1. Chapter 7 Carbohydrates

2.  Carbohydrates are the most abundant biomolecule in nature Chapter 7: Overview Functions of carbohydrates 1.Provide energy through their oxidation 2.Supply carbon for synthesis of cell components 3.Serve as stored form of chemical energy 4.Form structural elements of some cells and tissues

3. Classes of Carbohydrates (saccharides) Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011; http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/sugar.htm Hydrates of carbon C m (H 2 O) n characterized by having multiple functional groups Hydroxyl (alcohols) Carbonyl (aldehydes or ketones) Mono- and disaccharides are simple sugars glucose fructose galactose ribose deoxyribose sucrose lactose Fructo-oligosaccharides galactooligosaccharides starch glycogen cellulose chitin

4. Stereochemistry of Carbohydrates Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011; http://web.fccj.org/~ethall/stereo/stereo.htm Stereoisomers: Same molecular formula Same sequence of bonded atoms (constitution), Different 3-D orientations of their atoms in space. Enantiomers: Stereoisomers that are mirror images of each other. Chirality: “Handedness”. Refers to compounds that cannot be superimposed on mirror image. -Defined relative to central, chiral atom (carbon) enantiomers

5. Fischer Projections Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  When carbonyl is up:  D-family: OH (or NH 2 ) group of chiral C most distant from anomeric center projects to right  L-family: OH (or NH 2 ) group of chiral C most distant from anomeric center projects to left  Anomeric center: Carbonyl (aldehyde or ketone) carbon  Biological systems can only utilize the D- isomers. * * * * * * * * * * * *

6. Monosaccharides Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  Contain single polyhydroxy aldehyde or ketone unit  Sugars with an aldehyde functional group are aldoses  Sugars with an ketone functional group are ketoses  Further classified based on number of C atoms  Aldehydes contain prefix aldo-, ketones have prefix keto-

7. p. 528 chiral carbons in glucose Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011 Max # of possible stereoisomers = 2 n where n = number of chiral carbon atoms

8. Monosaccharides: D- aldoses Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011 Chiral C’s? 1 2 3 4 stereoisomers 2 4 8 16

9. Section 7.1: Monosaccharides  Cyclic Structure of Monosaccharides  Sugars with four or more carbons exist primarily in cyclic forms  Ring formation occurs because aldehyde and ketone groups react reversibly with hydroxyl groups in an aqueous solution to form hemiacetals and hemiketals Figure 7.5 Formation of Hemiacetals and Hemiketals

10. Monosaccharides: Chemical Properties Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  Depicted using Haworth structures  Ring is drawn with oxygen to the back, and anomeric carbon to the right.  Furanose ring: A 5-member ring containing an oxygen atom.  Pyranose ring: A 6-member ring containing an oxygen atom.

11. Glucose (Blood Sugar) Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  Beta-hydroxy group: OH attached to anomeric carbon above ring.  Alpha-hydroxy group: OH attached to anomeric carbon below ring. Hemiacetal (on C1) 64% 0.02% 36%

12. Section 7.1: Monosaccharides  Five-membered rings are called furanoses and six- membered rings are pyranoses  Cyclic form of fructose is fructofuranose, while glucose in the pyranose form is glucopyranose Figure 7.8 Furan and Pyran Figure 7.9 Fischer and Haworth Representations of D -Fructose

13. Section 7.1: Monosaccharides  Reaction of Monosaccharides  The carbonyl and hydroxyl groups can undergo several chemical reactions  Most important include oxidation, reduction, isomerization, esterification, glycoside formation, and glycosylation reactions

14. Joining Monosaccharides Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  Monosaccharide units can be joined together by glycosidic linkages to form glycosides.  Glycosidic linkages are same as adding alcohol to hemi- intermediates Hemiacetal intermediate acetal

15. Section 7.1: Monosaccharides  Naming of glycosides specifies the sugar component  Acetals of glucose and fructose are glucoside and fructoside  If an acetal linkage is formed between the hemiacetal hydroxyl of one monosaccharide and the hydroxyl of another, this forms a disaccharide  In polysaccharides, large numbers of monosaccharides are linked together through acetal linkages Figure 7.18 Methyl Glucoside Formation

16. Polymerization of Monosaccharide Subunits Through glycosidic linkages at the hemiacetal carbons, many monosaccharide subunits can be put together to form long, branching chains via 1,4 or 1,6 linkages. These can be between 2 α, 2 β, or between an α and a β.

