Carbohydrates

1. Carbohydrates Chapter 8

2. Carbohydrates • Of the macromolecules that we will cover in this class, those involving carbohydrates are the most abundant in nature. • Via photosynthesis, over 100 billion metric tons of CO2 and H2O are converted into cellulose and other plant products. • The term carbohydrate is a generic one that refers primarily to carbon-containing compounds that contain hydroxyl, keto, or aldehydic functionalities. • Carbohydrates can range in sizes, from simple monosaccharides (sugars) to oligosaccharides, to polysaccharides.

3. Carbohydrates • Carbohydrates constitute more than 1/2 of organic molecules • Main role of carbos in nature • Storage of energy • Structural support • Lipid and protein modification: • membranes asymmetry, recognition by IgG/fertilization/virus recognition/cell cell communication Definition: Carbohydrates, Sugars and Saccharides- are all polyhydroxy • (at least 2 OH) Cn(H20) n = hydrate of carbon • Notice that there are two distinct types of monosaccharides, ketoses and aldoses. • The number of carbons is important in general nomenclature (triose = 3, pentose = 5, hexose =6,

4. Basic facts Monosaccharides - Simple sugars • Single polyhydroxyl • Can’t be hydrolyzed to simpler form Trioses - Smallest monosaccharides have three carbon atoms Tetroses (4C) Pentose (5C) Hexoses (6C) Heptoses (7C) etc… Disaccharide - two sugars linked together. Can be the same molecule or two different sugars. Attached together via a glycosidic linkage Oligosaccharide - 2 to 6 monosaccharides Polysaccharides - straight or branched long chain monosaccharides. Bonded together by glycosidic linkages

5. The functional groups • Aldehyde:Consists of a carbon atom bonded to a hydrogen atom and double-bonded to an oxygen atom. • Polar. Oxygen, more electronegative than carbon, pulls the electrons in the carbon-oxygen bond towards itself, creating an electron deficiency at the carbon atom. • Ketone:Characterized by a carbonyl group (O=C) linked to two other carbon atoms or a chemical compound that contains a carbonyl group • A carbonyl carbon bonded to two carbon atoms distinguishes ketones from carboxylic acids, aldehydes, esters, amides, and other oxygen-containing compounds

6. Classification of monosaccharides • Monosaccharides are classified according to three different characteristics: • the placement of its carbonyl group, • the number of carbon atoms it contains • its chiral handedness. • If the carbonyl group is an aldehyde, the monosaccharide is an aldose • if the carbonyl group is a ketone, the monosaccharide is a ketose. • Monosaccharides with three carbon atoms are called trioses, those with four are called tetroses, five are called pentoses, six are hexoses, and so on.  • These two systems of classification are often combined. • For example, glucose is an aldohexose (a six-carbon aldehyde)

7. carbonyl group • A functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. • The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic  complex.

8. Classification of monosaccharides • D-glucose  • is an aldohexose with the formula (C·H2O)6. • The red atoms highlight the aldehyde group • the blue atoms highlight the asymmetric center furthest from the aldehyde; because this -OH is on the right of the Fischer projection, this is a D sugar.

9. Classification of monosaccharides • The α and β anomers of glucose. • Note the position of the hydroxyl group (red or green) on the anomeric carbon relative to the CH2OH group bound to carbon 5: • Either on the opposite sides (α) • Or the same side (β).

10. Steriochemistry - optically active molecules D or L as designated by furthest asymmetric carbon from ketone or aldehyde optical activity is independent of D or L designation D(-) fructose Increase in number of carbons increase possible stereoisomers: Van’t Hoff’s Rule: A compound with n asymmetric C atoms has a maximum of 2n possible stereoisomers Enantiomers: Stereoisomers that are non superimposable mirror images ex. L and D forms of sugars D-glyceraldehyde and L-glyceraldehyde

11. Chirality in Monosaccharides • Most simple monosaccharides have at least one chiral center. As in the case of amino acids, sugars are given D or L designations based on their similarity with D or L glyceraldehyde (shown on left). • Since some sugars contain many chiral centers, it is necessary to designate one chiral center that will act as the reference. This chiral center is designated as the one that is most distant from the carbon that bears the carbonyl functionality.

