For many years glycoproteins have been a subject of interest. However, it is in the second half of this century that they have aroused the interest of biochemists and biologists from a wide range of fields. This increased interest is partly due to the fact that glycoproteins were discovered to be abundant in living organisms. It is also due to the diverse functions of glycoproteins; glycoproteins appear in nearly every biological process studied.
Many glycoproteins have structural functions. One of many instances is their role as a constituent of the cell wall. Glycoproteins also form connective tissues such as collagen. They are also found in gastrointestinal mucus secretions. Glycoproteins are used as protective agents and lubricants. They are also found abundantly in the blood plasma where they serve many functions.
The diverse function of glycoproteins is a direct result of their structure. These macromolecules are composed of a peptide chain with one or more carbohydrate moieties. There are two broad categories of glycoprotein structure. The carbohydrates are either linked N-glycosidically or O-glycosidically to their constituent protein. Within these broader categories, there can be fine structural differences which account for the large diversity of functions among glycoproteins.
Controlling of glycoproteins is achieved through synthesis and degradation. Those processes are controlled by very specific enzymes. Not so much is yet researched on glycoprotein regulation in general.
Structurally, glycoproteins consist of a polypeptide covalently bonded to a carbohydrate moiety. The carbohydrate can make up anywhere from less than one percent to more than 80 percent of the total protein mass. The saccharide chains, referred to as glycans, can be linked to the polypeptide in two major ways. The first class of glycoproteins are the O-linked glycans. These usually contain an N-acetylgalactosamine which is attached through a glycosidic bond to the O-terminus of either threonine or serine. The other class of glycoproteins are the N-linked glycans. These involve a glycosidic bond between N-acetylglucosamine and the N-terminus of an asparagine residue (Mathews 291 and Schulz 228-230).
As stated above, O-linked glycans consist of N-acetylgalactosamine attached to the O-terminus of a threonine or serine residue. N-acetylgalactosamine is simply a galactose molecule with an amine group covalently bonded to the second carbon. This amine group is bonded to a carboxyl group. N-acetylgalactosamine attaches to the carboxyl group of the amino acid through the hydroxyl group of its anomeric carbon. Another type of O-linked glycan consists of a galactose or a glucosyl-galactose disaccharide linked to the hydroxyl of hydroxylysine. Yet another type of O-linkage involves the binding of arabinose to the hydroxyl of hydroxyproline. In all of the 0-linked glycans, there can be a variety of different monosaccharide or polysaccharide chains attached to the sugar that is bonded to the amino acid (Mathews 293 and Lennarz 6-10).
The other class of glycoproteins are N-linked glycans. These molecules consist of an N-acetylglucosamine bonded to the amide nitrogen of an asparagine molecule. N-acetylglucosamine is simply a glucose molecule which is bonded to an amine group. This amine group, in turn, is bonded to a hydroxyl group. The N-acetylglucosamine is bonded to the asparagine through its anomeric carbon. The asparagine must be surrounded by a specific amino acid sequence, or sequon. This sequence is -X-Asn-X-Thr; the X can be any amino acid. A large variety of polysaccharide side chains can be linked to the N-acetylglucosamine. A typical polysaccharide chain is Man2 a(1-6)-Man B(1-4)-GlcNAc B(1-4)-GlcNAc B(1-N) Asn. Adding on to this structure can create many different N-linked glycans (Mathews 293 and Lennarz 10-11).
The carbohydrate chains of glycoproteins can play a role in the structure of the polypeptide. For example, in human immunoglobulins, the carbohydrate chain wraps around one of the protein domains. By doing so it prevents contact of this domain with the neighboring domain. An experiment that was done by Koide et al illustrates how the carbohydrate can change the overall structure of the polypeptide. The carbohydrate side chains of a rabbit antibody were removed through glycosidase digestion. The result was that the domain where the carbohydrate had previously been attached could no longer perform its ordinary function. Because an immunoglobulin's function is determined to a large extent by its structure, it can be concluded that removing the carbohydrate affected the structure of the molecule (Lennarz 25-27).
Because carbohydrates and proteins by themselves serve in a vast number of biological functions, it should not be surprising that linking the two together results in a macromolecule with an extremely large number of functions. Because of this and their biologically ubiquitous nature, the best way to go about exploring glycoprotein function is to break it down into categories that are fairly general. The following is an attempt to do this.
