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GLYCOCHEMISTRY: OVERVIEW AND PROGRESS

Matthew Schombs and Jacquelyn Gervay-Hague

Department of Chemistry, University of California, Davis, Davis, CA, USA

1.1 INTRODUCTION

Officially, the International Union of Pure and Applied Chemistry (IUPAC) defines glycan as "synonymous with polysaccharides," meaning compounds consisting of a large number of monosaccharides linked to each other through glycosidic bonds [1]. Practically however, the term glycan is all encompassing and often used to describe the carbohydrate portion of glycoconjugates such as glycoproteins and glycolipids. Carbohydrates are the most abundant organic molecules on Earth and are the main products through which the energy of the sun is harnessed and stored. Glucose polysaccharides, such as starch in plants and glycogen in bacteria and animals, serve as a source of energy for essentially all organisms. However, the complex roles of carbohydrates are not limited to simply that of biological fuel stocks or biosynthetic starting materials. DNA and RNA, which transmit and store genetic information, have sugar backbones. Other carbohydrate polymers are essential structural and protective components of the cell walls of plants as cellulose, bacteria as peptidoglycan, and the exoskeletons of arthropods as chitin. They are important constituents of secreted and cell-surface proteins, membrane components in the form of glycolipids and gangliosides, as well as various types of extracellular matrix molecules [2]. The significance of the carbohydrate domains of glycoproteins and glycolipids is further exhibited in their roles as cell-surface recognition elements and as determinants in blood-group typing [3, 4]. Carbohydrates are also appended to various natural products including antibiotics [5]. As such, glycans mediate a wide range of biological processes from embryonic development to differentiation, signaling, host-pathogen interactions, metastasis, intracellular trafficking, and localization [6].

The many hydroxy groups that adorn the carbon backbone of glycans give rise to multiple stereoisomers, a fact that has been capitalized on for use as chiral synthons in organic synthesis [7]. The nine common monosaccharides found in mammalian cells can be linked in an astonishing number of ways, resulting in much higher complexity than is possible from amino acid or nucleotide building blocks. Unlike proteins and DNA, glycans encode immense biological information without being template driven or encoded by the genome. The first draft of the human genome revealed a relatively small number of genes associated with the human species-approximately 32,000-as compared to less complex organisms such as fly or worm, which encompasses roughly 13,000 or 18,000 genes, respectively [8-10]. While the origin of biological complexity remains a largely debated topic, one hypothesis accounting for this paradox is the posttranslational modifications of proteins.

Glycosylation is one of the most ubiquitous forms of posttranslational modification and is widely recognized as a modulator of protein structure, localization, and function. Because glycosylation is not under tight genetic control, often complex and unpredictable mixtures of glycoforms with varying properties are produced [11, 12]. Therefore, access to homogeneous glycolipids, glycopeptides, and glycoproteins is an essential step toward furthering our understanding of these important molecules. Over the past century, significant developments have occurred, from the establishment of a carbohydrate nomenclature to discovering the simple building blocks that make up oligosaccharides and how they combine to create unique structures. These advances have enabled studies that reveal the multifaceted roles of glycans.

1.2 NOMENCLATURE, STRUCTURES, AND PROPERTIES OF SUGARS

Most simple sugars have the general formula Cn(H2O)n, where n is between three and nine. Early nineteenth-century French chemists generically defined carbohydrates as "hydrates de carbone" because they were thought to consist solely of carbon and water in a 1?:?1 ratio. However, the term is used today in a much broader sense. Saccharides can be roughly split into two categories: monosaccharides and complex saccharides such as oligosaccharides and polysaccharides. Depending on their size, oligosaccharides and polysaccharides tend to exhibit different chemical and physical properties as compared to monosaccharides. Polysaccharides can form stable secondary and tertiary structures and are hydrolyzed into smaller subunits upon treatment with aqueous acid, while monosaccharides can be found in a variety of forms including linear and cyclic structures. Monosaccharides are the building blocks from which oligosaccharides and polysaccharides are constructed. They include polyhydroxyaldehydes (aldoses) and polyhydroxyketones (ketoses) as well as the resulting compounds derived thereof by either the reaction of the carbonyl group, via oxidation to form carboxylic acids, or by replacing one or more hydroxy groups with hydrogen, amino, acetamide, thiol, or other functional groups (Fig. 1.1).