12. Chirality in Monosaccharides • Sugars are frequently written as Fischer projections. Remember that atoms that lie on horizontal bonds are projecting towards you, while those on vertical bonds are projecting away from you. • Compare the top and bottom structures for glyceraldehyde. • The numbering of carbons in sugars begins at the end of the chain that is closer to the carbonyl functionality.

13. If the carbonyl group is an aldehyde the sugar is an aldose. • Arrows indicate Sterochemical relationships • Configuration around C2 (red) distinguishes the members of each pair of monosaccharides • Most common aldoses: • D-Ribose (Rib) • D-Glucose (Glc) • D-Mannose (Man) • D-Galactose (Gal) • D-Xylose (Xyl) • D-Arabinose (Ara) • ALL BASED ON THE STRUCTURE OF GLYCERALDEHYDE

14. If the carbonyl group is an ketone the sugar is an ketose • Arrows indicate Sterochemical relationships • Configuration around C3 (red) distinguishes the members of each pair of monosaccharides • Most common Ketoses: • D-Fructose (Fru) • D-Ribulose (Rib) • ALL BASED ON THE STRUCTURE OF Dihydroxyacetone

15. Epimers • Epimers are two sugars that differ only in the configuration around one carbon atom of their structures. • D-Mannose differs from D-glucose only in its configuration around carbon 2. • D-Galactose differs from D-glucose only in its configuration around carbon 4. • D-Galactose and D-Mannose are not epimers.

16. Conformational Structures Emil Fisher - Nobel Prize 1891 Organic chemist who found the structure of D glucose Fisher projections - place most oxidized carbon on top Haworth Structures: carbons counted from anomeric C to clockwise from the oxygen in the ring (pyranose) or the #2 C for furanose • OH in down position - a form • OH in up position - ß form

17. Cyclization of Monosaccharides • Carbohydrates of four carbons and more are found in cyclic forms. How? • Aldehydes and ketones react reversibly with -OH on sugars • 5 or 6 carbon rings most stable • 4 carbon membered ring furan - ribose • 5 carbon membered ring pyran - glucose - straight chains form hemiacetals and hemi ketals

18. Carbohydrates of four carbons and more are found in cyclic forms How? • C=O at reference carbon is reduced to OH • Formation of ring structure leads to an additional chiral carbon (anomeric carbon) This is the number one carbon in the pyranose structures, #2 for the furanoses

20. Cyclization of Monosaccharides • In hexoses, an attack of the hydroxyl group at C-5 onto the aldehyde functionality generates a cyclic structure containing six substituents. • One of the substituents is an oxygen, which acted as the nucleophile in the reaction.

21. Cyclization of Monosaccharides • Notice that two different stereochemical outcomes are possible, at the new chiral center that is generated (the hemiacetal). • The hydroxyl group can be on the same side (cis) of the ring as the CH2OH moiety (beta configuration), or it can be on the opposite side (trans) of the ring as the CH2OH moiety (alpha configuration). • Notice the formal names for each of the two possible configurations.

22. D sugar ring structures form and reform so there is an intra-conversion between the a and ß form. This is called mutarotation. Mutorotation occurs spontaneously or with the help of an enzyme (generally called a mutase) • The hemi groups are ordinarily unstable and revert back to the straight chain form

23. In solution: D-glucose (0.02%) a-D Glucopyranoside (37%) and ß-D glucopyranoside (64%) less than 1% of the furans • pyranose and furanose are the basis for nomenclature; glucose refers to the mixtures of the different forms. • Beta form - OH above plane of ring • Alpha form - OH below plane of ring

25. Reactions of monosaccharides Reducing sugars - a reduction reaction at the aldehyde or ketone groups of the sugar molecules. As sugar is being oxidized, something else is being reduced: Only those anomeric carbons that can mutorotate are available for reduction. ie. disaccarides only contain 1 reducing sugar • Formation of a carboxylic acid from an aldehyde is a 2-electron oxidation. • Notice that the sugar acts as the reductant of copper. Copper is reduced from the +2 state to the +1 state; however, the sugar is oxidized by 2 electrons. • In the presence of Benedict’s reagent (sodium citrate, sodium carbonate, and copper sulfate, reducing sugars will produce a brick-red precipitate of cuprous oxide. • There are enzymatic methods that can be used to quantify reducing sugars such as glucose, which forms the basis of blood sugar detection by diabetics.