Structural: Glycoproteins are found throughout matrices. They act as receptors on cell surfaces that bring other cells and proteins (collagen) together giving strength and support to a matrix (Ivatt).
Proteoglycan-linking glycoproteins cross links proteoglycan molecules and is involved in the formation of the ordered structure within cartilage tissue. In nerve tissue glycoproteins are abundant in gray matter and appear to be associated with synaptosomes, axons, and microsomes. Prothrombin, thrombin, and fibrinogen are all glycoproteins that play an intricate role in the blood clotting mechanism (Gottschalk). In certain bacteria the slime layer that surrounds the outermost components of cell walls are made up of glycoproteins of high molecular weight. In addition to forming these s-layers, glycoproteins also function as bacterial flagella. These are made up of bundles of glycoproteins protruding from the cell's surface. Their rotation provides propulsion. In plants, glycoproteins have roles in cell wall formation, tissue differentiation, embryogenesis, and sexual adhesion (certain algal species) (Montreuil).
Protection: High molecular weight polymers called mucins are found on internal epithelial surfaces. They form a highly viscous gel that protects epithelium form chemical, physical, and microbial disturbances. Examples of mucin sites are the human digestive tract, urinary tract, and respiratory tracts. "Cervical mucin" is a glycoprotein found in the cervix of animals that regulates access of spermatozoa to the upper reproductive tract. Recently it was discovered that mucins may be responsible for aiding in metastasis of transformed cancer cells (Ivatt). Mucins are also found on the outer body surfaces of fish to protect the skin (Gottschalk). Not only does mucin serve the function of protection, but it also acts as a lubricant. Human lacrimal glands produce a glycoprotein which protects the corneal epithelium from desiccation and foreign particles. Human sweat glands secrete glycoproteins which protect the skin from the other excretory products that could harm the skin (Gottschalk).
Reproduction: Glycoproteins found on the surface of spermatozoa appear to increase a sperm cell's attraction for the egg by altering the electrophoretic mobility of the plasma membrane. Actual binding of the sperm cell to the egg is mediated by )-linked glycoproteins serving as receptors on the surface of each the two membranes. The zona pellucida is an envelope made of glycoprotein that surrounds the egg and prevents polyspermy from occurring after the first sperm cell has penetrated the egg's plasma membrane (Ivatt). Hen ovalbumin is a glycoprotein found in egg white that serves as a food storage unit for the embryo (Mathews).
Adhesion: Glycoproteins serve to adhere cells to cells and cells to substratum. Cell-cell adhesion is the basis for the development of functional tissues in the body. The interactions between cells is mediated by the glycoproteins on those cell's surfaces. In different domains of the body, different glycoproteins act to unite cells. For example, nerve cells recognize and bind to one another via the glycoprotein N-CAM (nerve cell adhesion molecule). N-CAM is also found on muscle cells indicating a role in the formation of myoneural junctions. With cell-substratum adhesion, glycoproteins serve as cell surface receptors for certain adhesion ligands that mediate and coordinate the interaction of cells. Substrates with the appropriate receptor will bind to the cell related to that receptor. For example, a substrate containing the glycoprotein fibronectin will be recognized and adhered to by fibroblasts. The fibroblasts will then secrete adhesion molecules and continue to spread, producing a pericellular matrix (Ivatt).
Hormones: There are many glycoproteins that function as hormones such as human chorionic gonadotropin (HCG) which is present in human pregnancy urine. Another example is erythropoietin which regulates erythrocyte production (Gottschalk).
Enzymes: Glycoprotein enzymes are of three types. These are oxidoreductases, transferases, and hydrolases(Gottschalk).
Carriers: Glycoproteins can bind to certain molecules and serve as vehicles of transport. They can bind to vitamins, hormones, cations, and other substances.
Inhibitors: Many glycoproteins in blood plasma have shown antiproteolytic activity. For example, the glycoprotein a1-antichymotrypsin inhibits chymotrypsin.
Defense: In beetles pygidial glands secrete a glycoprotein disinfecting paste that covers the body and hardens. This shell provides protection against attack by bacteria and fungi (Gottschalk).
Freezing-point depression: Glycoproteins were found in the sera of antarctic fishes to decrease the freezing point due to their apparent interaction with water(Gottschalk).
Vision: In bovine visual pigment a glycoprotein forms the outer membranes of retinal rods (Gottschalk).
