Carbohydrates

1. Carbohydrates Andy HowardIntroductory Biochemistry, Fall 200816 September 2008 Biochemistry: Carbohydrates

2. Now we’ll study sugars! • Sugars are vital as energy sources, and they also serve as building blocks for lipid-carbohydrate and protein-carbohydrate complexes Biochemistry: Carbohydrates

3. Notes about upcoming midterm Sugar Concepts Monosaccharides Oligosaccharides Glycosides Polysaccharides Starch & glycogen Cellulose and chitin Glycoconjugates Proteoglycans Peptidoglycans Glycoproteins What we’ll discuss Biochemistry: Carbohydrates

4. Midterm is Tuesday 23 Sep • Internet students can take it between 9am Tuesday and 5pm Wednesday • Find a proctor or arrange to take it in class • Details about how the midterm works are in the Course Introduction document Biochemistry: Carbohydrates

5. What the midterm will cover • Everything up through today’s lecture • Thursday’s lecture will be on the second midterm • Exam syllabus will be posted by the weekend to help you study • Exam help-sheet too (don’t memorize what’s on the help sheet!) • Yes, I curve these exams; but the grade cutoffs are determined at the end of the course, not now Biochemistry: Carbohydrates

6. Carbohydrates • These are polyhydroxylated aldehydes and ketones, many of which can exist in cyclic forms • General monomeric formula (CH2O)m, 3 < m < 9 • With one exception (dihydroxyacetone) they contain chiral centers • Highly soluble • Can be oligomerized and polymerized • Oligomers may or may not be soluble • Most abundant organic molecules on the planet Biochemistry: Carbohydrates

7. How do we measure solubility for very soluble compounds? • (Note: this is not a serious chemical topic: it’s an example of how statistics can be abused…) • The assertion is that, with highly soluble compounds like sugars, it’s difficult to use conventional approaches to compare their solubilities • The suggestion is that we might use the amount of time it takes to dissolve (for example) 50g of solute in 100mL of cold water: if it’s fast, the solute is more soluble than if it’s slow. Biochemistry: Carbohydrates

8. Solubility measured by dissolution time 6 • Assertion: more polar groups means shorter dissolution time for a given class of compounds 5 4 3 # of polar groups 2 1 Time required for dissolution Biochemistry: Carbohydrates

9. What if we extrapolate to n=6? Extrapolated point 6 • We get a negative dissolution time! • That is, the solid goes into solution 6 seconds before we put it in the water! • This causes serious psychological problems (what if I change my mind?) and philosophical problems (is this pre-ordained?) 5 4 3 # of polar groups Observed points 2 1 Time required for dissolution Biochemistry: Carbohydrates

10. Whose idea is this? • Isaac Asimov, that’s who! • “The endochronic properties of resublimated thiotimolene”:Astounding Science Fiction, March1948 • My point: extrapolations and other misuses of statistics are dangerous • Benjamin Disraeli (popularized by Mark Twain):There are three kinds of untruth:lies, damn lies, and statistics. • Okay: let’s get back to the science. Biochemistry: Carbohydrates

11. Aldoses & ketoses • If the carbonyl moiety is at the end carbon (conventionally counted as 1), it’s an aldose • If carbonyl is one carbon away (counted as 2), it’s a ketose • If it’s two or more carbons from the end of the chain, it’s not a sugar Biochemistry: Carbohydrates

12. Simplest monosaccharides • Glyceraldehyde and dihydroxyacetone • Only glyceraldehyde is chiral:D-enantiomer is more plentiful in biosphere • All longer sugars can be regarded as being built up by adding-(CHOH)m-1 to either glyceraldehyde or dihydroxyacetone, just below C2 Biochemistry: Carbohydrates

13. How many aldoses are there? • Every -(CHOH) in the interior offers one chiral center • An m-carbon aldose has (m-2) internal -(CHOH) groups • Therefore: 2m-2 aldoses of length m • For m=3, that’s 21=2; for m=6, it’s 24=16. Biochemistry: Carbohydrates

14. How many ketoses are there? • Every -(CHOH) in the interior offers one chiral center • An m-carbon ketose has (m-3) internal-(CHOH) groups • Therefore: 2m-3 ketoses of length m • For m=3, that’s 20 = 1; for m=6, that’s 23=8. Biochemistry: Carbohydrates

15. Review: stereochemical nomenclature • Stereoisomers: compounds with identical covalent bonding apart from chiral connectivity • Enantiomers: compounds for which the opposite chirality applies at all chiral centers • Epimers: compounds that differ in chirality at exactly one chiral center • One chiral center: enantiomers are epimers. • > 1 chiral center: enantiomers are not epimers. Biochemistry: Carbohydrates

