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Carbohydrates II

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  1. Carbohydrates II Andy HowardIntroductory Biochemistry, Fall 2010 16 September 2010 Biochem: Carbohydrates II

  2. Mono-, oligo- and polysaccharides • These are the most abundant organic molecules on the planet, and they act as metabolites, components of complexes, and structural entities Biochem: Carbohydrates II

  3. Details of monosaccharide nomenclature Cyclic sugars Sugar derivatives Glycosides Polysaccharides Starch & glycogen Cellulose & chitin Glycoconjugates Proteoglycans Peptidoglycans Glycoproteins What we’ll discuss Biochem: Carbohydrates II

  4. Monosaccharide structures • Remember that there is just one 3-carbon ketose and two 3-carbon aldoses • Addition of each –CHOH group gives us one more chiral center • Unique names for each enantiomorphic monosaccharide Biochem: Carbohydrates II

  5. 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 Biochem: Carbohydrates II

  6. 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 Biochem: Carbohydrates II

  7. 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. Biochem: Carbohydrates II

  8. D-aldose family tree Biochem: Carbohydrates II

  9. D-ketose family tree Biochem: Carbohydrates II

  10. How many of these are important? • D-sugars are more prevalent than L-sugars • 3-, 5-, and 6-carbon sugars are the most important, but 4’s and 7’s play roles • Some 5’s and 6’s are obscure • Glucose, ribose, fructose, glyceraldehyde play more important roles than the others Biochem: Carbohydrates II

  11. 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 Biochem: Carbohydrates II

  12. Hemiacetals & hemiketals • Hemiacetals and hemiketals are compounds that have an –OH and an –OR group on the same carbon • Cyclic monosaccharides are hemiacetals & hemiketals Biochem: Carbohydrates II

  13. How do we cyclize a sugar? • Formation of an internal hemiacetal or hemiketal (see previous slide) 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 • Very low energy barriers between open-chain form and various cyclic forms Biochem: Carbohydrates II

  14. 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 Biochem: Carbohydrates II

  15. 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 Biochem: Carbohydrates II

  16. 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 Biochem: Carbohydrates II

  17. 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 Biochem: Carbohydrates II

  18. iClicker quiz, question 1 • Which of these is a furanose sugar? Biochem: Carbohydrates II

  19. 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. Biochem: Carbohydrates II

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

  21. 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 Biochem: Carbohydrates II

  22. b-D-glucopyranose • Differs from a-D-gluco-pyranose only at anomeric carbon Biochem: Carbohydrates II

  23. 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 Biochem: Carbohydrates II

  24. 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 Biochem: Carbohydrates II

  25. 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 Biochem: Carbohydrates II

  26. 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 Biochem: Carbohydrates II

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

  28. OH Amino sugars HO O HO OH HNCOCH3 GlcNAc • 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 Biochem: Carbohydrates II

  29. Acetals and ketals • Acetals and ketals have two —OR groups on a single carbon • Acetals and ketals are found in glycosidic bonds Biochem: Carbohydrates II

  30. 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 Biochem: Carbohydrates II

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

  32. 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 Biochem: Carbohydrates II

  33. 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 Biochem: Carbohydrates II

  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. Biochem: Carbohydrates II

  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 Biochem: Carbohydrates II

  36. 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 Biochem: Carbohydrates II

  37. 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 Biochem: Carbohydrates II

  38. Glycosides • Glycosides are covalent conjugates of a sugar with another species • Generally involve replacement of a sugar –OH group with a moiety that begins with an oxygen or a nitrogen • We describe them as N-linked and O-linked glycosides Biochem: Carbohydrates II

  39. 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 Biochem: Carbohydrates II

  40. 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 Biochem: Carbohydrates II

  41. 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)) Biochem: Carbohydrates II

  42. 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 Biochem: Carbohydrates II

  43. Amylose • Unbranched, a-14 linkages • Typically 100-1000 residues • Not soluble but can form hydrated micelles and may be helical • Amylases hydrolyze a-14 linkages Diagram courtesyLangara College Biochem: Carbohydrates II

  44. 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 • 300-6000 total glucose units per amylopectin molecule • One reducing end, many nonreducing ends Biochem: Carbohydrates II

  45. Glycogen • Principal storage form of glucose in human liver; some in muscle • Branched (a-14 + a few a-16) • More branches (~10%) • Larger than starch: 50000 glucose • One reducing end,many nonreducing ends • Broken down to G-1-P units • Built up fromG-6-P  G-1-P  UDP-Glucose units Biochem: Carbohydrates II

  46. Glycogen structure Biochem: Carbohydrates II

  47. Structural polysaccharides I • Insoluble compounds designed to provide strength and rigidity • Cellulose: glucose b-14 linkages • Rigid, flat structure: each glucose is upside down relative to its nearest neighbors (fig.7.27) • 300-15000 glucose units • Found in plant cell walls • Resistant to most glucosidases • Cellulases found in termites,ruminant gut bacteria • Chitin: GlcNAc b-14 linkages:exoskeletons, cell walls (fig. 7.26) Biochem: Carbohydrates II

  48. Structural polysaccharides II • Alginates: poly(-D-mannuronate),poly(-L-guluronate), linked 14 • Cellulose-like structure when free • Complexed to metal ions:3-fold helix (“egg-carton”) • Agarose: alternating D-gal, 3,6-anhydro-L-gal, with 6-methyl-D-gal side chains • Forms gels that hold huge amounts of H2O • Can be processed to use in the lab for gel exclusion chromatography • Glycosaminoglycans: see next section Biochem: Carbohydrates II