Figure 1.1 Common carbohydrate oxidation levels.

Monosaccharides are classified according to the number of carbons in their skeleton per IUPAC recommendations [13]. The suffix -ose is used to indicate an aldose, while -ulose denotes a ketose. Accordingly, the common aldoses in ascending order would be trioses, tetroses, pentoses, hexoses, etc. Prior to their structures being known and the subsequent systematization developed by Emil Fischer, carbohydrates were named based on either their sources (fructose for fruit sugar, glucose for grape sugar, lactose for milk sugar, and sucrose for cane sugar) or physical properties (dextrose for glucose because it rotates plane-polarized light in a clockwise manner (dextrorotation) and levulose for fructose because of its levorotatory nature). Note that each secondary carbon of the sugar alcohols is sp3 hybridized and represents a stereogenic or chiral center. A uniform method to visualize this tetrahedral geometry in two dimensions came in the form of the Fischer projection. While the Fischer proof is discussed later, this work largely eliminated inconsistencies in the representation and naming of sugars.

1.2.1 Fischer Projection

The Fischer projection is a convenient way of showing the configurations of the linear forms of monosaccharides. This convention depicts the concepts of stereochemistry established by Jacobus Henricus van 't Hoff and Joseph Achille Le Bel in a simplified form. While these abbreviated structural formulas are simple to write and easy to visualize, there are some guidelines that should be taken into account when converting a three-dimensional structure into a Fischer projection and in its manipulation (Fig. 1.2):

1. Orient the molecule in such a way that the chiral center is in the plane of the paper with its vertical bonds toward the back and the horizontal bonds coming out in front.
2. Position the carbon atoms of the chain on the vertical plane with the carbonyl group on top and the primary alcohol at the bottom. The hydrogen and hydroxy moieties should be oriented horizontally. Numbering of the carbon atoms begins with the carbonyl group in the case of aldoses or the terminal carbon closest to the carbonyl group in the case of ketoses.
3. Flatten the resulting model by "pulling" the vertical bonds toward the plane of the paper and "pushing" the horizontal bonds into the plane of the paper. The stereogenic carbon and the attached hydrogen atoms can then be omitted for clarity.
4. Define the most distant stereogenic center from the carbonyl group as the reference atom. The stereoisomer in which the highest priority substituent of the reference atom is pointed to the right is assigned the prefix "D" and to the left is assigned the prefix "L." This prefix designates the absolute configuration at the center of reference. A trivial name, on the other hand, is used to indicate the relative configuration of all other chiral centers in relation to the reference stereogenic center (Fig. 1.3). Consequently, the D- and L-isomers of a given trivial name are mirror images of each other.
5. Fischer projections must only be rotated in increments of 180° as a 90° rotation represents an inversion of configuration at the stereogenic center.

Figure 1.2 Fischer projection of glyceraldehyde and its manipulation.

Figure 1.3 The family tree of D-aldoses with the trivial and abbreviated names.

1.2.2 Linear Forms of Monosaccharides

Emil Fischer deduced the stereochemical relationship between monosaccharides using D-glyceraldehyde as the reference molecule. Ultimately, Fischer applied his proof to create the D-aldose family tree (Fig. 1.3), which is still in use to this day. The abbreviated names for aldopentoses and aldohexoses consist of the first three letters of their trivial names except only for "Glc," which is used for glucose ("Glu" had already been assigned to glutamic acid). The "D" (or "L") prefix in the abbreviated names may be omitted when referring to the more abundant isomer. Epimers are...