16. Example: 2 chiral centers • Chiral centers u,v; compounds A,B,C,D Biochemistry: Carbohydrates

17. Properties • Enantiomers have identical physical properties (MP,BP, solubility, surface tension…) except when they interact with other chiral molecules • (Note!: water isn’t chiral!) • Stereoisomers that aren’t enantiomers can have different properties; therefore, they’re often given different names Biochemistry: Carbohydrates

18. Sugar nomenclature • All sugars with m ≤ 7 have specific names apart from their enantiomeric(L or D) designation,e.g. D-glucose, L-ribose. • The only 7-carbon sugar that routinely gets involved in metabolism is sedoheptulose, so we won’t try to articulate the names of the others Biochemistry: Carbohydrates

19. Fischer projections Emil Fischer • Convention for drawing open-chain monosaccharides • If the hydroxyl comes off counterclockwise relative to the previous carbon, we draw it to the left; • Clockwise to the right. Biochemistry: Carbohydrates

20. Cyclic sugars • Sugars with at least four carbons can readily interconvert between the open-chain forms we have drawn and five-membered(furanose) or six-membered (pyranose) ring forms in which the carbonyl oxygen becomes part of the ring • There are no C=O bonds in the ring forms Biochemistry: Carbohydrates

21. 1 Furanoses 5 2 4 3 furan • Formally derived from structure of furan • Hydroxyls hang off of the ring; stereochemistry preserved there • Extra carbons come off at 2 and 5 positions Biochemistry: Carbohydrates

22. 1 Pyranoses 6 2 3 5 • Formally derived from structure of pyran • Hydroxyls hang off of the ring; stereochemistry preserved there • Extra carbons come off at 2 and 6 positions 4 pyran Biochemistry: Carbohydrates

23. How do we cyclize a sugar? • Formation of an internal hemiacetal or hemiketal (see a few slides from here) by conversion of the carbonyl oxygen to a ring oxygen • Not a net oxidation or reduction;in fact it’s a true isomerization. • The molecular formula for the cyclized form is the same as the open chain form Biochemistry: Carbohydrates

24. Family tree of aldoses • Simplest: D-, L- glyceraldehyde (C3) • Add —CHOH: D,L-threose, erythrose (C4) • Add —CHOH:D,L- lyxose, xylose, arabinose, ribose (C5) • Add —CHOH:D,L-talose, galactose, idose, gulose,mannose, glucose, altrose, allose (C6) Biochemistry: Carbohydrates

25. Family tree of ketoses • Simplest: dihydroxyacetone (C3) • Add —CHOH: D,L-erythrulose (C4) • Add —CHOH:D,L- ribulose, xylulose(C5) • Add —CHOH:D,L-sorbose, tagatose, fructose, psicose (C6) Biochemistry: Carbohydrates

26. Haworth projections • …provide a way of keeping track the chiral centers in a cyclic sugar, as the Fischer projections enable for straight-chain sugars Sir Walter Haworth Biochemistry: Carbohydrates

27. O The anomeric carbon C O • In any cyclic sugar (monosaccharide, or single unit of an oligosaccharide, or polysaccharide) there is one carbon that has covalent bonds to two different oxygen atoms • We describe this carbon as the anomeric carbon Biochemistry: Carbohydrates

28. iClicker quiz, question 1 • Which of these is a furanose sugar? Biochemistry: Carbohydrates

29. iClicker quiz, question 2 • Which carbon is the anomeric carbon in this sugar? • (a) 1 • (b) 2 • (c) 5 • (d) 6 • (e) none of these. Biochemistry: Carbohydrates

30. iClicker, question 3 • How many 7-carbon D-ketoses are there? • (a) none. • (b) 4 • (c) 8 • (d) 16 • (e) 32 Biochemistry: Carbohydrates

31. a-D-glucopyranose • One of 2 possible pyranose forms of D-glucose • There are two because the anomeric carbon itself becomes chiral when we cyclize Biochemistry: Carbohydrates

32. b-D-glucopyranose • Differs from a-D-gluco-pyranose only at anomeric carbon Biochemistry: Carbohydrates

33. Count carefully! • It’s tempting to think that hexoses are pyranoses and pentoses are furanoses; • But that’s not always true • The ring always contains an oxygen, so even a pentose can form a pyranose • In solution: pyranose, furanose, open-chain forms are all present • Percentages depend on the sugar Biochemistry: Carbohydrates

34. Substituted monosaccharides • Substitutions on the various positions retain some sugar-like character • Some substituted monosaccharides are building blocks of polysaccharides • Amination, acetylamination, carboxylation common O O- OH HO O HO O HO HO OH OH D-glucuronic acid(GlcUA) GlcNAc HNCOCH3 HO Biochemistry: Carbohydrates

