Chapter 12 Sugar, Carbohydrates, and Glycobiology （糖生物学）

1. Carbohydrates are aldehyde or ketone compounds with multiple hydroxyl groups or substances that can yield such compounds on hydrolysis (p. 293) 1.1 Carbohydrates are the most abundant biomolecules on earth and have multiple roles in all forms of life Carbohydrates serve as energy stores (e.g., starch in plants, glycogen in animals), fuels (e.g., glucose), and metabolic intermediates (e.g., ATP, many coenzymes).

1.1.2 Carbohydrates serve as structural elements in cell walls of plants (cellulose) or bacteria (peptidoglycans), exoskeletons of arthropods (chitin), and extracellular matrixes of vertebrate animals (proteoglycans).

1.1.3 Carbohydrates serve as recogntion signals in glycoproteins and glycolipids determining cell-cell recognition, intracellular location, and metabolic fates of proteins (thus sugars, like nucleic acids and proteins, are also information rich! But codes unknown) Carbohydrates (ribose and deoxyribose) form part of the structural framework of RNA and DNA.

1.2 Carbohydrates can be categorized into monosaccharides, oligosaccharides, and polysacchrides Monosacchrides are simple sugars consisting of a single polyhydroxyl aldehyde or ketone unit (e.g., glyceraldehyde, dihydroxyacetone, ribose, glucose, galatose, ribulose, and fructose).

1.2.2 Oligosaccharides contain two (disaccharides) or a few monosaccharides joined by glycosidic bonds (e.g., lactose, sucrose, maltose, some covalently linked sugars in glycoproteins and glycolipids) Polysaccharides contain long chains of (hundreds to thousands) monosaccharide units joined by glycosidic bonds (e.g., glycogen, starch, cellulose, chitin, and glycosaminoglycans).

2. Monosacchrides contain one carbonyl group and two or more hydroxyl groups. (p. 294) 2.1 Monosacchrides can be divided into two families: aldoses and ketoses Aldoses have their carbonyl groups at the ends of the carbon chains, thus being an aldehyde Ketoses have their carbonyl groups at places other than the ends,, thus being ketones The simplest aldose is glyceraldehyde, and the simplest ketose is dihyoxyacetone, both being triose.

2.1.4 Monosacchrides containing four, five, and six carbon atoms in their backbones are called tetroses, pentoses (e.g., ribose and deoxyribose), and hexoses (e.g., glucose and fructose), respectively. Each has both aldoses and ketoses Hexoses are the most common monosacchrides in nature, including D-glucose, D-mannose, D-galactose, D-fructose.

2.2 All the monosacchrides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms The configuration of an asymmetric carbon in an open chain monosacchride is usually indicated by the Fischer projection formulas (fig.) The carbon atoms of a sugar are numbered starting at the end nearest to the carbonyl group.

2.2.3 Glyceraldehyde is conventionally used as the standard for defining D and L configurations: D-glyceraldehyde has the -OH group on the right, L-glyceraldehyde has the -OH group on the left. (fig.) For sugars with more than one asymmetric carbon atom, the D- and L- symbols refer to the absolute configuration of the asymmetric carbon farthest to the carbonyl group (e.g., in D-glucose, the -OH on C-5 has the same configuration as the asymmetric carbon in D- glyceraldehyde, therefore, D- and L- glucoses are not enantiomers but stereoisomers!)

2.2.5 (p. 295) Most of the monosaccharides found in living organisms are the D-isomers (e.g., D-ribose, D-glucose, D-galactose, D-mannose, D- fructose) Each stereoisomer has a different conventional name, ending with “-ose” suffix Ketoses are often named by inserting an “ul” into the name of the corresponding aldoses (e.g., aldopentose is named as ribose, the ketopentose is named as ribulose.