Immunological: The interaction of blood group substances with antibodies is determined by the glycoproteins on erythrocytes. Adding or removing just one monosaccharide from a blood group structure, the antigenicity and therefore a person's blood type can be altered (Ivatt). Many immunoglobulins are actually glycoproteins (Gottschalk). Soluble immune mediators such as helper, suppressor, and activator cell have been shown to bind to glycoproteins found on the surface of their target cells. B and T cells contain surface glycoproteins that attract bacteria to these sites and bind them. In much the same manner, glycoproteins can direct phagocytosis. Because the HIV virus recognizes the receptor protein CD4, it binds to helper T cells which contain it. (Montreuil, et al).
Regulation and control of glycoproteins is not as straight forward as some might think. "If someone was to understand regulation in glycoproteins he would have to look at the enzymes that are involved in the biosynthesis pathway of these molecules; what is affecting the enzyme activity (could be hormonal). The big interest in this area is in the cloning of the genes, the cloning of the enzymes that are involved in the synthesis of the oligosaccharide on the glycoproteins. Unfortunately, not that much is known on the above subject" Dr. Roger Bretthauer
On the other hand, control of glycoproteins can be seen through biosynthesis and degradation. Here, the biosynthesis and degradation of the N-linked glycoproteins will be emphasized.
The protein part of the glycoprotein is formed at the ribosomes, where all proteins are synthesized, on template represented by RNA and DNA. As a result, its structure can change only through the mutation of the genetic material of the cell. On the other hand, the carbohydrate component of a glycoprotein is not a product of the ribosome; it is synthesized somewhere in the cytoplasm, but the exact site of synthesis has not yet been established. Since it is not directly genetically controlled, the oligosaccharide part shows a much greater variation (Scientific American).
In contrast with O-linked glycoproteins, where oligosaccharide assemble occurs on the polypeptide chain, N-linked glycoproteins assemble their oligosaccharide portions on a lipid linked intermediate, dolichol phosphate. The first step in oligosaccharide synthesis is the formation of that intermediate. In subsequent steps, sugars, the first being always N-acetylglucosamine (GlcNAc), are chain-like connected to dolichol phosphate. One more GlcNAc and three more mannose sugars are linked to the first GlcNAc to form the typical core of N-linked glycoproteins. Addition of any other sugar can be in any possible combination, according to the desired resulting functions. The specificity of the enzymes is very important in the synthesis process. Every sugar added, is catalyzed by a different enzyme. These group of enzymes are called glycosyltransferases (example Name). The first addition of GlcNAc to dolichol phosphate is catalyzed by the specific enzyme example) that cleave peptide bonds, and glycosidases (e.g example). enzymes that remove sugars one at a time from the end of an oligosaccharide chain. Both of these groups are contained in lysozomes. The lysosome attaches to a phagocyte, which has engulfed a substance that needs to be broken down, and releases its enzymes in it(Gottschalk). Next, these enzymes begin their catalytic action. In degradation, ad in synthesis, the enzymes involved are very specific. After the glycoprotein is broken down, its amino acid and sugar components are either metabolized or can be used in the formation of another glycoprotein. Enzymatic degradation can provide much information about the structure of the oligosaccharide chains, as well as about the carbohydrate peptide linkage. For example, if a glycoprotein is treated with mannosidase (removes mannose), and mannose is released, one can conclude that mannose residues were located at the periphery of the molecule since glycosidases remove sugars from the end of the oligosaccharide chain (Scientific American).
Regulation and control is the organism's ultimate tool to monitor and adjust the production or degradation of different molecules. Like a factory, it would be useless and inefficient to produce excess products when there is no need for them. On the other hand, shortage of products could be a problem too. Hopefully, further research will put some light in the area of glycoprotein regulation.
For a more detailed analysis on the biosynthesis and regulation of glycoproteins, the reader is referred to one of the journal articles in the bibliography.
Evans, Ronald M. "The steroid and Thyroid Hormone Receptor Superfamily." Science. 40 (1988): 889-894.
Gottschalk, Alfred. Glycoproteins. Their Composition, Structure, and function. Elsevier Publishing Company: New York, 1972.
Herrera H., and E.M Rodriguez. "Secretory Glycoproteins of the Rat Subcommisoral Organ Are N-Linked Complex-Type Glycoproteins. Demonstration by Combined Use of Lectins and Specific Glycosidases, and by the Administration of Tunicamycin. Histochemistry. 93 (1990): 607-615.