35. 6 5 Sugar acids (fig. 7.10) 4 1 D--gluconolactone 2 3 • Gluconic acid: • glucose carboxylated @ 1 position • In equilibrium with lactone form • Glucuronic acid:glucose carboxylated @ 6 position • Glucaric acid:glucose carboxylated @ 1 and 6 positions • Iduronic acid: idose carboxylated @ 6 Biochemistry: Carbohydrates

36. Sugar alcohols (fig.7.11) • Mild reduction of sugars convert aldehyde moiety to alcohol • Generates an additional asymmetric center in ketoses • These remain in open-chain forms • Smallest: glycerol • Sorbitol, myo-inositol, ribitol are important Biochemistry: Carbohydrates

37. Sugar esters (fig. 7.13) Glucose 6-phosphate • Phosphate esters of sugars are significant metabolic intermediates • 5’ position on ribose is phosphorylated in nucleotides Biochemistry: Carbohydrates

38. OH Amino sugars HO O HO OH GlcNAc HNCOCH3 • Hydroxyl at 2- position of hexoses is replaced with an amine group • Amine is often acetylated (CH3C=O) • These aminated sugars are found in many polysaccharides and glycoproteins Biochemistry: Carbohydrates

39. Acetals and ketals • Hemiacetals and hemiketals are compounds that have an –OH and an –OR group on the same carbon • Cyclic monosaccharides are hemiacetals & hemiketals • Acetals and ketals have two —OR groups on a single carbon • Acetals and ketals are found in glycosidic bonds Biochemistry: Carbohydrates

40. Oligosaccharides and other glycosides • A glycoside is any compound in which the hydroxyl group of the anomeric carbon is replaced via condensation with an alcohol, an amine, or a thiol • All oligosaccharides are glycosides, but so are a lot of monomeric sugar derivatives, like nucleosides Biochemistry: Carbohydrates

41. Sucrose: a glycoside • A disaccharide • Linkage is between anomeric carbons of contributing monosaccharides, which are glucose and fructose Biochemistry: Carbohydrates

42. Other disaccharides • Maltose • glc-glc with -glycosidic bond from left-hand glc • Produced in brewing, malted milk, etc. • Cellobiose • -glc-glc • Breakdown product from cellulose • Lactose: -gal-glc • Milk sugar • Lactose intolerance caused by absence of enzyme capable of hydrolyzing this glycoside Biochemistry: Carbohydrates

43. Reducing sugars • Sugars that can undergo ring-opening to form the open-chain aldehyde compounds that can be oxidized to carboxylic acids • We describe those as reducing sugars because they can reduce metal ions or amino acids in the presence of base • Benedict’s test:2Cu2+ + RCH=O + 5OH-Cu2O + RCOO- + 3H2O • Cuprous oxide is red and insoluble Biochemistry: Carbohydrates

44. Ketoses are reducing sugars • In presence of base a ketose can spontaneously rearrange to an aldose via an enediol intermediate, and then the aldose can be oxidized. Biochemistry: Carbohydrates

45. Sucrose: not a reducing sugar • Both anomeric carbons are involved in the glycosidic bond, so they can’t rearrange or open up, so it can’t be oxidized • Bottom line: only sugars in which the anomeric carbon is free are reducing sugars Biochemistry: Carbohydrates

46. Reducing & nonreducing ends • Typically, oligo and polysaccharides have a reducing end and a nonreducing end • Non-reducing end is the sugar moiety whose anomeric carbon is involved in the glycosidic bond • Reducing end is sugar whose anomeric carbon is free to open up and oxidize • Enzymatic lengthening and degradation of polysaccharides occurs at nonreducing end or ends Biochemistry: Carbohydrates

47. Nucleosides • Anomeric carbon of ribose (or deoxyribose) is linked to nitrogen of RNA (or DNA) base (A,C,G,T,U) • Generally ribose is in furanose form • This is an example of an N-glycoside Diagram courtesy of World of Molecules Biochemistry: Carbohydrates

48. Polysaccharides • Homoglycans: all building blocks same • Heteroglycans: more than one kind of building block • No equivalent of genetic code for carbohydrates, so long ones will be heterogeneous in length and branching, and maybe even in monomer identity Biochemistry: Carbohydrates

49. Categories of polysaccharides • Storage homoglycans (all Glc) • Starch: amylose ((14)Glc) , amylopectin • Glycogen • Structural homoglycans • Cellulose ((14)Glc) • Chitin ((14)GlcNac) • Heteroglycans • Glycosaminoglycans (disacch.units) • Hyaluronic acid (GlcUA,GlcNAc)((1  3,4)) Biochemistry: Carbohydrates

50. Storage polysaccharides • Available sources of glucose for energy and carbon • Long-chain polymers of glucose • Starch (amylose and amylopectin):in plants, it’s stored in 3-100 µm granules • Glycogen • Branches found in all but amylose Biochemistry: Carbohydrates