Glycans interact with many types of proteins including enzymes, antibodies, and lectins. Protein recognition of glycans represents a major way in which the information contained in glycan structures is deciphered and promotes biological activities. This chapter describes approaches to study the kinetics and thermodynamics of interactions between glycans and glycan-binding proteins (GBPs).
Fungi are a fascinating group of predominantly multicellular organisms. Fungal species, such as Saccharomyces cerevisiae, have been instrumental in defining the fundamental processes of glycosylation, but their glycobiology is significantly different from animal or plant systems. This chapter describes the glycan structures that compose the fungal cell wall, offers some insights into novel glycobiology revealed through studying fungal systems, addresses the use of fungi as experimental and synthetic systems, and delineates the relationships of several important glycoconjugates to fungal biology and pathogenesis.
Parasitic protozoans and helminths (worms) synthesize glycans with structures often different from those typically found in vertebrates and are typically antigenic. Parasites also express glycan-binding proteins (GBPs) involved in host invasion and parasitism. As part of the disease process, parasite glycans can trigger the host's innate immune system, which can lead to the induction of adaptive immune responses. This chapter discusses the major roles of glycoconjugates in parasitic infections.
Galectins are among the most widely expressed class of lectins in all organisms. They typically bind β-galactose-containing glycoconjugates and share primary structural homology in their carbohydrate-recognition domains (CRDs). Galectins have many biological functions, including roles in development, regulation of immune cell activities, and microbial recognition as part of the innate immune system. This chapter describes the diversity of the galectin family and presents an overview of what is known about their biosynthesis, secretion, and biological roles.
This chapter describes the variable components of N-glycans, O-glycans, and glycolipids attached to the core of each glycan class and presented in Chapters 9, 10, and 11. The glycan extensions of these cores form the mature glycan and may include human blood group determinants. The terminal sugars of the mature glycan often regulate the function(s) or recognition properties of a glycoconjugate. Also discussed are milk oligosaccharides, that carry many of the same extensions on a lactose core.
C-type lectins (CTLs) are Ca++-dependent glycan-binding proteins (GBPs) that share primary and secondary structural homology in their carbohydrate-recognition domains (CRDs). The CRD of CTLs is more generally defined as the CTL domain (CTLD), because not all proteins with this domain bind either glycans or Ca++. CTLs include collectins, selectins, endocytic receptors, and proteoglycans, some of which are secreted and others are transmembrane proteins. They often oligomerize, which increases their avidity for multivalent ligands. CTLs differ significantly in the types of glycans that they recognize with high affinity. These proteins function as adhesion and signaling receptors in many pathways, including homeostasis and innate immunity, and are crucial in inflammatory responses and leukocyte and platelet trafficking.
The R-type lectins are members of a superfamily of proteins that contain a carbohydrate-recognition domain (CRD) that is structurally similar to the one in ricin. Ricin is considered the first lectin to be discovered, and it is thus the prototypical lectin in this category. R-type lectins are present in plants, animals, and bacteria, and the lectin domain in some cases is associated with a separate subunit that is a potent toxin. The structure–function relationships of this group of proteins are discussed in this chapter.
Several classes of successful commercial products are based on isolated or synthetic glycans. This chapter summarizes the use of glycans as vaccines and therapeutics. Applications of glycan mimics as drugs are also discussed.
Antibodies, lectins, microbial adhesins, viral agglutinins, and other proteins with carbohydrate-binding modules, collectively termed glycan-recognizing probes (GRPs), are widely used in glycan analysis because their specificities enable them to discriminate among a diverse variety of glycan structures. The native multivalency of many of these molecules promotes high-affinity avidity binding to the glycans and cell surfaces containing those glycans. This chapter describes the variety of commonly used GRPs, the types of analyses to which they may be applied, and cautionary principles that affect their optimal use.
This chapter focuses on the nematode (roundworm) Caenorhabditis elegans as an example of the phylum Nematoda. C. elegans provides a powerful genetic system for studying glycans during embryological development and in primitive organ systems.
The L-type lectins occur in the seeds of leguminous plants, and they have structural motifs that are present in a variety of glycan-binding proteins (GBPs) from other eukaryotic organisms. The structures of many of these lectins have been characterized, and many L-type lectins are used in a wide range of biomedical and analytical procedures. This chapter discusses the structure–function relationships of these lectins and the various biological roles they have in different organisms.
N-Glycans affect glycoprotein folding because of their hydrophilic nature. In the endoplasmic reticulum (ER), the processing of N-glycans yields a series of truncated N-glycans that serve as checkpoints that dictate the life or death of many newly made membrane and secreted proteins. Other glycan modifications also may affect glycoprotein folding in the ER. This chapter describes glycan-mediated quality-control processes in the ER and Golgi apparatus and what happens to glycoproteins that fail their “final folding examination.”