CHAPTER 20: GLYCOPROTEINS AND GLYCOLIPIDS

Glycoproteins: These macromolecules are found in most organisms, from bacteria and viruses to mammals. They are present, for example, in lysosomes, blood, mucus, the extracellular matrix, cell membranes, the nucleus and cytoplasm, and they are associated with several important physiologic processes. Many enzymes, structural proteins, polypeptide hormones, immunoglobulins, antigens, receptors, transport proteins, and blood group substances have covalently linked oligosaccharide (glycan) chains, thus establishing them as glycoproteins. In many cases, precise roles for their carbohydrate moieties are not clearly established. For example, a major problem in cancer is metastasis, where problematic cells leave their tissue of origin (e.g., the liver), and migrate through blood to some distant site (e.g., the lung or brain), growing there in an unregulated fashion. Reports indicate that alterations in the oligosaccharide chains of glycoproteins on the surface of metastatic cancer cells may contribute to this phenomenon. Another proposal regarding the significance of carbohydrate attachment to protein is that the oligosaccharide chains become the identifying chemical markers used to tag proteins that are destined to be utilized outside of the cell, or in the membranous network of the cell. Thus, most proteins that are retained for use in the cytoplasm of the cell are nonglycosylated.

Although about two-hundred monosaccharides are found in nature, only eight are commonly found in the oligosaccharide chains of glycoproteins. A widely held belief is that these chains encode considerable biologic information, depending upon their sequences. The carbohydrate content of glycoproteins can range from 1% to over 80% by weight, however, less than 100 monosaccharide residues are generally found associated with any one protein residue. This seemingly limited number of glycosidic bonds does not necessarily limit diversity, for these eight monosaccharides may be linked to each other to form several thousand different oligosaccharide chains.

Oligosaccharide chains are attached to protein through either N-glycosidic or O-glycosidic bonds. N-Glycosidic bonds are formed between the amide group of an asparagine (Asn) chain, and the anomeric carbon of N-acetylglucosamine (GlcNAc). The enzyme catalyzing formation of this bond recognizes Asn residues in the protein having the sequence Asn-X-Thr (threonine), or Asn-X-Ser (serine), where X represents any amino acid. Although some Asn residues with this sequence do not become glycosylated, this sequence is necessary for defining glycosylation sites.

Fucose (Fuc), which is sometimes found attached to the GlcNAc residues of glycoproteins, is one of the few monosaccharides of the L-configuration found in plants and animals.

O-Glycosidic bonds are formed between the side chain hydroxyl group of either Ser or Thr, and the anomeric carbon of either GalNAc or xylose (Xyl). There is a special type of O-glycosidic linkage found in collagen, where Gal is attached to the hydroxyl group of hydroxylysine. The carbohydrate attached to collagen is always the Gal-Glc disaccharide.

Lectins, which were first discovered in plants and microbes, are carbohydrate-binding proteins that agglutinate cells or precipitate glycoconjugates, and several are known to be glycoproteins. Some enzymes, toxins, and transport proteins can be classified as lectins if they have multiple binding sites. Animal lectins (or selectins such as the C-, S-, P-, and I-types), participate in cell-cell interactions that occur in pathologic conditions such as inflammation and cancer metastasis.

Mucins (or mucoproteins) are glycoproteins that are distributed on the surfaces of epithelial cells of the respiratory, gastrointestinal, and reproductive tracts, and help to lubricate and protect these surfaces. Those with 4% or more carbohydrate are sometimes termed mucoproteins because they exhibit a high viscosity.

Ovulated eggs are surrounded by an extracellular coat called the zona pellucida (ZP). O linked oligosaccharides containing a terminal Gal unit are attached to ZP3, a glycoprotein in this coat, and are recognized by a receptor on the sperm surface. Binding triggers release of proteases and hyaluronidases from the sperm, thus dissolving the ZP and allowing fertilization. Another example of the importance of the carbohydrate moiety of glycoproteins can be found in the interaction of lymphocytes and lymph nodes. Circulating lymphocytes tend to migrate toward their sites of origin. This return, or "homing", is known to be mediated by specific interactions between receptors on the surface of lymphocytes, and carbohydrates on the endothelial lining of lymph nodes. When sialic acid residues from the endothelial surface are enzymatically removed, adhesion of lymphocytes is prevented.