2.2.8 Two sugars differing in configuration at a single asymmetric carbon is called epimers to each other (e.g., D-glucose and D-mannose are epimers at C-2; D-glucose and D-galactose are epimers at C-4) Monosaccharides easily form intramolecular hemiacetals (in aldoses) or hemiketals (in ketoses) in aqueous solutions. (fig.) The optical activity of D-glucose slowly changes when dissolved in water.

(p. 297) An aldehyde can react with one alcohol to form a hemiacetal (two alcohol to form acetal), a ketone with an alcohol to form a hemiketal In the open chain form of glucose, the aldehyde group at C-1 and the hydroxyl group at C-5 react to form two six-membered pyran-like cyclic stereoisomers: the  -D-glucopyranose (the -OH group attached to C-1 locates on a different side from the C-6 atom) and the  -D- glucospyranose (-OH o C-1 on the same side of the plane as C-6), thus being specifically called anomers to each other. (fig.)

The ring structures are commonly shown by Haworth Perspective Formulas: carbon atoms not explicitly shown, ring plane (actually not planar!) perpendicular to the plane of the paper, heavy line projecting (protruding) toward the reader, -OH groups below the ring are at the right side of a Fischer projection.

The  and  anomers interconvert through the open chain form in aqueous solution to give an equilibrium mixture, a process being called mutarotation Similarly, the C-2 keto group can interact with the C-5 hydroxyl group in D- fructose to form the a and  -D-fructosefuranoses (five-membered ring, furan-like) D-ribose and 2-deoxy-D-ribose also form corresponding  (and  furanoses. (This is what’s in DNA and RNA.)

2.4 The six-membered pyranose ring and five- membered furanose ring are not planar The six-membered pyranose rings usually take the chair form of conformation with bulky substituents in equatorial positions, which makes them less hindered (comparing with the boat conformation).

2.4.2 The five-membered furanose rings usually take the envelop form of conformations with four carbons in the same plane and the fifth out of the plane. (fig.) In the furan ribose rings, either C-2 or C-3 is out of the plane and locates on the same side of C The furanose rings can interconvert rapidly between different conformations states (thus pentoses are chosen as components of RNA and DNA).

2.5 The free carbonyl carbons (in open chains) in monosaccharides can reduce Cu2+ (cupric ion) to Cu+ (cuprous ion), which in turn forms a red cuprous oxide precipitate. (oxidoreduction reaction) (p. 301) Such monosaccharides are called reducing sugars This color reaction is the basis of Fehling’s reaction, a qualitative test for the presence of reducing sugars, was also used to estimate the glucose levels in blood and urine of diabetes patients.

2.6 A variety of hexose derivatives exist in living organisms. (p. 299) These derivatives include mainly amino sugars (e.g., glucosamine, galactosamine, and mannosamine, N-acetyl-glucosamine), deoxy sugars (e.g., fucose--deoxygenated galactose, deoxyribose), and acidic sugars (e.g., gluconate-- an aldonic acid, glucuronate--an uronic acid).

2.6.2 Amino sugars are often found in structural polysaccharides (e.g., bacterial cell walls contain a heterosaccharide made of alternating N-acetyl-b-D-glucosamine and N- acetylmuramic acid units; arthropod chitin is a homopolysaccharide made of N-acetyl-b-D- glucosamine).

2.6.3 Glucuronate is attached to bilirubin (the degraded product of heme) to solubilize it in bile N-acetylneuraminic acid (sialic acid), an acidic sugar, is a component of many glycoproteins and glycolipids in higher animals, playing roles in molecular and cellular recognition Sugar phosphates are common metabolic intermediates in sugar metabolism.

2.7 Abbreviations of common monosaccharides and their derivatives are often used in describing oligo- and polysaccharides Glucose, galactose, fructose are abbreviated as Glc, Gal, Fru, respectively. (others?)

3. Monosaccharide units can link with each other through O-glycosidic bonds to form oligo- and polysaccharides. 3.1 Carbohydrates are joined to alcohols and amines by glycosidic bonds An anomeric carbon atom, being a hemiacetal, can react with an alcohol to form an acetal The C-O bond thus formed is called an O-glycosidic bond An anomeric carbon atom can also be linked to the nitrogen atom of an amine by an N-glycosidic bond.

