12/26/2023 0 Comments Non reducing anomeric carbonThe overall configuration (D or L) of each sugar is determined by the absolute configuration of the stereogenic center furthest from the carbonyl group (i.e., with the highest numbered asymmetric carbon atom this is C-5 in hexoses and C-4 in pentoses). The aldehyde carbon is referred to as C-1 and the carbonyl group in ketoses is referred to as C-2. The numbering of carbon atoms follows the rules of organic chemistry nomenclature. The chiral central carbon in glyceraldehyde gives rise to two possible configurations of the molecule, termed D and L. Structures of glyceraldehyde and dihydroxyacetone. For example, an aldohexose with the general formula C 6H 12O 6 and four asymmetric carbon atoms (i.e., four (CHOH) groups) can be described in 16 possible isomeric forms (Figure 2.1). The number of stereoisomers corresponds to 2 k, where k equals the number of asymmetric carbon atoms. With the exception of dihydroxyacetone, all monosaccharides have at least one asymmetric carbon atom, the total number being equal to the number of internal (CHOH) groups ( n–2 for aldoses and n–3 for ketoses with n carbon atoms). The structures of glyceraldehyde and dihydroxyacetone are distinct in that glyceraldehyde contains an asymmetric (chiral) carbon atom (Figure 2.1), whereas dihydroxyacetone does not. Glyceraldehyde is the simplest aldose and dihydroxyacetone is the simplest ketose (Figure 2.1). As mentioned briefly in Chapter 1, all monosaccharides consist of a chain of chiral hydroxymethylene units, which terminates at one end with a hydroxymethyl group and at the other with either an aldehyde group (aldoses) or an α-hydroxy ketone group (ketoses). All simple monosaccharides have the general empirical formula C x(H 2O) n, where n is an integer ranging from 3 to 9. The classification of monosaccharide structures began in the late 19th century with the pioneering work of Emil Fischer. MONOSACCHARIDES: BASIC STRUCTURES AND STEREOISOMERISM 2015 in Further Reading at the end of this chapter, and Online Appendix 1B). A more detailed and comprehensive listing of carbohydrate nomenclature rules has been published (see McNaught 1997 and Varki et al. Additional nomenclature issues are covered in this chapter and Chapter 3. The term complex carbohydrates includes glycoconjugates, whereas the term carbohydrates per se would not. However, even so-called simple glycans, such as cellulose and starch, often have very complex molecular structures in three dimensions. The glucose-based polymer cellulose is an example of a “simple” carbohydrate, whereas a galactomannan polysaccharide, composed of both galactose and mannose, is an example of a complex carbohydrate. Just as is observed with proteins in nature, additional structural diversity can be imparted to glycans by modifying their hydroxyl groups with phosphate, sulfate, or acetyl esters, and/or their amino groups with acetyl or sulfate groups.Ī carbohydrate may be termed “complex” if it contains more than one type of monosaccharide building unit. The prefix “glyco” and the suffixes “saccharide” and “glycan” indicate the presence of carbohydrate constituents (e.g., glycoproteins, glycolipids, and proteoglycans). The term “glycoconjugate” is often used to describe a macromolecule that contains monosaccharides covalently linked to proteins or lipids. Thus, the relationship of monosaccharides to oligosaccharides or polysaccharides is analogous to that of amino acids and proteins or nucleotides and nucleic acids (polynucleotides). The term “polysaccharide” is typically used to denote any linear or branched polymer consisting of monosaccharide residues, such as cellulose ( Chapters 14 and 24). Typically, the term “oligosaccharide” refers to any glycan that contains less than 20 monosaccharide residues connected by glycosidic linkages. Monosaccharides are joined together to give rise to oligosaccharides or polysaccharides. Monosaccharides are the simplest of these polyhydroxylated carbonyl compounds (saccharide is derived from the Greek word for sugar or sweetness). X(H 2O) n that also possess a carbonyl group, either an aldehyde or a ketone. In the 19th century, sugar-based substances were referred to as carbohydrates, or “hydrates of carbon,” that are based on the general formula C Still, a host of names are commonly used to refer to sugar polymers in other textbooks and the literature. In this book, as well as in the earlier editions, the term glycan is used.
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