Several diseases involving abnormalities in the synthesis and/or degradation of glycoproteins are recognized. As previously indicated, cell surface glycoproteins of metastatic cancer cells can exhibit differences in the structures of their oligosaccharide chains. In one lysosomal disease, for example, lysosomal membrane proteins are not properly targeted to the lysosome because of an absence of their Man 6-P recognition signal owing to a genetically determined deficiency of GlcNAc phospohtransferase. Other relatively rare diseases are due to genetic deficiencies in the activities of specific glycoprotein lysosomal hydrolases. These include a- and b-mannosidosis, fucosidosis, sialidosis, aspartylglycosamminuria, and Schindler's disease. Glycoprotein abnormalities are also thought to be involved in several other disease processes, including influenza, AIDS, and rheumatoid arthritis.

Glycolipids: Glycolipids are carbohydrate-containing lipids with a sphingosine backbone. Also attached to the sphingosine moiety is a long-chain fatty acid unit. Glycosphingolipids typically account for about 5-10% of lipid in plasma membranes, and are important in intercellular communication.

Cerebrosides are perhaps the simplest glycolipids, in which a singular carbohydrate unit, either glucose (Glc) or galactose (Gal), appears. Without the carbohydrate unit, cerebrosides become ceramides. More complex glycolipids, such as gangliosides, are found to contain up to seven carbohydrate residues in a branched chain, and include NeuAc and GalNAc in addition to Glc and Gal.

Studies have shown that the carbohydrate residues of membrane glycolipids and glycoproteins are normally located on the exterior surface of cell membranes. This occurs because carbohydrates are hydrophilic, thus preferring the aqueous outside surface of plasma membranes over the more lipid-rich, hydrocarbon core. This orientation assists in maintaining the asymmetric nature of cell membranes, and also allows carbohydrate moieties of these macromolecules to function as cell attachment-recognition sites.

Certain diseases of animals are characterized by abnormal quantities of glycolipids in tissues, often the central nervous system. They may be generally classified into two groups: 1) true demyelinating diseases, and 2) lysosomal storage disorders (e.g., gangliosidoses, sphingolipidoses, and leukodystrophies).

In demyelinating disorders, there may be loss of both phospholipids and sphingolipids from neurons of the brain and spinal cord. Affected animals reportedly demonstrate progressive paraparesis and pelvic limb ataxia. The etiology of these conditions is not well established; however, a hereditary basis is suspected.

Ganglioside storage diseases are inherited disorders of lysosomal hydrolase enzymes, that result in accumulation of gangliosides and glycolipid substrates of these hydrolases within lysosomes of neurons and glia throughout the nervous system. These diseases are particularly evident in the brain and spinal cord, but also sometimes manifest themselves in peripheral nerves as well. Leukodystrophies are caused by a deficiency of b-galactocerebrosidase, and result in accumulation of galactocerebroside within the nervous system. Effective treatments for demyelinating disorders and lysosomal storage diseases in animals have not been developed, and therefore long-term prognosis is poor.

SUMMARY

Chapter 20 discusses the role of glycoproteins and glycolipids in various physiological processes. Glycoproteins are macromolecules found in organisms and are associated with important functions such as enzyme activity, hormone regulation, and cell signaling. The carbohydrate chains attached to glycoproteins may play a role in cancer metastasis and protein identification. Only eight common monosaccharides are found in glycoprotein chains, but they can be linked in various ways to create thousands of different chains. Glycoproteins can be attached to proteins through N-glycosidic or O-glycosidic bonds. Lectins, which are carbohydrate-binding proteins, and mucins, which are glycoproteins that protect and lubricate surfaces, are examples of glycoproteins. Glycolipids are carbohydrate-containing lipids that play a role in intercellular communication. They are found on the exterior surface of cell membranes and can be simple or complex, containing multiple carbohydrate residues. Abnormalities in glycoproteins and glycolipids are associated with diseases such as cancer, lysosomal storage disorders, and demyelinating disorders. Effective treatments for these diseases have not been developed.

