Stephen G. Withers

Glycosyltransferases: Structures, Functions, and Mechanisms

Glycosyltransferases catalyze glycosidic bond formation using sugar donors containing a nucleoside phosphate or a lipid phosphate leaving group. Only two structural folds, GT-A and GT-B, have been identified for the nucleotide sugar-dependent enzymes, but other folds are now appearing for the soluble domains of lipid phosphosugar-dependent glycosyl transferases. Structural and kinetic studies have provided new insights. Inverting glycosyltransferases utilize a direct displacement S(N)2-like mechanism involving an enzymatic base catalyst. Leaving group departure in GT-A fold enzymes is typically facilitated via a coordinated divalent cation, whereas GT-B fold enzymes instead use positively charged side chains and/or hydroxyls and helix dipoles. The mechanism of retaining glycosyltransferases is less clear. The expected two-step double-displacement mechanism is rendered less likely by the lack of conserved architecture in the region where a catalytic nucleophile would be expected. A mechanism involving a short-lived oxocarbenium ion intermediate now seems the most likely, with the leaving phosphate serving as the base.

X-ray structures along the reaction pathway of cyclodextrin glycosyltransferase elucidate catalysis in the alpha-amylase family.

Cyclodextrin glycosyltransferase (CGTase) is an enzyme of the α-amylase family, which uses a double displacement mechanism to process α-linked glucose polymers. We have determined two X-ray structures of CGTase complexes, one with an intact substrate at 2.1 Å resolution, and the other with a covalently bound reaction intermediate at 1.8 Å resolution. These structures give evidence for substrate distortion and the covalent character of the intermediate and for the first time show, in atomic detail, how catalysis in the α-amylase family proceeds by the concerted action of all active site residues.

Crystal structure of the retaining galactosyltransferase LgtC from Neisseria meningitidis in complex with donor and acceptor sugar analogs.

Many bacterial pathogens express lipooligosaccharides that mimic human cell surface glycoconjugates, enabling them to attach to host receptors and to evade the immune response. In Neisseria meningitidis, the galactosyltransferase LgtC catalyzes a key step in the biosynthesis of lipooligosaccharide structure by transferring α-d-galactose from UDP-galactose to a terminal lactose. The product retains the configuration of the donor sugar glycosidic bond; LgtC is thus a retaining glycosyltranferase. We report the 2 Å crystal structures of the complex of LgtC with manganese and UDP 2-deoxy-2-fluoro-galactose (a donor sugar analog) in the presence and absence of the acceptor sugar analog 4′-deoxylactose. The structures, together with results from site-directed mutagenesis and kinetic analysis, give valuable insights into the unique catalytic mechanism and, as the first structure of a glycosyltransferase in complex with both the donor and acceptor sugars, provide a starting point for inhibitor design.

Structural analysis of the sialyltransferase CstII from Campylobacter jejuni in complex with a substrate analog

Sialic acid terminates oligosaccharide chains on mammalian and microbial cell surfaces, playing critical roles in recognition and adherence. The enzymes that transfer the sialic acid moiety from cytidine-5′-monophospho-N-acetyl-neuraminic acid (CMP-NeuAc) to the terminal positions of these key glycoconjugates are known as sialyltransferases. Despite their important biological roles, little is understood about the mechanism or molecular structure of these membrane-associated enzymes. We report the first structure of a sialyltransferase, that of CstII from Campylobacter jejuni, a highly prevalent foodborne pathogen. Our structural, mutagenesis and kinetic data provide support for a novel mode of substrate binding and glycosyl transfer mechanism, including essential roles of a histidine (general base) and two tyrosine residues (coordination of the phosphate leaving group). This work provides a framework for understanding the activity of several sialyltransferases, from bacterial to human, and for the structure-based design of specific inhibitors.

High-throughput screening methodology for the directed evolution of glycosyltransferases.

Engineering of glycosyltransferases (GTs) with desired substrate specificity for the synthesis of new oligosaccharides holds great potential for the development of the field of glycobiology. However, engineering of GTs by directed evolution methodologies is hampered by the lack of efficient screening systems for sugar-transfer activity. We report here the development of a new fluorescence-based high-throughput screening (HTS) methodology for the directed evolution of sialyltransferases (STs). Using this methodology, we detected the formation of sialosides in intact Escherichia coli cells by selectively trapping the fluorescently labeled transfer products in the cell and analyzing and sorting the resulting cell population using a fluorescence-activated cell sorter (FACS). We screened a library of >106 ST mutants using this methodology and found a variant with up to 400-fold higher catalytic efficiency for transfer to a variety of fluorescently labeled acceptor sugars, including a thiosugar, yielding a metabolically stable product.