3.2 Disaccharides consist of two monosaccharides linked through an O-glycosidic bond Sucrose, lactose and maltose are the most abundant disaccharides in nature In sucrose (common table sugar), the anomeric carbon of one  -D-glucose is joined to the hydroxyl oxygen atom on C-2 of an  -D- fructose.

3.2.3 In lactose (found mainly in milk) the anomeric carbon of one  -D-galactose is joined to the hydroxyl oxygen atom on C-4 of an D-glucose In maltose (being the hydrolysis product of starch), the anomeric carbon of one  - D-glucose is joined to the hydroxyl oxygen atom on C-4 of another glucose.

3.2.5 Sucrose, lactose, and maltose can be abbreviated as Glc(  1-2  )Fru, (or Fru(  2- 1  )Glc), Gal(  1-4)Glc, and Glc(  1-4)Glc, respectively Both lactose and maltose have a free anomeric carbon (not involved in glycosidic bonding) that can be oxidized, thus being reducing sugars The end of an oligo- and polysaccharide having a free anomeric carbon is called the reducing end.

3.2.8 Sucrose does not have a reducing end (the anomeric carbons of both saccharide units are involved in glycosidic bonding!) The three disaccharides can be hydrolyzed into two monosaccharide units by specific sucrase (also called invertase), lactase (  - galactosidase in bacteria), and maltase existing on the outer surface of epithelial cells lining the small intestines. (milk allergy is due to lack of lactase in the intestines).

3.3 Glycogen and starch are mobilizable stores of glucose in animals and plants respectively Glycogen (mainly in liver and skeleton muscles) is a polymer of (  1-4) linked glucose units with (  1-6) linked branches (occurring about once every 10 glucose residues).

3.3.2 Starch can be linear or branched polymers of glucose, called amylose and amylopectin, respectively Amylose consists of D-glucose residues in (  1-4) linkage Amylopectin has about one (  1-6) branch per 30 (  1-4) linkages Amylopectin is like glycogen except for its lower degree of branching.

3.3.6 The (  1-4) linkages of glycogen, amylose, and amylopectin cause these polymers (of thousands of glucose units) to assume a tightly coiled helical structure, which produce dense granules in many animal or plant cells.

3.3.7 Each amylose has one nonreducing one and one reducing one, but each amylopectin and glycogen has one reducing end and many nonreducing ends Starch and glycogen ingested in the diet are hydrolyzed by  -amylase (present in saliva and intestinal juice) that break the  1,4 glycosidic linkages between glucose units. (starting from the nonreducing ends).

3.4 Cellulose and chitin are structural homopolysaccharides with similar composition and structures Cellulose, like amylose, is a linear homopolysaccharide of 10,000 or 15,000 D-glucose residues, but with (  1-4) linkages Chitin is a linear homopolysaccharide composed of N-acetyl-D-glucosamine residues also with (  1-4) linkages.

3.4.3 The only chemical difference between cellulose and chitin is the replacement of a hydroxyl group at C-2 with an acetylated amino group The (  1-4) linkage allow the polysaccharide chains of cellulose and chitin to take an extended conformation forming parallel fibers through intrachain and interchain hydrogen bonding.

3.4.5 Most animals lack enzymes to hydrolyze cellulose but some (like termites and ruminant animals) can use cellulose because of the cellulase secreted by symbiotic microorganisms.

4. Highly negatively charged glycosaminoglycans existing as proteoglycans are found in the extracellular matrixes of vertebrates. 4.1 Glycosaminoglycan chains are made of repeating disaccharide units One of the monosaccharide unit is always a derivative of either glucosamine or galactosamine The other monosaccharide is often a uronic acid The sulfate group is often found forming esters with certain -OH groups, thus making glycosaminoglycans highly negatively charged. (function?).