OUTLINE

• Glycoproteins are macromolecules found in most organisms and are associated with important physiological processes
• Many proteins have covalently linked oligosaccharide chains, making them glycoproteins
• The precise roles of carbohydrate moieties in glycoproteins are not always clear, but they may contribute to processes like cancer metastasis and protein tagging
• Only eight commonly found monosaccharides are found in the oligosaccharide chains of glycoproteins
• Oligosaccharide chains are attached to proteins through N-glycosidic or O-glycosidic bonds
• Lectins are carbohydrate-binding proteins that can be classified as glycoproteins and play a role in cell-cell interactions
• Mucins are glycoproteins that help lubricate and protect surfaces of epithelial cells
• Glycoproteins are involved in processes like fertilization and lymphocyte adhesion
• Abnormalities in the synthesis and degradation of glycoproteins can lead to diseases like cancer, lysosomal diseases, and influenza
• Glycolipids are carbohydrate-containing lipids with a sphingosine backbone and are important in intercellular communication
• Glycolipids can be simple, like cerebrosides, or more complex, like gangliosides
• Carbohydrate residues of glycolipids and glycoproteins are located on the exterior surface of cell membranes
• Diseases characterized by abnormal quantities of glycolipids in tissues include demyelinating disorders and lysosomal storage disorders
• Effective treatments for these diseases in animals have not been developed, leading to a poor long-term prognosis.

QUESTIONS

Qcard 1:

Question: What are glycoproteins?

Answer: Glycoproteins are macromolecules found in most organisms that have covalently linked oligosaccharide chains. They are associated with several important physiological processes and can be found in lysosomes, blood, mucus, cell membranes, and more.

Qcard 2:

Question: What is the significance of carbohydrate attachment to proteins?

Answer: Carbohydrate attachment to proteins can serve as identifying chemical markers used to tag proteins that are destined to be utilized outside of the cell or in the membranous network of the cell. It is also believed that alterations in the oligosaccharide chains of glycoproteins can contribute to phenomena such as cancer metastasis.

Qcard 3:

Question: How are oligosaccharide chains attached to proteins?

Answer: Oligosaccharide chains can be attached to proteins through either N-glycosidic or O-glycosidic bonds. N-glycosidic bonds are formed between the amide group of an asparagine chain and the anomeric carbon of N-acetylglucosamine. O-glycosidic bonds are formed between the side chain hydroxyl group of either serine or threonine and the anomeric carbon of either GalNAc or xylose.

Qcard 4:

Question: What are lectins?

Answer: Lectins are carbohydrate-binding proteins that agglutinate cells or precipitate glycoconjugates. They can be classified as glycoproteins and are involved in cell-cell interactions that occur in pathologic conditions such as inflammation and cancer metastasis.

Qcard 5:

Question: What are glycolipids?

Answer: Glycolipids are carbohydrate-containing lipids with a sphingosine backbone. They are important in intercellular communication and can be found in plasma membranes. Cerebrosides are the simplest glycolipids, while gangliosides are more complex and contain multiple carbohydrate residues.

Qcard 6:

Question: Where are the carbohydrate residues of membrane glycolipids and glycoproteins located?

Answer: The carbohydrate residues of membrane glycolipids and glycoproteins are normally located on the exterior surface of cell membranes. This helps maintain the asymmetric nature of cell membranes and allows the carbohydrate moieties to function as cell attachment-recognition sites.