26. Reactions of monosaccharides Phosphorylation - can form anhydride phosphoester bond. phosphorylation alters ionic character. • Locks molecule in cell. • Nucleotides are phosphorylated (ATP, GTP…)

27. Reactions of monosaccharides Deoxy sugars - without oxygen common case - ribose RNA vs. DNA

28. Reactions of monosaccharides Amino sugars - Sugars with OH replaced by NH2. Amination usually occurs at C2 and often is acetylated Glycosaminoglycans both amino and sulfated sugars. Heparin inhibits action of thrombin in blood clotting - sugars as this will often have added C chains (acylated), sulfur groups (sulfanated) and aminated

29. Important monosaccharides • Glucose - preferred source of energy for brain cells and cells without mitochondria • Fructose - ketose, 2x as sweet as sucrose. Sperm use this as major sugar/energy source for motility • Galactose - important for lacotose and glycolipid production • galactosemia - genetic disorder in galactose metabolism leads to accumulation of galactose-1-phosphate in liver results in liver damage. Another version of the disease results due to lack of galactose metabolism. Galactose concentration builds up in blood leading to cataracts. - Can result in severe mental retardation. Identification and galactose free diet helps

30. Modification of monosaccharides • A uronic acid is a sugar acid with both a carbonyl and a carboxylic acid function. • It is best thought of as a sugar in which the terminal carbon's hydroxyl function has been oxidized to a carboxylic acid • Some of these compounds have important biochemical functions; for example, many wastes in the human body are excreted in the urine as their glucuronate salts • Yes – peeing on plants is good for them!!!!!!!!! D-glucose a-D-glucuronic acid

31. Disaccharide Nomenclature • When named, structures are considered to have their reducing ends on the right. Locate the reducing ends of the structures on the left if appropriate. • The configuration of the anomeric carbon joining the first monosaccharide unit to the second is given (reading left to right). • The non-reducing residue is named, and five-and six-membered ring structures are distinguished by using “furano” or “pyrano” prefixes. • Non-reducing disaccharides are named as glycosides rather than glycoses. Note that a double-headed arrow is used to denote sugars that are joined by their anomeric carbons, AND, it is necessary to specifiy the stereochemistry at both anomeric carbons.

32. Disaccharide Nomenclature • The two carbons joined by the glycosidic bond are indicated in parentheses, with an arrow connecting the two numbers. • The second residue is then named. • If there are subsequent residues, the subsequent glycosidic bonds are described by the same conventions. • Non-reducing disaccharides are named as glycosides rather than glycoses. Note that a double-headed arrow is used to denote sugars that are joined by their anomeric carbons, AND, it is necessary to specifiy the stereochemistry at both anomeric carbons.

33. Important Disaccharides • sucrose = table sugar - glucose and fructose (alpha linkage) • lactose = milk sugar - galactose and glucose (beta linkage) • Maltose - malt sugar, from breakdown of starch - 2 glucoses

34. Lactose intolerance • inability to metabolize lactose • lack of the required enzyme lactase in the digestive system. • estimated that 75% of adults worldwide show some decrease in lactase activity during adulthood. • The frequency of decreased lactase activity ranges from as little as 5% in northern Europe, up to 71% for Sicily, to more than 90% in some African and Asian countries

35. Lactose intolerance • Disaccharides cannot be absorbed through the wall of the small intestine into the bloodstream • so in the absence of lactase • lactose present in ingested dairy products remains uncleaved and passes intact into the colon. • Remember that there are bacteria in the human digestive system! • Remember the LAC operon?

36. Lactase • Part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers. • Lactase is present predominantly along the brush border membrane of the differentiated enterocytes lining the villi of the small intestine.

37. The LAC operon beta-galactosidase: This enzyme hydrolyzes the bond between the two sugars, glucose and galactose. It is coded for by the gene LacZ. Lactose Permease: This enzyme spans the cell membrane and brings lactose into the cell from the outside environment. The membrane is otherwise essentially impermeable to lactose. It is coded for by the gene LacY. Thiogalactoside transacetylase: The function of this enzyme is not known. It is coded for by the gene LacA.