Herscovics Annete, and Peter Orlean. "Glycoprotein Biosynthesis in Yeast." FASEB Journal 7 (1993): 540-550.
Ivatt, Raymond J. The Biology of Glycoproteins. Plenum Press: New York, 1984. Kornfeld Rosaline, and Stuart Kornfeld. "Assembly of Asparagine-Linked Oligosaccharide." Annual Review of Biochemistry 54 (1985): 631-664.
Langan Thomas J., and Mary C. Slater. "Isoprenoids and Astroglial Cell Cycling: Diminished Mevalonate Availability and Inhibition of Dolichol-Linked Glycoprotein Synthesis Arrest Cycling Through Distinct Mechanisms". Journal of Cellular Physiology 149 (1991): 284- 292.
Lennarz, William J. The Biochemistry of Glycoproteins and Proteoglycans. Plenum Press: New York, 1980.
Mathews, Christopher K., and K.E. van Holde. Biochemistry. The Benjamin/Cummings Publishing Company, Inc.: Redwood City, 1990.
Montreuil, J., Vliegenthart, J.F.G., and Schachter, H. Glycoproteins. Elsevier: Amsterdam, 1995. Qiu Zhiyong, and Frank Tufaro and Shirley Gillam. "Brefeldin A and Monensin Arrest Cell Surface Expression of Membrane Glycoproteins and Release of Rubella Virus". Journal of General Virology 76 (1995): 885-863.
Sharon, Nathan. Glycoproteins." Scientific American. May (1974): 78-86.
Schulz, Georg E. And R.H. Schirmer. Principles of Protein Structure. Springer-Verlag: New York, 1979.
Zhu Xiaying, and Yucheng Zeng, and Mark A. Lehrman. "Evidence That the Hamster Tunicamycin Resistence Gene Encodes UDP-GlcNac:Dolichol Phosphate N- Acetylglucosamine-1-phosphate Transferase". The Journal of Biological Chemistry 267 (1992): 8895-8902.
Glycoproteins play an important part in hormone function. The action of hormones depends on the initial binding of the hormone to a protein receptor molecule. In many cases this molecule is a glycoprotein. Many hormones bind to receptors in the cell membrane; these hormones never actually enter the cell. Steroids, on the other hand, bind to an intracellular protein receptor. There is still controversy over whether the steroid hormone receptor is found in the nucleus or the cytosol. However, it is clear that after the steroid binds, the hormone-receptor complex moves to the nucleus. The hormone binding domain of the receptor is found at the C-terminus. The amino acid sequence of this region is highly diverse; it is not conserved from one protein to the next. Binding of the hormone stimulates a conformational change in the hormone receptor. This change allows the hormone-receptor complex to bind to the DNA. The DNA binding domain is highly conserved and is found within the central core of the protein. The central core contains a very basic amino acid sequence. In the cellular environment, these bases will tend to pick up a hydrogen and become positively charged. The positive charge will attract the hormone-receptor complex to the negatively charged DNA. Binding of the complex to the DNA stimulates transcription (Evans 890-891 and Mathews 808-809).
Cytochromes have features that are quite similar to transferrins, a particular class of glycoproteins. First of all, cytochromes make up some of the integral proteins found in membranes, as do glycoproteins. Second of all, the main function of cytochromes is the binding of ion cations. Via oxidoreduction with these cations of 2+state, cytochromes bind iron to a porphyrin prosthetic group, forming a heme group (Mathews). The function is to transport cations in and out of the cell. Transferrin is an iron binding protein as well. It interacts with iron cations of the 2+state which also bind to a porphyrin group, Protoporphyrin IX, to form a heme group. Transferrins carry ions to many different locations in the body including in and out of cells (Gottschalk). Another group of molecules that serve a function related to glycoproteins are the metalloenzymes. These enzymes, as their name implies, bind to metal ions via the same type of heme groups just discussed (Gottschalk). One class of glycoprotein enzymes, the oxidoreductases, contain an enzyme called chloroperoxidase. Like transferrin, chloroperoxidase contains protophyrin IX and forms the heme group with iron 3+ (Mathews). Eicosanoids, are also related to glycoproteins because they recognize the receptors on cells which are glycoproteins.