Chemoenzymatic Methods for the Synthesis of Glycoproteins

Glycosylation is one of the most prevalent posttranslational modifications that profoundly affects the structure and functions of proteins in a wide variety of biological recognition events. However, the structural complexity and heterogeneity of glycoproteins, usually resulting from the variations of glycan components and/or the sites of glycosylation, often complicates detailed structure-function relationship studies and hampers the therapeutic applications of glycoproteins. To address these challenges, various chemical and biological strategies have been developed for producing glycan-defined homogeneous glycoproteins. This review highlights recent advances in the development of chemoenzymatic methods for synthesizing homogeneous glycoproteins, including the generation of various glycosynthases for synthetic purposes, endoglycosidase-catalyzed glycoprotein synthesis and glycan remodeling, and direct enzymatic glycosylation of polypeptides and proteins. The scope, limitation, and future directions of each method are discussed.

The ENGases: versatile biocatalysts for the production of homogeneous N-linked glycopeptides and glycoproteins

The endo-β-N-acetylglucosaminidases (ENGases) are an enzyme class (EC 3.2.1.96) produced by a range of organisms, ranging from bacteria, through fungi, to higher order species, including humans, comprising two-sub families of glycosidases which all cleave the chitobiose core of N-linked glycans. Synthetic applications of these enzymes, i.e. to catalyse the reverse of their natural hydrolytic mode of action, allow the attachment of N-glycans to a wide variety of substrates which contain an N-acetylglucosamine (GlcNAc) residue to act as an 'acceptor' handle. The use of N-glycan oxazolines, high energy intermediates on the hydrolytic pathway, as activated donors allows their high yielding attachment to almost any amino acid, peptide or protein that contains a GlcNAc residue as an acceptor. The synthetic effectiveness of these biocatalysts has been significantly increased by the production of mutant glycosynthases; enzymes which can still catalyse synthetic processes using oxazolines as donors, but which do not hydrolyse the reaction products. ENGase biocatalysts are now finding burgeoning application for the production of biologically active glycopeptides and glycoproteins, including therapeutic monoclonal antibodies (mAbs) for which the oligosaccharides have been remodelled to optimise effector functions.

Advances in Enzymatic Glycoside Synthesis

A robust platform for facile defined glycan synthesis does not exist. Yet the need for such technology has never been greater as researchers seek to understand the full scope of carbohydrate function, stretching beyond the classical roles of structure and energy storage to encompass highly nuanced cell signaling events. To comprehensively explore and exploit the full diversity of carbohydrate functions, we must first be able to synthesize them in a controlled manner. Toward this goal, traditional chemical syntheses are inefficient while nature's own synthetic enzymes, the glycosyl transferases, can be challenging to express and expensive to employ on scale. Glycoside hydrolases represent a pool of glycan processing enzymes that can be either used in a transglycosylation mode or, better, engineered to function as "glycosynthases," mutant enzymes capable of assembling glycosides. Glycosynthases grant access to valuable glycans that act as functional and structural probes or indeed as inhibitors and therapeutics in their own right. The remodelling of glycosylation patterns in therapeutic proteins via glycoside hydrolases and their mutants is an exciting frontier in both basic research and industrial scale processes.

Synthesis of a hybrid type N-glycan heptasaccharide oxazoline for Endo M catalysed glycosylation

Endo-β-N-acetylglucosaminidases (ENGases) are versatile biocatalysts that allow access to a wide variety of defined homogenous N-linked glycoconjugates in a convergent manner. A hybrid-type N-glycan was accessed by total synthesis, converted to an oxazoline, and used as a donor substrate with both wild type Endo M and an N175Q glycosynthase Endo M mutant allowing the convergent synthesis of a glycosylated amino acid bearing a hybrid N-glycan structure.

Current Approaches to Engineering N-Linked Protein Glycosylation in Bacteria

N-Linked protein glycosylation is a common posttranslational protein modification in eukaryotes involved in many biological processes. As glycosylation is also important for the function and the pharmacokinetic properties of many protein therapeutics, there is an increasing interest in expression systems able to produce glycoproteins of well-defined structure. Bacterial expression hosts generally do not glycosylate proteins at all. However, the discovery of bacterial N-glycosylation systems has opened up a new route for the production of therapeutically interesting glycoproteins in glyco-engineered bacteria. This review offers an introduction to the many efforts taken to engineer bacteria in order to produce N-glycoproteins with defined eukaryotic glycan structures, completely novel protein glycoconjugates as well as to establish screening approaches for improvement and adaptation of the glycosylation machinery to specific applications.

Endohexosaminidase-catalyzed synthesis of glycopeptides and proteins

The synthetic application of endohexosaminidase enzymes (e.g., Endo A, Endo M, Endo D) promises to allow ready access to a wide variety of defined homogeneous glycoproteins and glycopeptides. The use ofN-glycan oligosaccharides that are activated at the reducing terminus as oxazolines allows their high-yielding attachment to almost any amino acid, peptide, or protein that contains a GlcNAc residue as an acceptor. A wide variety of oxazoline donors are readily available, either by total synthesis or by isolation of the corresponding oligosaccharide from natural sources and then conversion to the oxazoline in water. The synthetic potential of the enzymes is particularly augmented by the production of mutant glycosynthases, the use of which allows the synthesis of a wide variety of glycopeptides and glycoproteins bearing defined homogeneousN-glycan structures.