17. Section 7.1: Monosaccharides  Glycosylation Reactions attach sugars or glycans (sugar polymers) to proteins or lipids  Catalyzed by glycosyl transferases, glycosidic bonds are formed between anomeric carbons in certain glycans and oxygen or nitrogen of other types of molecules, resulting in N- or O-glycosidic bonds

18. Chemical Reactions of Sugars Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011  Open chain forms of monosaccharides (aldehydes, hydroxyketones) can be readily oxidized.  Ex. Use of Benedict’s reagent to oxidize aldehydes and ketones with hydroxy group on adjacent carbon.  At the same time, the cyclic forms are converted to open-chain forms and also react.  Reducing sugars: Monosaccharides that can be oxidized  Oxidation of carbohydrates to CO 2 and H 2 O very important at cellular level, serves as source of heat and energy. Reducing sugar + Cu(II)  oxidized cmpd + Cu 2 O Deep blue solution Red-orange precipitate

19. Section 7.1: Monosaccharides  Glycation: Reaction of reducing sugars with nucleophilic nitrogen atoms in a nonenzymatic reaction  Most researched example of the glycation reaction is the nonenzymatic glycation of protein (Maillard reaction)

20. Section 7.1: Monosaccharides Figure 7.20 The Maillard Reaction  The Schiff base that forms rearranges to a stable ketoamine, called the Amadori product  Can further react to form advanced glycation end products (AGEs)  Promote inflammatory processes and involved in age- related diseases

22. Section 7.1: Monosaccharides  Important Monosaccharides  Glucose (D-Glucose) —originally called dextrose  The primary fuel for living cells  Precursor for Vitamin C synthesis  Preferred energy source for brain cells and cells without mitochondria (erythrocytes)  Modified subunits can form long polymer chains  starch, cellulose, glycogen Figure 7.21  - D -glucopyranose

23. Section 7.1: Monosaccharides  Fructose ( D -Fructose) is often referred to as fruit sugar, because of its high content in fruit  On a per - gram basis, it is twice as sweet as sucrose; therefore, it is often used as a sweetening agent in processed food Figure 7.22  -D-fructofuranose

24. Fructose (D-fructose) http://upload.wikimedia.org/wikipedia/commons/1/1e/Fructose-isomers.jpg  Fruit sugar  Absorbed directly into bloodstream during digestion (like glucose and galactose)  Anomeric carbon is C2 70% 22% Hemiketal (on C2) 1 2 3 4 5 6

25. Other Important Monosaccharides Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011 Galactose  Synthesized in mammary gland, incorporated into milk lactose  Component of the antigens present on blood cells that determine blood type  Used to synthesize other biomolecules  Galactosemia is a genetic disorder resulting from a missing enzyme in galactose metabolism

26. Section 7.2: Disaccharides  Disaccharides  Two monosaccharides linked by a glycosidic bond  Linkages are named by  - or  -conformation and by which carbons are connected (e.g.,  (1,4) or  (1,4)) Figure 7.27 Glycosidic Bonds

27. Disaccharides Maltose LactoseSucrose Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011

28. Section 7.2: Disaccharides  Disaccharides Continued  Lactose (milk sugar) is the disaccharide found in milk  One molecule of galactose linked to one molecule of glucose (  (1,4) linkage)  It is common to have a deficiency in the enzyme that breaks down lactose (lactase)  Lactose is a reducing sugar Figure 7.28  - and  -lactose

29. Section 7.2: Disaccharides  Disaccharides Continued  Sucrose is common table sugar (cane or beet sugar) produced in the leaves and stems of plants  One molecule of glucose linked to one molecule of fructose, linked by an ,  (1,2) glycosidic bond  Glycosidic bond occurs between both anomeric carbons  Sucrose is a nonreducing sugar Figure 7.31 Sucrose

30. Other Important Monosaccharides Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7 th Edition, 2011: http://www.istpace.org/Web_Final_Report/WP_4_chem_subsystems/descr_chem_subsystems/pna_intro/pna_intro.html Ribose and Deoxyribose  Used to synthesize RNA and DNA  Used in protein synthesis Phosphodiester bond

31. Section 7.3: Polysaccharides  Polysaccharides (glycans) are composed of large numbers of monosaccharides connected by glycosidic linkages  Smaller glycans made of 10 to 15 monomers called oligosaccharides, most often attached to polypeptides as glycoproteins  Two broad classes : N- and O-linked oligosaccharides

32. Section 7.3: Polysaccharides  O-Glycosidic linkages attach glycans to the side chain hydroxyl of serine or threonine residues or the hydroxyl oxygens of membrane lipids Figure 7.32 Oligosaccharides Linked to Polypeptides  N-linked oligosaccharides attached to polypeptides by an N-glycosidic bond with side chain amide nitrogen from the asparagine  Three major types of asparagine-linked oligosaccharides: high mannose, hybrid, and complex

33. Section 7.3: Polysaccharides  Homoglycans  Have one type of monosaccharide and are found in starch, glycogen, cellulose, and chitin (glucose monomer)  Starch and glycogen are energy storage molecules while chitin and cellulose perform structural roles  Chitin is part of the cell wall of fungi and arthropod exoskeleton  Cellulose is the primary component of plant cell walls  No fixed molecular weight, because the size is a reflection of the metabolic state of the cell producing them

