Carbohydrates

1. Carbohydrates 42Mo Andy HowardBiochemistry Lectures, Spring 2019 Tuesday 26 March 2019

2. Carbohydrates are more than just energy sources • We’ll introduce names carbohydrate monomers and oligomers • Carbohydrates can be cyclic or derivatized, and they can be oligomerized and polymerized. • Glycoconjugates are complexes involving sugar and peptide oligomers or polymers Carbohydrates

3. Carbohydrates Names & depictions Cyclic Forms Sugar Derivatives Acetals & ketals Glycosides Oligosaccharides Polysaccharides Storage Structural Glycoconjugates Proteoglycans Peptidoglycans Glycoproteins What we’ll discuss Carbohydrates

4. 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; or • If the chiral carbon is up, then it’s L if the OH is on the left Carbohydrates

5. Cyclic sugars • Aldoses with ≥ 4 carbons & ketoses with ≥ 5 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 • Remember that 5- and 6-atom rings form with little or no strain; 4- and 3- are much harder • There are no C=O bonds in the ring forms Carbohydrates

6. 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 • Note that there are four carbons in this 5-membered ring Carbohydrates

7. 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 • Note that there are 5 carbons in this 6-membered ring 4 pyran Carbohydrates

8. How do we cyclize a sugar? • Formation of an internal hemiacetal or hemiketal (see a few slides from here) by conversion of the terminal hydroxyl to a ring oxygen while the carbonyl becomes a hydroxyl • 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 Carbohydrates

9. 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) Carbohydrates

10. 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) Carbohydrates

11. Relative significance (?) Glucose, fructose, galactose,mannose, ribose, glyceraldehyde, xylose, arabinose Carbohydrates

12. Haworth projections • …provide a way of keeping track the chiral centers in a cyclic sugar, as the Fischer projections enable for straight-chain sugars (cf. fig. 16.9) Sir Walter Haworth Carbohydrates

13. 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 Carbohydrates

14. iClicker question #1 1. Which is the anomeric carbon in b-D-fructopyranose? • (a) 1 • (b) 2 • (c) 3 • (d) 6 • (e) none of the above. Carbohydrates

15. a-D-gluco-pyranose • One of 2 possible pyranose forms of D-glucose • There are two because the anomeric carbon itself becomes chiral when we cyclize • Anomeric carbon is not chiral in open-chain form Carbohydrates

16. b-D-glucopyranose • Differs froma-D-glucopyranose only at anomeric carbon Carbohydrates

17. Why is glucose special? • All aldohexoses are fairly similar • Glucose is the only aldohexose that, when built into the pyranose form, can be drawn with all its bulky substituents(–OH, –CH2OH) in equatorial positions • This minimizes steric clashes • This matters enough that some hexoses are actually marginally more stable as furanoses, e.g. fructose Carbohydrates

18. 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 Carbohydrates

19. 5-carbon example:cyclic D-arabinose • Note that the pyranose forms of 5-carbon sugars have no non-ring carbons Carbohydrates

20. Substituted monosaccharides (cf. §16.2) • Substitutions on the various positions retain some sugar-like character • Some substituted monosaccharides are building blocks of polysaccharides Carbohydrates

21. Specific substitutions O O- OH HO O HO O HO HO OH OH D-glucuronic acid(GlcUA) GlcNAc HNCOCH3 HO Amination, acetylamination common Reduction to alcohols, oxidation to carboxylates Carbohydrates

22. Sugar acids • 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: idosecarboxylated @ 6 D--gluconolactone Carbohydrates

23. iClicker question #2 2. Gluconic acid requires a 2-electron oxidation of carbon 1 of glucose. How many electrons of oxidation are required to convert glucose to glucuronic acid? • (a) 0 • (b) 1 • (c) 2 • (d) 4 • (e) none of the above. Carbohydrates

24. Sugar alcohols • Mild reduction of sugars convert aldehyde or ketone moiety to alcohol • Generates an additional asymmetric center in ketoses with > 3 carbons • These remain in open-chain forms • Smallest: glycerol • Sorbitol, myo-inositol, ribitol are important Carbohydrates

25. Sugar esters • Phosphate esters of sugars are significant metabolic intermediates • 5’, 3’ positions on ribose is phosphorylated in nucleotides • Hexoses are typically phosphorylated at 1 or 6 Glucose 6-phosphate Carbohydrates

