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Glycerides (glycerol-containing lipids)
Edited by Patricia Kuwabara. Last revised October 17, 2006. Published December 18, 2006. This chapter should be cited as: Berninsone, P.M. Carbohydrates and glycosylation (December 18, 2006), , ed. The Research Community, WormBook, doi/10.1895/wormbook.1.125.1, .
Recently developed methods have allowed the identification of 304 proteins containing N-glycans and the simultaneous determination of the glycosylation sites (; ; ). These studies employed lectin affinity chromatography to isolate glycopeptides generated by tryptic digestion of protein fractions, followed by a variety of mass spectrometry approaches. All but four of the N-glycosylation sites identified correspond to the conventional Asn-X-Ser/Thr. Three peptides contain Asn-X-Cys (; ). The identified N-glycoproteins comprise soluble and hydrophobic proteins, and many of them are extracellular matrix components which have been implicated in cell adhesion or are components of basement membranes. Among them, , (the two integrin subunits), (the integrin subunit), , , , (the four laminin subunit gene products), (nidogen) and (dystroglycan) contain at least one N-linked glycan [see ; for complete lists].
Complex lipids (lipoproteins, glycolipids)
, a member of the ADAM (a disintegrin and metalloprotease) family, regulates directional migration of the distal tip cell (DTC) (). Sensitivity to N-glycanase indicated that is N-glycosylated (). A mutation in causes defective DTC migration and genetically interacts with . is a membrane-bound nucleoside diphosphatase (NDPase) required for glycosylation and proper localization of (). NDPase activities in the lumen of the Golgi apparatus generate nucleoside monophosphates required for translocation of nucleotide sugars from the cytosol into the Golgi lumen (). Nucleotide sugars are the activated donors for glycosylation reactions in the lumen of the endoplasmic reticulum and Golgi apparatus (). Genetic studies in showed that guanosine diphosphatase, an NDPase, is required for protein and sphingolipid glycosylation in the Golgi lumen (). Hence, likely affects DTC migration through its role in the transport of nucleotide sugars into the lumen of the Golgi apparatus. Although the mutation may affect the glycosylation of other proteins, appears to regulate DTC migration through glycosylation and correct localization of .
In individual studies, some proteins have been demonstrated to contain N-linked oligosaccharides. , a member of the /Notch family of receptor proteins essential for distal tip cell (DTC) control of germline proliferation () is a glycoprotein ().
- orcytosolic and glycoproteins- have.
O-linked GlcNAc at Ser and Thr residues is an evolutionary conserved modification emerging as a key regulator of nuclear and cytoplasmic protein activity (). The GlcNAc transferase (OGT) responsible for this modification was the first glycopeptide-forming enzyme to be localized outside the secretory apparatus (). O-GlcNAc addition may compete with O-phosphorylation for certain Ser/Thr target sites, suggesting a potential regulatory cycle in which cytosolic -N-acetylglucosaminidase plays a key role (). O-GlcNAc addition is driven in part by the levels of UDP-GlcNAc derived from the hexosamine biosynthetic pathway, a nutrient-sensing pathway implicated in cellular signaling (). In mammals, the OGT gene is essential for embryonic and stem cell development and produces multiple transcripts (; ; ). O-GlcNAc addition has been implicated in mammalian insulin resistance (; ; ; ) histone remodeling, transcription, proliferation, apoptosis and proteosomal degradation.
An obvious function of the extracellular mucins, which form an important component of the mucous layer covering some epithelial cells (eg, lining the gastrointestinal and respiratory tracts), is protection of the underlying epithelium from insult. The O-glycans play a major role in this protective function, not only in the formation of large oligomers, which makes the mucous layer viscous, but also in serving as receptors for invading microorganisms. The membrane-associated MUC1 mucin can also have a protective function but, like the selectin ligands, can also affect cell-cell adhesion.
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The primary function of the glycerides is energy storage.
Proteoglycans are composed of glycosaminoglycan chains (GAGs) bound to serine residues in a protein core, via a xylose-galactose-galactose bridge. Unlike mucin-type O-glycosylation, threonine residues do not seem to act as acceptors. All GAGs (chondroitin sulfate, dermatan sulfate, heparin and heparan sulfate) except hyaluronic acid are secreted as components of proteoglycans. The post-translational processing of the core protein occurs in the Golgi apparatus (19), where, after chain initiation, monosaccharides are added stepwise from the appropriate UDP-sugars. To attain their final shape, chains are then carried through a series of modifications, including sulfation (see O-Linked Oligosaccharides). There has been more progress in isolating the genes coding for and defining the core proteins of proteoglycans than in isolating the genes coding for the enzymes involved in the biosynthesis of the carbohydrate moieties added to these proteins. The GAGs are large, and where many chains are added there may be only 10% protein in the proteoglycan.
Glycerides can be subdivided into two categories.
Emphasizing the importance of cell phenotype in defining profiles of glycosylation, the changes in levels of enzymes in malignancy differ in various cancers. In leukemias, the core 2 enzyme can be increased, whereas it may be decreased in breast cancers (18). In breast cancer, there is also an increase in the a2,3 sialyl-transferase that adds sialic acid to core 1, resulting in an increase in the major epitope for sialoadhesin expressed on macrophages. In the colon, where the O-glycans added to the mucins are very large and complex, changes occur in carcinomas that result in increased expression of the sialylated Le a and Lex structures—the ligands for P and E selectins, respectively.
The first group, theneutral glycerides are nonionic and nonpolar.
The initial, rate-limiting step in the synthesis of GAGs is the transfer of xylose from UDP-xylose to a serine residue. This step is catalyzed by UDP-D-xylose: proteoglycan core protein b-D-xylosyl transferase (xyloseT). Attempts to define a sequence in the core protein that will determine whether xylose will be added suggest that, although some limitations can be placed on the sequence flanking the serine (glycine must follow toward the carboxyl end), protein secondary structure may play an important role (20). The linking galactose moieties are then added, and the type of GAG to be synthesized is determined by the first hexosamine to be added, which begins the biosynthesis of the main chain.
The most prevalent and most importantare the triglycerides.
Changes in the expression of the core 2 enzyme have been characterized both in differentiation of T cells and in cancer. Leukosialin is a major glycoprotein expressed on leukocytes, and changes in its composition of added O-glycans occur on activation of human T cells. In resting cells, the dominant O-glycan is a tetrasaccharide (disialylated core 1), whereas in activated T cells the expression of the core 2 enzyme is increased and core 2-based structures are added (17). A similar increase in the activity of the core 2 enzyme is seen in leukocytes from patients with immunodeficiency, as such that seen in Wiscott-Aldrich syndrome (9). Furthermore, the core 2 enzyme is differentially expressed in the thymus, where expression is high in the subcapsular and cortical thymocytes and low in the medullary thymocytes. In this case, the differential expression of core 2 structures correlates with the ability of the cells to interact with a lectin (galectin) synthesized by thymic epithelial cells.
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