Recent advances in glycotechnology for glycoconjugate synthesis using microbial endoglycosidases

Biotechnology associated with synthesis of glycopeptides and glycoproteins has recently advanced as glycotechnology. Studies toward glycotechonology include the artificial modification of sugar chains in glycoconjugates to improve their function because the physiological importance of sugar chains in living organisms is well recognized. Methods involving addition of oligosaccharides to peptides and proteins have attracted attention as efficient techniques in glycotechnology, especially those involving the transglycosylation activities of microbial endoglycosidases. The exploration of oligosaccharide oxazolines as donor substrates for the transglycosylation of endoglycosidases has significantly enhanced the efficiency of these processes. Moreover, discovery of novel endoglycosidase mutants with glycosynthase-like activity has made it possible to effectively synthesize large quantities of glycopeptides, as well as homogeneous glycoprotein. The use of mutant enzymes and oligosaccharide oxazolines has led to development of practical applications for the synthesis of bioactive glycopeptides and therapeutic glycoproteins as bio-medicines.

A two-step enzymatic glycosylation of polypeptides with complex N-glycans

A chemoenyzmatic method for direct glycosylation of polypeptides is described. The method consists of two site-specific enzymatic glycosylation steps: introduction of a glucose moiety at the consensus N-glycosylation sequence (NXS/T) in a polypeptide by an N-glycosyltransferase (NGT) and attachment of a complex N-glycan to the glucose primer by an endoglycosidase (ENGase)-catalyzed transglycosylation. Our experiments demonstrated that a relatively small excess of the UDP-Glc (the donor substrate) was sufficient for an effective glucosylation of polypeptides by the NGT, and different high-mannose and complex type N-glycans could be readily transferred to the glucose moiety by ENGases to provide full-size glycopeptides. The usefulness of the chemoenzymatic method was exemplified by an efficient synthesis of a complex glycoform of polypeptide C34, a potent HIV inhibitor derived from HIV-1 gp41. A comparative study indicated that the Glc-peptide was equally efficient as the natural GlcNAc-peptide to serve as an acceptor in the transglycosylation with sugar oxazoline as the donor substrate. Interestingly, the Glc-Asn linked glycopeptide was completely resistant to PNGase F digestion, in contrast to the GlcNAc-Asn linked natural glycopeptide that is an excellent substrate for hydrolysis. In addition, the Glc-Asn linked glycopeptide showed at least 10-fold lower hydrolytic activity toward Endo-M than the natural GlcNAc-Asn linked glycopeptide. The chemoenzymatic glycosylation method described here provides an efficient way to introducing complex N-glycans into polypeptides, for gain of novel properties that could be valuable for drug discovery.

Remarkable transglycosylation activity of glycosynthase mutants of endo-D, an endo-β-N-acetylglucosaminidase from Streptococcus pneumoniae

Endo-β-N-acetylglucosaminidase from Streptococcus pneumoniae (Endo-D) is an endoglycosidase capable of hydrolyzing the Fc N-glycan of intact IgG antibodies after sequential removal of the sialic acid, galactose, and internal GlcNAc residues in the N-glycan. Endo-D also possesses transglycosylation activity with sugar oxazoline as the donor substrate, but the transglycosylation yield is low due to enzymatic hydrolysis of the donor substrate and the product. We report here our study on the hydrolytic and transglycosylation activity of recombinant Endo-D and its selected mutants. We found that Endo-D preferred core-fucosylated N-glycan for hydrolysis but favored nonfucosylated GlcNAc acceptor for transglycosylation. Several mutants showed significantly enhanced transglycosylation efficiency over the wild type enzyme. Two mutants (N322Q and N322A) were identified as typical glycosynthases that demonstrated remarkable transglycosylation activity with only marginal or no product hydrolysis activity. Kinetic studies revealed that the N322Q [corrected]and N322A glycosynthases had much higher catalytic efficiency for glycosylating the nonfucosylated GlcNAc acceptor. In comparison, the N322Q was much more efficient than N322A for transglycosylation. However, N322Q and N322A [corrected] could not take more complex N-glycan oxazoline as substrate for transglycosylation, indicating their strict substrate specificity. The usefulness of the N322Q glycosynthase was exemplified by its application for efficient glycosylation remodeling of IgG-Fc domain.

Emerging technologies for making glycan-defined glycoproteins

Protein glycosylation is a common and complex posttranslational modification of proteins, which expands functional diversity while boosting structural heterogeneity. Glycoproteins, the end products of such a modification, are typically produced as mixtures of glycoforms possessing the same polypeptide backbone but differing in the site of glycosylation and/or in the structures of pendant glycans, from which single glycoforms are difficult to isolate. The urgent need for glycan-defined glycoproteins in both detailed structure-function relationship studies and therapeutic applications has stimulated an extensive interest in developing various methods for manipulating protein glycosylation. This review highlights emerging technologies that hold great promise in making a variety of glycan-defined glycoproteins, with a particular emphasis in the following three areas: specific glycoengineering of host biosynthetic pathways, in vitro chemoenzymatic glycosylation remodeling, and chemoselective and site-specific glycosylation of proteins.