26. Amino sugars • Hydroxyl at 2- position of hexoses is replaced with an amine group • Amine is often acetylated (CH3C=O) • These aminated and N-acetylated sugars are found in many polysaccharides and glycoproteins Carbohydrates

27. Hemiacetals and hemiketals • Hemiacetals and hemiketals are compounds that have an –OH and an –OR group on the same carbon • Cyclic monosaccharides are automatically hemiacetals & hemiketals (cf. fig. 16.7) Carbohydrates

28. Oligosaccharides and other glycosides (§16.3) • 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 Carbohydrates

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

30. 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 Carbohydrates

31. Lactose: another disaccharide • Lactose: -gal-glc • Milk sugar • Lactose intolerance caused by absence of enzyme capable of hydrolyzing this glycoside Carbohydrates

32. 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 Carbohydrates

33. Benedict’s test Stanley R. Benedict (courtesy Wikimedia) Benedict’s test:2Cu2+ + RCH=O + 5OH-Cu2O + RCOO- + 3H2O Cuprous oxide is red and insoluble Carbohydrates

34. 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. Carbohydrates

35. 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 Carbohydrates

36. Why does this matter? • Partly historical: this cuprate reaction was one of the first well-characterized tools for characterizing these otherwise very similar compounds • But it also gives us a convenient way of distinguishing among types of glycosidic arrangements, even if we never really use Cu2+ ions in experiments Carbohydrates

37. 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 Carbohydrates

38. Nucleosides • Anomeric carbon of ribose (or deoxyribose) is linked to nitrogen of RNA (or DNA) base (A,C,G,T,U) • Ribose is in furanose form • This is an example of an N-glycoside Adenosine Carbohydrates

39. Acetals and ketals • Acetals and ketals have two —OR groups on a single carbon • Acetals and ketals are found in glycosidic bonds, e.g. in oligosaccharides & nucleosides aldehyde alcohol hemiacetal acetal Carbohydrates

40. Polysaccharides (§16.4) • 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 Carbohydrates

41. Polysaccharides: homoglycans • Storage homoglycans (all monomers are glucose (Glc)) • Starch: amylose ((14)Glc) , amylopectin • Glycogen • Structural homoglycans • Cellulose ((14)Glc) • Chitin ((14)GlcNAc) • Others Carbohydrates

42. Polysaccharides: heteroglycans Found in glycosaminoglycans (disaccharide units) Hyaluronic acid:GlcUA,GlcNAc) ((1  3,4)) Glycosylations of proteins Carbohydrates

43. Storage polysaccharides • 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 • Typically 70% amylopectin, but it varies with the plant from which it’s taken • Glycogen: animals, some bacteria • Branches found in all but amylose Carbohydrates

44. Amylose (fig. 8.22) • Unbranched, a-14 linkages • Typically 100-3000 residues • Not soluble but can form hydrated micelles and may be helical • Amylases hydrolyze a-14 linkages Diagram courtesyLangara College Carbohydrates

45. Amylopectin • Mostly a-14 linkages; 4% a-16 • Each sidechain has 15-25 glucose moieties • a-16 linkages broken down by debranching enzymes • Usually 300-50000 total glucose units per amylopectin molecule • One reducing end, many nonreducing ends Carbohydrates

46. Glycogen • Principal storage form of glucose in human liver; some in muscle • Branched (a-14 + a few a-16) • More branches (~10%) • Often larger than starch: 50000 glucose • One reducing end,many nonreducing ends Carbohydrates

47. Preview: glycogen metabolism Broken down to G-1-P units Built up fromG-6-P  G-1-P  UDP-Glucose units Carbohydrates

48. Glycogen structure Carbohydrates

49. Structural polysaccharides I • Insoluble compounds designed to provide strength and rigidity • Cellulose: glucose b-14 linkages • Chitin: GlcNAcb-14 linkages:exoskeletons, cell walls Carbohydrates

50. Cellulose: details Rigid, flat structure: each glucose is upside down relative to its nearest neighbors 300-15000 glucose units Found in plant cell walls Resistant to most glucosidases Cellulases found in termites,ruminant gut bacteria Carbohydrates