38. Lactose intolerance • The operons of enteric bacteria quickly switch over to lactose metabolism, and results in in vivo fermentation.  • produces copious amounts of gas (a mixture of hydrogen, carbon dioxide, and methane). • This, in turn, may cause a range of abdominal symptoms, including stomach cramps, nausea, bloating, acid reflux and flatulence. • In addition, as with other unabsorbed sugars (such as sorbitol, mannitol, and xylitol), the presence of lactose and its fermentation products raises the osmotic pressure of the colon contents.

39. Lactose intolerance • Symptoms • Abdominal bloating • Abdominal cramps • Diarrhea • Floating stools • Foul-smelling stools • Gas (flatulence) • Malnutrition • Nausea • Slow growth • Weight loss

40. Lactose intolerance • Most people with low lactase levels can tolerate 2 - 4 ounces of milk at one time (up to one-half cup). Larger (8 oz.) servings may cause problems for people with some amount of milk intolerance. • These milk products may be easier to digest: • Buttermilk and cheeses (they have less lactose than milk) • Fermented milk products, such as yogurt • Goat's milk (but drink it with meals, and make sure it is supplemented with essential amino acids and vitamins if you give it to children) • Ice cream, milkshakes, and aged or hard cheeses • Lactose-free milk and milk products • Lactase-treated cow's milk for older children and adults • Soy formulas for infants younger than 2 years • Soy or rice milk for toddlers

41. Sucralose • zero-calorie artificial sweetener • Approximately 600 times as sweet as sucrose • It is stable under heat and over a broad range of pH conditions • Belongs to a class of compounds known as organochlorides (or chlorocarbons). Some organochlorides, particularly those that accumulate in fatty tissues, are toxic to plants or animals, including humans. • Sucralose, however, is not known to be toxic in small quantities and is extremely insoluble in fat; it cannot accumulate in fat like chlorinated hydrocarbons. • In addition, sucralose does not break down or dechlorinate

42. Sucralose • Some concern has been raised about the effect of sucralose on the thymus. •  Two studies on rats, both of which found "a significant decrease in mean thymus weight" at high doses. • The sucralose dose which caused the effects was 3000 mg/kg/day for 28 days. • For a 150 lb (68.2 kg) human, this would mean an intake of nearly 205 grams of sucralose a day, which is equivalent to more than 17,200 individual Splenda packets/day for approximately one month

43. Important Polysaccharides: Starch - energy reservoir in plants - made of two polysaccharides Amylose -long unbranched glucose a (1,4) with open reducing end large tight helical forms. Test by iodination..

44. Important Polysaccharides: Starch - energy reservoir in plants - made of two polysaccharides • Amylose -long unbranched glucose a (1,4) with open reducing end large tight helical forms. Test by iodination. • Amylopectin - polymer of a(1,4) and a (1,6) branches. Not helical.

45. Glycogen - storage of carbohydrates in vertebrates greatest conc. in muscles and liver. • Similar to amylopectin with more branch points - takes up less space

46. Cellulose • Linear glucan chains of unbranched (1-4)-b-linked-D-glucose in which every other glucose residue is rotated 180° with respect to its two neighbors and contrasts with other glucan polymers such as: • starch (1-4-a-glucan) • callose (1-3-b-glucan).

47. Cellulose • This means that cellobiose, and not glucose, is the basic repeating unit of the cellulose molecule. Groups of 30 to 40 of these chains laterally hydrogen-bond to form crystalline or para-crystalline microfibrils.

48. Chitin • Chitin is a linear homopolysaccharide composed of N-acetylglucosamine residues in b linkages. • Chitin differs chemically from cellulose only in the acetylated amino substituent at carbon 2. • It forms extended fibers that are similar to those of cellulose, and is found principally in hard exoskeletons of arthropods. • Also occurs naturally in both parallel and antiparallel stacking arrangements.

49. Glycoproteins • proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. • The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. • In proteins that have segments extending extracellularly, the extracellular segments are often glycosylated. • Glycoproteins are often important integral membrane proteins, where they play a role in cell-cell interactions. • Glycoproteins also occur in the cytosol, but their functions and the pathways producing these modifications in this compartment are less well-understood

50. Glycoproteins • There are two types of glycoproteins: • In N-glycosylation , the addition of sugar chains can happen at the amide nitrogen on the side chain of  asparagine. • In O-glycosylation, the addition of sugar chains can happen on the hydroxyl oxygen on the side chain of hydroxylysine, hydroxyproline, serine, or threonine. asparagine hydroxyproline