Convergent synthesis of homogeneous Glc1Man9GlcNAc2-protein and derivatives as ligands of molecular chaperones in protein quality control

A detailed understanding of the molecular mechanism of chaperone-assisted protein quality control is often hampered by the lack of well-defined homogeneous glycoprotein probes. We describe here a highly convergent chemoenzymatic synthesis of the monoglucosylated glycoforms of bovine ribonuclease (RNase) as specific ligands of lectin-like chaperones calnexin (CNX) and calreticulin (CRT) that are known to recognize the monoglucosylated high-mannose oligosaccharide component of glycoproteins in protein folding. The synthesis of a selectively modified glycoform Gal(1)Glc(1)Man(9)GlcNAc(2)-RNase was accomplished by chemical synthesis of a large N-glycan oxazoline and its subsequent enzymatic ligation to GlcNAc-RNase under the catalysis of a glycosynthase. Selective removal of the terminal galactose by a β-galactosidase gave the Glc(1)Man(9)GlcNAc(2)-RNase glycoform in excellent yield. CD spectroscopic analysis and RNA-hydrolyzing assay indicated that the synthetic RNase glycoforms maintained essentially the same global conformations and were fully active as the natural bovine ribonuclease B. SPR binding studies revealed that the Glc(1)Man(9)GlcNAc(2)-RNase had high affinity to lectin CRT, while the synthetic Man(9)GlcNAc(2)-RNase glycoform and natural RNase B did not show CRT-binding activity. These results confirmed the essential role of the glucose moiety in the chaperone molecular recognition. Interestingly, the galactose-masked glycoform Gal(1)Glc(1)Man(9)GlcNAc(2)-RNase also showed significant affinity to lectin CRT, suggesting that a galactose β-1,4-linked to the key glucose moiety does not significantly block the lectin binding. These synthetic homogeneous glycoprotein probes should be valuable for a detailed mechanistic study on how molecular chaperones work in concert to distinguish between misfolded and folded glycoproteins in the protein quality control cycle.

Unusual transglycosylation activity of Flavobacterium meningosepticum endoglycosidases enables convergent chemoenzymatic synthesis of core fucosylated complex N-glycopeptides

Structurally well defined, homogeneous glycopeptides and glycoproteins are indispensable tools for functional glycomics studies. By screening of various endo-β-N-acetylglucosaminidases through the use of appropriate synthetic donor and acceptor substrates, we have found that the Flavobacterium meningosepticum endo-β-N-acetyl-glucosaminidases (GH family 18), including Endo-F2 and Endo-F3, were able to glycosylate α-1,6-fucosylated GlcNAc derivative to provide natural, core-fucosylated complex-type N-glycopeptides. The Endo-F2 and Endo-F3 were efficient for transferring both sialylated and asia-lylated glycans and were highly specific for an α-1,6-fucosylated GlcNAc-peptide as acceptor for transglycosylation, showing only marginal activity with non-fucosylated GlcNAc-peptides. In contrast, we found that the commonly used endoglycosidases such as Endo-A and Endo-M, which belong to GH family 85, were unable to take α-1,6-fucosyl-GlcNAc derivative as acceptors for transglycosylation. The novel activity of Endo-F2 and Endo-F3 was successfully applied for a highly convergent chemoenzymatic synthesis of a full-sized CD52 glycopeptide antigen carrying both terminal sialic acid and core fucose. This is the first report on endoglycosidases that are able to glycosylate α-1,6-fucosylated GlcNAc derivatives to form natural core-fucosylated glycopeptides.

The Amazing Transglycosylation Activity of Endo-β-N-acetylglucosaminidases

Major advances have been made in exploring the transglycosylation activity of endo-β-N-acetylglucosaminidases (ENGases) for synthetic purpose. The exploration of synthetic sugar oxazolines as donor substrates for the ENGase-catalyzed transglycosylation has expanded the substrate availability and significantly enhanced the overall transglycosylation efficiency. On the other hand, site-directed mutagenesis in combination with activity screening has led to the discovery of the first generation ENGase-based glycosynthases that can use highly active sugar oxazolines as substrates for transglycosylation but lack hydrolytic activity on the ground-state products. ENGases have shown amazing flexibility in transglycosylation and possess much broader substrate specificity than previously thought. Now the ENGase-based chemoenzymatic method has been extended to the synthesis of a range of complex carbohydrates, including homogeneous glycopeptides, glycoproteins carrying well-defined glycans, novel oligosaccharide clusters, unusually glycosylated natural products, and even polysaccharides. This article highlights recent advances related to ENGase-catalyzed transglycosylation with a focus on their synthetic potential.