Until recently it was not recognized that nature can employ sugars for the synthesis of highly specific compounds that can act as carriers of biological information. This capability arises from the fact that a large number of structures can be formed from a small number of monomers. In other words, monosaccharides can serve as letters in a vocabulary of biological specificity, where the words are formed by variations in the nature of the sugars present, the type of linkage and the presence or absence of branch points. It is now known that the specificity of many natural polymers is written in terms of sugars, not amino acids or nucleotides. This idea is not entirely novel, but it has only recently become well established.
The central dogma of molecular biology limits the downstream flow of genetic information to proteins.
The central paradigm of modern molecular biology is that biological information flows from DNA to RNA to protein. The power of this concept lies not only in its template-driven precision, but also in the ability to manipulate any one class of molecules based on knowledge of another. () Progress from the last two decades of research on cellular glycoconjugates justifies adding the enzymatic production of glycan antennae with information-bearing determinants to this famous and basic pathway.
In the first part of this century, the chemistry, biochemistry, and biology of carbohydrates were very prominent matters of interest. However, during the initial phase of the modern revolution in molecular biology, studies of glycans lagged far behind those of other major classes of molecules. In the 1920's it was still believed that the specific information in biological polymers was carried only by proteins.
Between 1925 and 1937 Oswald T. Avery of the Rockefeller Institute, together with Michael Heidelberger and Walther F. Goebel, demonstrated that pure polysaccharides can carry specific immunological messages as antigens: substances that stimulate the production of an antibody specific to themselves.
It is well established that carbohydrates are ideally suited for the formation of specificity determinants that can be recognized by complementary structures, which presumably are carbohydrate- binding proteins, on other cells or molecules. An impressive variety of regulatory processes including cell growth and apoptosis, folding and routing of glycoproteins and cell adhesion/migration have been unraveled and found to be mediated or modulated by specific protein (lectin)-carbohydrate interactions. Currently, the potential for medical applications in anti-adhesion therapy or drug targeting is one of the major driving forces fuelling progress in glycosciences.
The first indication that sugars serve as specificity determinants came from the discovery in 1941 by George K. Hirst in New York and by Ronald Hare in Toronto that the influenza virus caused red blood cells to agglutinate, or clump. The molecular basis of this phenomenon was for a time obscure. Mainly as a result of the efforts of Alhed Gottschalk in Australia it was shown that the influenza virus binds to the red blood cell through sialic acid units on the cell surface. If the sialic acid is removed from the cell surface by the enzyme neuraminidase, the influenza virus will no longer bind to the cell.
The role of carbohydrates in recognition has been best demonstrated in the control of the lifetime of glycoproteins in the circulatory system and their uptake into the liver- and of the uptake of lysosomal enzymes by cells. As often happens, these exciting discoveries originated with an unexpected observation, this one made in 1966 by G. Gilbert Ashwell of the National Institute of Arthritis, Metabolism, and Digestive Diseases and by Anatol G. Morell of the Albert Einstein College of Medicine in the course of an effort to understand the biological role of ceruloplasmin, a copper-transport protein found in the blood serum of man and other animals. When Ashwell and Morell removed sialic acid from rabbit ceruloplasmin and reinjected the modified ceruloplasmin into the animals, it almost completely disappeared from the circulatory system within 15 minutes. This was in striking contrast to the native glycoprotein, almost all of which remained in circulation after the same length of time.
Galactose hence serves as a recognition marker that determines the survival time of many serum glycoproteins in the circulatory system of man, the rabbit and the mouse. In bird and reptile species the recognition marker appears to be primarily acetylglucosamine. Clearance systems in which fucose and mannose are the markers have also been found.
A particularly interesting marker is mannose-6-phosphate, a sugar derivative that has recently been shown to act mainly in directing the intracellular traffic of glycoprotein enzymes normally present in lysosomes. This finding had its origins in Neufeld's discovery that the enzyme deficiencies in cells from patients afflicted by mucopolysaccharidoses such as Hurler's and Hunter's syndromes can be corrected by providing the cells with the missing enzymes. In 1974 she showed further that uptake into the cells depended on the presence in the enzymes of a carbohydrate -recognition marker. In 1977 William S. Sly of the Washington University School of Medicine and Arnold Kaplan of the Saint Louis University School of Medicine identified the recognition marker as a phosphorylated sugar unit: mannose-6-phosphate. The function of the marker is apparently to prevent the secretion of the enzymes from the cells and to direct them into the lysosomes. When the enzymes are supplied from the outside, it is this recognition signal that promotes their binding to the cell surface; without binding they cannot enter the cells and reach the lysosomes.
By the covalent (electron- sharing) attachment of carbohydrates to proteins or by a modification of the sugars in glycoproteins it may thereby be possible to control the proteins' lifetime in the circulation and to direct them to the liver and perhaps also to other organs, as well as into lysosomes. Such techniques will have far-reaching uses for enzyme replacement therapy in cases of genetic disease and also for delivering drugs accurately into target organs and cells.
The first proof that sugars could serve as specificity determinants came from the discovery that influenza virus could agglutinate red cells only in the presence of the membrane bound sialic acids. If these were removed, the virus no longer binds to the cell. Removal of sialic acid exposes the terminal underlying galactose unit and results in the rapid clearance of the treated cells from the bloodstream. Sugars on cell surfaces also seem to determine the distribution of the circulating cells within the body. Radioactively treated rat lymphocytes will migrate to the spleen when re-injected into the animal.
However if the sugar fucose is removed from the surface of the cells before reintroduction, the cells migrated to the liver instead, as if "the fucose served as a ZIP code- directing the calls where to go." It was not until 1953 that Morgan and Watkins demonstrated that the specificity of the ABO blood group-system was determined by sugars. For example, the difference between blood types A and B lies in a simple sugar unit that sticks out from the end of a carbohydrate chain of a glycoprotein or glycolipid. In blood A the determinant is N-acetylgalactosamine and in group B it is galactose.
Several toxins of bacteria and plants are known to recognize carbohydrate structures present in various classes of cell surface molecules.
When a lectin contains multiple binding sites, they can interconnect large numbers of cells, causing them to clump together or agglutinate. Each molecule of a lectin has two or more regions, perhaps clefts or grooves, each of which fits a complementary molecule of a sugar or several sugar units of an oligosaccharide. It is by means of these combining sites that the lectin attaches itself to the sugars on cell surfaces.