4.1.4 In hyaluronate (a sulfate free glycosaminoglycan), the disaccharide unit (containing a D-glucuronic acid and N- acetylglucosamine in (  1-3) linkage) links each other in (  1-4) linkages Chondroitin sulfate, Keratan sulfate, heparin are also common glycosaminoglycans in extracellular matrix, usually covalently bound to proteins Hyaluronate does not contain sulfate and exists as a free polysaccharide.

4.2 Glycosaminoglycans interacts with proteins (noncovalently and covalently) to form complex proteoglycans Proteoglycans consist of one or more core proteins and one or more glycosaminoglycans.

4.2.2 The best-characterized proteoglycan is the one found in cartilage consisting of a single long molecule of hyaluronate associating noncovalently with many molecules of core proteins, each containing covalently bound other glycosaminoglycans (including chondroitin sulfate and keratan sulfate).

4.2.3 Proteoglycans provide viscosity, lubrication, and resilience to the extracellular matrix Proteoglycans also have important roles in mediating cell adhesion (through integrins and fibronectins), docking proteins (that stimulating cell growth and proliferation), morphogenesis and other other unknown ones.

Proteoglycan structure, showing the trisaccharide bridge, Xylose anomeric carbon to the hydroxyl of serine

Proteoglycan structure of an integral membrane protein. Shown is syndecan. Trisaccharide linkers in blue.

A proteoglycan aggregate of the extracellular matrix. 100 core protein aggrecan. Link proteins mediate interaction between hyaluronate backbone and core protein.

5. Oligosaccharides are attached to integral membrane proteins and many secreted proteins to form glycoproteins. 5.1 The number of possible arrangements and combinations of monosaccharide types and glycosidic linkages in an oligosaccharide can be astronomical Sugar residues commonly found in glycoproteins include Fuc, Gal, Man, GalNAc, and Sia (or NeuNAc).

5.1.2 Glycosidic linkages can be (1-2), (1-3), (1-4), (1-6), (2-3), (2-6), …etc Many more oligosaccharides can be formed from four monosaccharides than oligopeptides from four amino acids Oligosaccharides can be extremely information rich!

5.2 The oligosaccharide can be covalently linked to Ser and Thr residues on proteins by O- glycosidic bonds, or to Asn residue by N- glycosidic bonds glycophorin, a well-studied membrane glycoprotein, contains 15 O-linked oligosaccharides and one N-linked one N-linked oligosaccharides usually contain a common pentasaccharide core consisting of three Man and two GlcNAc residues.

5.3 Carbohydrate-binding proteins mediate many biological recognition processes The terminal sugar residues on a glycoprotein can serve as a signal that directs liver cells to remove the protein from the blood (e.g., exposed Gal residues of a trimmed glycoproteins are detected by the asialoglycoprotein receptors in the plasma membranes of liver cells).

5.3.2 Plants contain many specific carbohydrate-binding proteins called lectins, which contain two or more sites that bind specific carbohydrate units Some viruses gain entry into host cells by adhering to cell-surface carbohydrates (e.g., influenza virus contains a hemagglutinin protein that recognizes sialic acid residues on cells lining the respiratory tract).

5.3.4 Carbohydrate are critical in the interaction bewteen sperm and ovulated egg (a carbohydrate on the surface of the egg is recognized by a receptor on the sperm) Carbohydrate play important roles in cell adhesions The carbohydrate codes are waiting to be deciphered!

Interactions bwt cells and extracellular matrix. Mediated by integrin and fibronectin. Note the close association of collagen with fibronectin.

Oligosaccharide linkages in glycoproteins. Common examples

Common among bacteria Determinant of serotype of bacterium

Helicobacter pylori cells colonize on the surface of the gastric epithelium, causing ulcers. Lectin binds Le b oligosaccharides.

Lectins are in light purple. They mediate cell-cell interactions.