Bacterial Glycosidases in Pathogenesis and Glycoengineering

Jonathan Sjögren; Mattias Collin

Disclosures

Future Microbiol. 2014;9(9):1039-1051. 

In This Article

Abstract and Introduction

Abstract

Glycosylation is a common post-translational protein modification and many key proteins of the immune system are glycosylated. As the true experts of our immune system, pathogenic bacteria produce enzymes that can modify the carbohydrates (glycans) of the defense mechanisms in order to favor bacterial survival and persistence. At the intersection between bacterial pathogenesis and glycobiology, there is an increased interest in studying the bacterial enzymes that modify the protein glycosylation of their colonized or infected hosts. This is of great importance in order to fully understand bacterial pathogenesis, but it also presents itself as a valuable source for glycoengineering and glycoanalysis tools. This article highlights the role of bacterial glycosidases during infections, introduces the use of such enzymes as glycoengineering tools and discusses the potential of further studies in this emerging field.

Introduction

In the microenvironment where pathogenic bacteria reside within their host, immune effector molecules, immune cells, mucins and epithelial cells are glycosylated to varying extents. The glycans play a fundamental role in many functions of the immune system, and it is therefore not surprising that these carbohydrates are targets for bacterial interactions and/or modifications. By studying the mechanisms by which pathogenic bacteria interact with host glycosylation, new insights can be gained into bacterial pathogenesis and into the importance of protein glycosylation in the immune system. Furthermore, discoveries of new glycan-hydrolyzing enzymes could serve as a source for novel glycoengineering and glycan analysis tools.

Glycans in the immune system play a variety of different roles: protecting from proteases, mediating protein interactions and contributing to protein stability.[1] In humans, protein glycosylation is present in two major forms: N-linked and O-linked glycosylation. The consensus sequence for N-linked glycosylation is Asn–X–Ser/Thr, where X can be any amino acid except proline. O-linked glycosylation involves Ser or Thr residues, but there is no consensus sequence. N-linked glycans can be high-mannose, hybrid- or complex-type structures, all carrying the core Man3N-acetylglucoasamines (GlcNAc)2 structure (Figure 1). The most common O-linked glycans are core 1–4, starting with an α-linked N-acetylgalactosamine (GalNAc) and additional galactose or GlcNAcs (Figure 1). Recent advances in glycan analysis have revealed that not only eukaryote organisms, but also bacteria, especially mucosal-associated bacteria, have protein glycosylation machineries.[2]

Figure 1.

Types of glycans attached to asparagine (N-linked) and the four most common core structures attached to serine or threoinin (O-linked).
GalNAc: N-acetylgalactosamine; GlcNAc: N-acetylglucoasamine.

The well-studied glycans of the antibody IgG are located at Asn-297 on each of the two heavy chains of the Fc region (Figure 2A). The glycans of IgG gives the Fc region its horseshoe-like structure and are of major importance for the antibody's interaction with the Fc receptors and for its communication with immune cells.[3] The impact of the IgG glycan is highlighted by the importance of the correct glycoform of the therapeutic monoclonal antibodies in which glycan engineering and selection are used to improve antibody-based therapeutics.[4] The various glycoforms present on the heavy chains of IgG give different Fc conformations and affect the binding affinities to the Fc receptors, leading to changes in the elicited effector functions. For example, it has been postulated that fully sialylated glycans on IgG increases the anti-inflammatory properties of IgG, while IgGs carrying agalactosylated (G0) glycans display increased antiviral activities.[5,6]

Figure 2.

Overview of bacterial glycosidases interacting with the Fc-glycan of IgG. (A) A model of IgG1 with the Fc-glycan depicted in yellow. The model was generated from a deposition in the Protein Data Bank by M Clark (Cambridge University, UK) using VMD 1.9.1. (B) A schematic picture of an N-linked glycan and examples of bacterial enzymes that cleave carbohydrate residues, along with three examples of enzymes in the transit from bacterial pathogenesis to glycoengineering applications.
Endo: Endoglycosidase; GlcNAc: N-acetylglucoasamine; mAb: Monoclonal antibody; PNGaseF: Peptide-N-glycosidase F; VMD: Visual molecular dynamics.

Enzymes that catalyze the hydrolysis of carbohydrates on N- and O-linked glycans are either exo- or endo-glycosidases. Exoglycosidases act on the terminal residue of the glycan and cleave one specific residue, whereas endoglycosidases have activity located within the glycan structure and can release several residues in one reaction. Peptide-N-glycosidase F (PNGaseF) is not a true glycosidase (rather, it is an amidase), but it is still a very important enzyme in this context, since it catalyzes the hydrolysis of the amide bond between the innermost GlcNAc and the asparagine residue, resulting in the release of the complete N-linked oligosaccharide from the glycosylation site on the protein. Details of the mechanisms of action of exo- and endo-glycosidases are outside the scope of this review, but can be found in a review by Bojarová and Kren.[7] Enzymes that catalyze the reaction of hydrolysis of the glycosidic bonds are called glycoside hydrolases (GHs) and are subdivided into families based on amino acid sequence similarities. Today, there are more than 130 families described in the Carbohydrate-Active Enzymes (CAZy) database.[8]

The first section of this article will provide examples of the involvement of bacterial glycosidases in bacterial pathogenesis through immunomodulation, adherence and acquirement of nutrients. The second section describes the applications for bacterial glycosidases in antibody glycan engineering and glycan analysis of glycoproteins. Following the concluding section, we present our future perspectives on bacterial glycosidases in infectious diseases and as tools for glycobiology research and drug development. We hope that this glimpse into the world of bacterial glycosidases will inspire further enquiries within this expanding field of study.

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