both His(234) and Asp(232) with glycine reduced the rate of hydrolysis of the fluorogenic substrate 4'-methylumbelliferyl N-acetyl-β-D-glucosaminide 1900- and 4500-fold, respectively, and rendered activity pH-independent in the alkaline range consistent with a role of these residues in acid/base catalysis. N-Acetylglucosaminyl enzyme intermediate accumulated in the H234G mutant and β-azide product was formed in the presence of sodium azide in both mutants. The Asp-His dyad is conserved within β-N-acetylglucosaminidases but otherwise absent in β-glycosidases of family 3, which instead carry a "classical" glutamate acid/base catalyst. The acid/base glutamate of Hordeum vulgare exoglucanase (Exo1) superimposes with His(234) of the dyad of BsNagZ and, in contrast to the latter, protrudes from a second domain of the enzyme into the active site. This is the first report of an Asp-His catalytic dyad involved in hydrolysis of glycosides resembling in function the Asp-His-Ser triad of serine proteases. Our findings will facilitate the development of mechanism-based inhibitors that selectively target family 3 β-N-acetylglucosaminidases, which are involved in bacterial cell wall turnover, spore germination, and induction of β-lactamase.