Chapter 10 – Carbohydrates Most carbohydrates are produced photo-synthetically by bacteria, algae and plants Carbohydrates (“hydrate of carbon”) have the empirical formula of (CH2O)n , where n ≥ 3. Monosaccharides: one monomeric unit Oligosaccharides: ~2-20 monosaccharides Polysaccharides: > 20 monosaccharides Glycoconjugates: linkages to proteins or lipids

Saccharides of varying length Aldoses and Ketoses Trioses – three carbon sugars Figure 8.1 Fischer projections of (a) glyceraldehyde and (b) dihydroxyacetone. The designations L (for left) and D (for right) for glyceraldehyde refer to the configuration of the hydroxyl group of the chiral carbon (C-2). Dihydroxyacetone is achiral. Ketotriose Aldotriose

Saccharides of varying length Aldoses and Ketoses Tetroses – 4 carbon sugars Figure 8.3 Fischer projections of the three- to six-carbon D-aldoses. The aldoses shown in blue are the most important in our study of biochemistry. Chiral designation comes from the most distant chiral carbon from the carbonyl D-sugars dominate in nature

Saccharides of varying length Aldoses and Ketoses Figure 10.2 Pentoses – 5 carbon sugars Hexoses – 6 carbon sugars Chiral designation comes from the most distant chiral carbon from the carbonyl

Figure 10.1 Isomeric forms of carbohydrates

Epimers Epimers – sugars that differ at only one of several chiral centers. example: D-Mannose is an epimer of D-Glucose

D-fructose to form a- and b- fructofuranose Figure 10.4 Cyclization of D-fructose to form a- and b- fructofuranose anomeric carbon Figure 8.9 Cyclization of D-ribose to form - and -D-ribopyranose and - and -D-ribofuranose.

Fig 10.3 Cyclization of D-glucose to form glucopyranose Figure 8.8 Cyclization of D-glucose to form glucopyranose. The Fischer projection (top left) is redrawn as a three-dimensional representation (top right). Rotation of the bond between C-4 and C-5 brings the C-5 hydroxyl group close to the C-1 aldehyde group. Reaction of the C-5 hydroxyl group with one side of C-1 gives -D-glucopyranose; reaction of the hydroxyl group with the other side gives -D-glucopyranose. The glucopyranose products are shown as Haworth projections in which the lower edges of the ring (thick lines) project in front of the plane of the paper and the upper edges project behind the plane of the paper. In the -D anomer of glucose, the hydroxyl group at C-1 points down; in the -D anomer, it points up. In aqueous solutions hexoses and pentoses will cyclize forming alpha (a) and beta (b) forms C1 is called the anomeric carbon

In aqueous solutions, it is the ring structures that dominate These rings are NOT planar Figure 10.5 Figure 8.10 Conformations of -D-ribofuranose. (a) Haworth projection. (b) C2’-endo envelope conformation. (c) C3’-endo envelope conformation. (d) Twist conformation. In the C2’-endo conformation, C-2 lies above the plane defined by C-1, C-3, C-4, and the ring oxygen. In the C3’-endo conformation, C-3 lies above the plane defined by C-1, C-2, C-4, and the ring oxygen. In the twist conformation shown, C-3 lies above and C-2 lies below the place defined by C-1, C-4, and the ring oxygen. The planes are shown in yellow.

Figure 10.7 modified monosaccharides 6-deoxy-L-galactose Important in structural glycans Important in metabolic pathways

Disaccharides and other Glycosides Glycosidic bond – the primary structural linkage in all polymers of monsaccharides Glycosides – glucose provides the anomeric carbon Figure 8.20 Structures of (a) maltose, (b) cellobiose, (c) lactose, and (d) sucrose. The oxygen atom of each glycosidic bond is shown in red. Maltose is in malt and is used in brewing of beer Cellobiose – cellulose in plants released during cellulose degradation Figure 10.8 Structures of disaccharides: maltose

More structures of disaccharides: lactose and sucrose Most abundant sugar Major carbohydrate in milk Sucrase, lactase and maltase are found on the outer surface of the epithelial cells lining the small intestine. Figure 8.20 Structures of (a) maltose, (b) cellobiose, (c) lactose, and (d) sucrose. The oxygen atom of each glycosidic bond is shown in red. Sucrose links two anomeric carbons together Lactose is an epimer of cellobiose Comes from the breakdown of starch and glycogen