34. Section 7.3: Polysaccharides  Starch — the energy reservoir of plant cells and a significant source of carbohydrate in the human diet  Two polysaccharides occur together in starch: amylose and amylopectin  Amylose is composed of long, unbranched chains of D - glucose with  (1,4) linkages between them Figure 7.33 Amylose

35. Section 7.3: Polysaccharides  Amylose typically contains thousands of glucose monomers and a molecular weight from 150,000 to 600,000 Da  The other form is amylopectin, which is a branched polymer containing both  (1,6) and  (1,4) linkages  Branch points occur every 20 to 25 residues Figure 7.33 Amylose

36. Components of plant starch built from glucose residues. Plant starches Amylose (10-20%) No branching Amylopectin (80-90%) Branching 24-30 residues

37. Section 7.3: Polysaccharides  Glycogen is the carbohydrate storage molecule in vertebrates found in greatest abundance in the liver and muscle cells  Up to 8–10% of the wet weight of liver cells and 2–3% in muscle cells  Similar in structure to amylopectin, with more branch points  More compact and easily mobilized than other polysaccharides

38. Structure of glycogen Polymer built from glucose subunits. Glucose in Glycogen are connected via α-1,4 or α-1,6 linkage. α-1,4 linkage makes a linear chain, α-1,6 linkage makes a branch (~every 10 residues). The end glucose residues without open 1’-OH is called nonreducing ends. Branches provide more non-reducing ends for rapid degradation.

39. Section 7.3: Polysaccharides Figure 7.34 (a) Amylopectin and (b) Glycogen

40. Glycogen breakdown Glycogen cleaved from nonreducing ends of chain by the enzyme glycogen phosphorylase. Produces monomers of glucose-1-phosphate, which is then converted to glucose 6-phosphate by phosphoglucomutase. G6P monomers produced have three possible fates:  G6P can continue on the glycolysis pathway and be used as fuel.  G6P can enter the pentose phosphate pathway to produce NADPH and 5-carbon sugars.  In liver and kidney, G6P can be dephosphorylated back to glucose by the enzyme glucose 6-phosphatase. This is the final step in the gluconeogenesis (formation of glucose) pathway.

41. Most important structural polysaccharide and single most abundant organic compound on earth. Provides strength and rigidity to plant cell walls. Wood is ~50% cellulose. Contains 300-3000 glucose subunits. Form extended straight chains that hydrogen bond with parallel chains, creating long, rigid fibers. Undigestible by humans.Cellulose

42. Section 7.3: Polysaccharides  Pairs of unbranched cellulose molecules (12,000 glucose units each) are held together by hydrogen bonding to form sheetlike strips, or microfibrils  Each microfibril bundle is tough and inflexible with a tensile strength comparable to that of steel wire  Important for dietary fiber, wood, paper, and textiles Figure 7.36 Cellulose Microfibrils

43. Section 7.3: Polysaccharides  Heteroglycans  High-molecular-weight carbohydrate polymers that contain more than one type of monosaccharide  Major types: N- and O-linked glycosaminoglycans (glycans), glycosaminoglycans, glycan components of glycolipids, and GPI (glycosylphosphatidylinositol) anchors  GPI anchors and glycolipids will be discussed in Chapter 11

44. Section 7.3: Polysaccharides  Heteroglycans Continued  N- and O-Glycans — many proteins have N- and O- linked oligosacchaarides  N-linked (N-glycans) are linked via a  -glycosidic bond  O-linked (O-glycans) have a disaccharide core of galactosyl-  -(1,3)-N-acetylgalactosamine linked via an  -glycosidic bond to the hydroxyl of serine or threonine residues  Glycosaminoglycans (GAGs) are linear polymers with disaccharide repeating units  Five classes: hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparin and heparin sulfate, and keratin sulfate  Varying uses based on repeating unit

45. Section 7.4: Glycoconjugates  Glycoconjugates result from carbohydrates being linked to proteins and lipids  Proteoglycans  Heavily glycosylated proteins; a core protein with one or more GAG chains; high carbohydrate content (about 95%)  Occur on cell surfaces or are secreted to the extracellular matrix Figure 7.38 Proteoglycan Aggregate From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

46. Section 7.4: Glycoconjugates  Glycoproteins  Proteins covalently linked to carbohydrates through N- and O-linkages  Carbohydrate could be 1%–85% of total weight  Glycoproteins can act as cell surface receptors

47. Section 7.4: Glycoconjugates  The glycocalyx is a glycoprotein-polysaccharide covering that surrounds the cell membranes of some bacteria, epithelia and other cells. Most animal epithelial cells have a fuzz-like coat on the external surface of their plasma membranes. https://en.wikipedia.org/wiki/Glycocalyx