Mechanisms of glycosyl transferases and hydrolases

Abstract Glycosidases and glycosyl transferases fall into two major mechanistic classes; those that hydrolyse the glycosidic bond with retention of anomeric configuration and those that do so with inversion. There are, however, two classes of transferases: those that use nucleotide phosphosugars (NP-sugar-dependent) and those that simply transglycosylate between oligosaccharides or polysaccharides (transglycosylases). The latter are mechanistically similar to retaining glycosidases while the mechanisms of NP-sugar-dependent transferases are far from clear. Retaining glycosidases and the transglycosylases employ a mechanism involving a covalent glycosyl–enzyme intermediate formed and hydrolysed with acid/base catalytic assistance via oxocarbenium ion-like transition states. This intermediate has been trapped on glycosidases in two distinct ways, either by modification of the substrate through fluorination, or of the enzyme through mutation of key residues. A third method has been developed for trapping the intermediate on transglycosylases involving the use of incompetent substrates that allow formation of the intermediate, but prohibit its transfer as a consequence of their acceptor hydroxyl group being removed. Three-dimensional structures of several of these glycosyl–enzyme complexes, along with those of Michaelis complexes, have been determined through X-ray crystallographic analysis, revealing the identities of important amino acid residues involved in catalysis. In particular they reveal the involvement of the carbonyl oxygen of the catalytic nucleophile in strong hydrogen bonding to the sugar 2-hydroxyl for the β-retainers or in interactions with the ring oxygen for α-retainers. The glucose ring in the −1 (cleavage) site in the intermediates formed on several cellulases and a β-glucosidase adopts a normal 4 C 1 chair conformation. By contrast the xylose ring at this site in a xylanase is substantially distorted into a 2,5 B boat conformation, an observation that bears significant stereoelectronic implications. Substantial distortion is also observed in the substrate upon binding to several β-glycosidases, this time to a 1 S 3 skew boat conformation. Much less distortion is seen in the substrate bound on an α-transglycosylase. Finally an efficient catalyst for synthesis, but not hydrolysis, of glycosidic bonds has been generated by mutation of the glutamic acid catalytic nucleophile of a β-glucosidase to an alanine. When used with α-glucosyl fluoride as a glycosyl donor, along with a suitable acceptor, oligosaccharides up to five sugars in length have been made with yields of up to 90% on individual steps. These new enzymes have been named Glycosynthases.

Glycosyl fluorides in enzymatic reactions.

Glycosyl fluorides have considerable importance as substrates and inhibitors in enzymatic reactions. Their good combination of stability and reactivity has enabled their use as glycosyl donors with a variety of carbohydrate processing enzymes. Moreover, the installation of fluorine elsewhere on the carbohydrate scaffold commonly modifies the properties of the glycosyl fluoride such that the resultant compounds act as slow substrates or even inhibitors of enzyme action. This review covers the use of glycosyl fluorides as substrates for wild-type and mutant glycosidases and other enzymes that catalyze glycosyl transfer. The use of substituted glycosyl fluorides as inhibitors of enzymes that catalyze glycosyl transfer and as tools for investigation of their mechanism is discussed, including the labeling of active site residues. Synthetic applications in which glycosyl fluorides are used as glycosyl donors in enzymatic transglycosylation reactions for the synthesis of oligo- and polysaccharides are then covered, including the use of mutant glycosidases, the so-called glycosynthases, which are able to catalyze the formation of glycosides without competing hydrolysis. Finally, a short overview of the use of glycosyl fluorides as substrates and inhibitors of phosphorylases and phosphoglucomutase is given.

Covalent inhibitors of glycosidases and their applications in biochemistry and biology

Glycoside hydrolases are important enzymes in a number of essential biological processes. Irreversible inhibitors of this class of enzyme have attracted interest as probes of both structure and function. In this review we discuss some of the compounds used to covalently modify glycosidases, their use in residue identification, structural and mechanistic investigations, and finally their applications, both in vitro and in vivo, to complex biological systems.

Dissection of nucleophilic and acid–base catalysis in glycosidases

A startling array of added anions have been observed to function as replacement catalytic nucleophiles in mutant glycosidases, including formate, azide, fluoride and other halides. Likewise, the mechanism of acid-base catalysis is somewhat plastic. The carboxylic acids can be substituted by a sulfenic acid or by ascorbate, and the effective acid strength enhanced by the introduction of strong hydrogen bonds. These studies provide an interesting bridge between enzymes and models thereof.

The Search for Novel Human Pancreatic α‐Amylase Inhibitors: High‐Throughput Screening of Terrestrial and Marine Natural Product Extracts

Specific inhibitors of human pancreatic α‐amylase (HPA) have potential as oral agents for the control of blood glucose levels in the treatment of diabetes and obesity. In a search for novel inhibitors, a library of 30 000 crude biological extracts of terrestrial and marine origin has been screened. A number of inhibitory extracts were identified, of which the most potent was subjected to bioassay‐guided purification. A family of three glycosylated acyl flavonols, montbretins A–C, was thereby identified and characterized as competitive amylase inhibitors, with Ki values ranging from 8.1–6100 nM. Competitive inhibition by myricetin, which corresponds to the flavone core, and noncompetitive inhibition by a second fragment, ethyl caffeiate, suggest a binding mode for these inhibitors.