Mind Map: Chapter 20: Glycoproteins and Glycolipids

Central Idea: Glycoproteins and glycolipids are important macromolecules found in organisms, with various roles in physiological processes.

Glycoproteins

• Found in most organisms
• Present in lysosomes, blood, mucus, extracellular matrix, cell membranes, nucleus, and cytoplasm
• Associated with important physiological processes
• Examples: enzymes, structural proteins, polypeptide hormones, immunoglobulins, antigens, receptors, transport proteins, blood group substances

Roles of Carbohydrate Moieties in Glycoproteins

• Role in cancer metastasis
• Role in identifying proteins for utilization outside the cell or in the membranous network of the cell
• Nonglycosylated proteins are retained for use in the cytoplasm of the cell

Carbohydrate Content and Diversity

• Only eight common monosaccharides in oligosaccharide chains
• Carbohydrate content ranges from 1% to over 80% by weight
• Less than 100 monosaccharide residues generally associated with one protein residue
• Eight monosaccharides can form several thousand different oligosaccharide chains

Types of Glycosidic Bonds

• N-Glycosidic bonds: formed between asparagine (Asn) and N-acetylglucosamine (GlcNAc)
• O-Glycosidic bonds: formed between serine (Ser) or threonine (Thr) and GalNAc or xylose (Xyl)
• Special O-glycosidic linkage found in collagen: Gal attached to hydroxylysine

Lectins and Mucins

• Lectins: carbohydrate-binding proteins that agglutinate cells or precipitate glycoconjugates
• Animal lectins (selectins) participate in cell-cell interactions in inflammation and cancer metastasis
• Mucins: glycoproteins on epithelial cell surfaces, lubricate and protect surfaces

Importance of Carbohydrate Moiety in Glycoproteins

• Zona pellucida and fertilization
• Lymphocyte interaction with lymph nodes
• Removal of sialic acid residues prevents adhesion of lymphocytes

Diseases involving Glycoprotein Abnormalities

• Altered glycoproteins on metastasis

Study Plan: Chapter 20: Glycoproteins and Glycolipids

Day 1: Introduction and Overview

• Read the chapter introduction to understand the importance of glycoproteins and glycolipids in various biological processes.
• Focus on the roles of glycoproteins in physiologic processes and their association with diseases like cancer metastasis.
• Take notes on the different types of glycoproteins and their carbohydrate moieties.
• Understand the significance of glycosidic bonds and the attachment of oligosaccharide chains to proteins.
• Study the role of lectins, mucins, and glycoproteins in cell-cell interactions and protection of epithelial surfaces.

Day 2: Glycoproteins and Oligosaccharide Chains

• Review the eight commonly found monosaccharides in the oligosaccharide chains of glycoproteins.
• Understand the concept of encoding biologic information in the sequences of oligosaccharide chains.
• Learn about N-glycosidic and O-glycosidic bonds and their formation with specific amino acid residues.
• Focus on the glycosylation sites and the importance of glycosylation in defining protein function.
• Study the role of fucose and its attachment to GlcNAc residues in glycoproteins.

Day 3: Specific Examples and Functions

• Explore specific examples of glycoproteins and their carbohydrate moieties, such as ZP3 in the zona pellucida and lymphocyte receptors.
• Understand the importance of carbohydrate moieties in fertilization, lymphocyte homing, and adhesion.
• Learn about diseases associated with abnormalities in glycoprotein synthesis and degradation.
• Focus on the role of glycoproteins in diseases like metastatic cancer, lysosomal diseases, and other disease processes like influenza, AIDS, and rheumatoid arthritis.

Day 4: Glycolipids and Intercellular Communication

• Begin studying the section on glycolipids and their role in intercellular communication.
• Understand the structure of glycolipids with a sphingosine backbone and long-chain fatty acid units.
• Focus on the different types of glycolipids, including cerebrosides and gangliosides.
• Learn about the location of carbohydrate residues on the exterior surface of cell membranes and their function as cell attachment-recognition sites.
• Explore diseases characterized by abnormal