Reducing and Nonreducing ends of sugars In linear polymeric chains of monosaccharides there is usually one reducing end (containing the free anomeric carbon) and one nonreducing end Branched polysaccharides have a number of nonreducing ends, but only one reducing end Nonreducing end Beta form tends to dominate. Reducing end (anomeric carbon) Read Clinical Insight pg 161

Polysaccharides - Homoglycans – homopolysaccharides containing only one type of monosaccharide. Heteroglycans – heteropolysaccharides containing residues of more than one type of monosaccharide The lengths and compositions of a polysaccharide may vary within a population of these molecules e.g.: starch and glycogen – storage polysaccharides cellulose and chitin – structural polysaccharides Kytin = chitin

Starch D-glucose is stored intracellularly in polymeric forms - plants and fungi store glucose as starch - Animals store glucose as glycogen Starch is a mixture of amylose (unbranched) and amylopectin (branched every 25 sugars) Amylose is a linear polymer containing only a-1,4-glycosidic bonds. (b) Amylopectin is a branched polymer also contains a-1,6-bonds Figure 10.12

Starch is stored by plants and used as fuel. Prentice Hall c2002 Chapter 8

a-amylase on amylopectin - a-amylase is found in plants and animals and is a hydrolase it is an endoglycosidase – hydrolyzes internal a-(14) glycosidic bonds

Glycogen is is stored by animals and used as fuel. Prentice Hall c2002 Chapter 8

Glycogen Glycogen is the main storage polysaccharide of humans Glycogen is a polysaccharide of glucose residues connected by a-(14) linkages with a-(16) branches (one branch every 8 to 12 residues) - Glycogen is present in large amounts in liver and skeletal muscle

Cellulose – a structural polysaccharide that is a Major component of cell walls of plants Cellulose has b-(14) glycosidic bonds Each glucose residue is rotated 180o relative to the next residue Cellulases Pectin vs Cellulose Pectin is a water soluble polysaccharide using galacturonate-an sugar acid Extended hydrogen bonding between chains leads to bundles or fibrils

Figure 10.14 Glycosidic bond Determines polysaccharide Structure. Bent structure Straight structure

- Humans digest starch and glycogens ingested in their diet using amylases, enzymes that hydrolyze a-(14) glycosidic bond Humans cannot hydrolyze b-(14) linkages of cellulose. Therefore cellulose is not a fuel source for humans. It is fiber. Certain microorganisms have cellulases, enzymes that hydrolyze b-(14) linkages of cellulose. - cattle have these organisms in their rumen - termites have them in their intestinal tract

Carbohydrates attached to proteins form glycoproteins Many glycoproteins are found as components of cell membranes (chapter 11) and take part in cell adhesion and binding. or Thr Mucins or mucoproteins are proteins which has N-acetylgalactosamine attached. This glycoprotein is found in mucus and is a lubricant. 1st class is glycoprotein where the protein is the largest component of the sugar protein unit. Proteoglycans-larger component is the polysaccharide Mucins and the amino sugar Two methods to anchor the protein to carbohydrates

Figure 10.15 Glycosidic bonds between proteins and carbohydrates asparagine serine Thr: CH3CH(OH)- Thr is also used to make O-linkages Two methods to anchor the protein to carbohydrates

Figure 10.16 N-linked oligosaccharides All N-linkages have a common core shown in grey. Complex structure High mannose Fuc-fructose-methyl group on C6, Sia-sialic acid-N-aceyl glucosamine acid Protein residue Protein residue Additional sugars can be attached to these cores to make diverse and unique structures

Proteoglycans are glycoproteins were the protein is bound to a special class of polysaccharides called a glycosaminoglycan. This class of glycoprotein are used as structural components and lubricants. -In proteoglycans nearly 95% of the mass comes from the polysaccharide. -Proteoglycans function as lubricants, structural components in tissue and mediate the adhesion of cells to the extracellular matrix. 1st class is glycoprotein where the protein is the largest component of the sugar protein unit. Proteoglycans-larger component is the polysaccharide Mucins and the amino sugar

Assignment Read Chapter 10 Read Chapter 11 Topics not covered: Section 10.4 Lectins