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Carbohydrates II Andy Howard Introductory Biochemistry, Fall 2010 16 September 2010 Biochem: Carbohydrates II 09/16/2010 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 09/16/2010 Biochem: Carbohydrates II p. 2 of 48 What we’ll discuss Details of monosaccharide nomenclature Cyclic sugars Sugar derivatives Glycosides 09/16/2010 Polysaccharides Starch & glycogen Cellulose & chitin Glycoconjugates Biochem: Carbohydrates II Proteoglycans Peptidoglycans Glycoproteins p. 3 of 48 Monosaccharide structures Remember that there is just one 3carbon ketose and two 3-carbon aldoses Addition of each –CHOH group gives us one more chiral center Unique names for each enantiomorphic monosaccharide 09/16/2010 Biochem: Carbohydrates II p. 4 of 48 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 09/16/2010 Biochem: Carbohydrates II p. 5 of 48 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 09/16/2010 Biochem: Carbohydrates II p. 6 of 48 Fischer projections Convention for drawing openchain monosaccharides If the hydroxyl comes off counterclockwise relative to the previous carbon, we draw it to the left; Clockwise to the right. 09/16/2010 Biochem: Carbohydrates II Emil Fischer p. 7 of 48 D-aldose family tree 09/16/2010 Biochem: Carbohydrates II p. 8 of 48 D-ketose family tree 09/16/2010 Biochem: Carbohydrates II p. 9 of 48 How many of these are important? D-sugars are more prevalent than Lsugars 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 09/16/2010 Biochem: Carbohydrates II p. 10 of 48 Cyclic sugars Sugars with at least four carbons can readily interconvert between the openchain forms we have drawn and fivemembered(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 09/16/2010 Biochem: Carbohydrates II p. 11 of 48 Hemiacetals & hemiketals Hemiacetals and hemiketals are compounds that have an –OH and an –OR group on the same carbon Cyclic monosaccharides are hemiacetals & hemiketals 09/16/2010 Biochem: Carbohydrates II p. 12 of 48 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 openchain form and various cyclic forms 09/16/2010 Biochem: Carbohydrates II p. 13 of 48 Furanoses Formally derived from structure of furan Hydroxyls hang off of the ring; stereochemistry preserved there Extra carbons come off at 2 and 5 positions 09/16/2010 Biochem: Carbohydrates II 1 5 2 4 3 furan p. 14 of 48 1 Pyranoses 6 Formally derived from structure of pyran Hydroxyls hang off of the ring; stereochemistry preserved there Extra carbons come off at 2 and 6 positions 09/16/2010 Biochem: Carbohydrates II 2 3 5 4 pyran p. 15 of 48 Haworth projections …provide a way of keeping track the chiral centers in a cyclic sugar, as the Fischer projections enable for straight-chain sugars 09/16/2010 Biochem: Carbohydrates II Sir Walter Haworth p. 16 of 48 O The anomeric carbon C 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 09/16/2010 Biochem: Carbohydrates II O p. 17 of 48 iClicker quiz, question 1 Which of these is a furanose sugar? 09/16/2010 Biochem: Carbohydrates II p. 18 of 48 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. 09/16/2010 Biochem: Carbohydrates II p. 19 of 48 iClicker, question 3 How many 7-carbon D-ketoses are there? (a) none. (b) 4 (c) 8 (d) 16 (e) 32 09/16/2010 Biochem: Carbohydrates II p. 20 of 48 a-Dglucopyranose One of 2 possible pyranose forms of Dglucose There are two because the anomeric carbon itself becomes chiral when we cyclize 09/16/2010 Biochem: Carbohydrates II p. 21 of 48 b-Dglucopyranose Differs from aD-glucopyranose only at anomeric carbon 09/16/2010 Biochem: Carbohydrates II p. 22 of 48 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, openchain forms are all present Percentages depend on the sugar 09/16/2010 Biochem: Carbohydrates II p. 23 of 48 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 OOH HO HO HO O GlcNAc HNCOCH OH 3 09/16/2010 HO D-glucuronic acid HO (GlcUA) Biochem: Carbohydrates II O OH p. 24 of 48 6 Sugar acids (fig. 7.10) Gluconic acid: 5 4 1 3 D--gluconolactone 2 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 09/16/2010 Biochem: Carbohydrates II p. 25 of 48 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 09/16/2010 Biochem: Carbohydrates II p. 26 of 48 Sugar esters (fig. 7.13) Phosphate esters of sugars are significant metabolic intermediates 5’ position on ribose is phosphorylated in nucleotides 09/16/2010 Biochem: Carbohydrates II Glucose 6phosphate p. 27 of 48 OH Amino sugars HO HO GlcNAc O OH 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 09/16/2010 Biochem: Carbohydrates II p. 28 of 48 Acetals and ketals Acetals and ketals have two —OR groups on a single carbon Acetals and ketals are found in glycosidic bonds 09/16/2010 Biochem: Carbohydrates II p. 29 of 48 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 09/16/2010 Biochem: Carbohydrates II p. 30 of 48 Sucrose: a glycoside A disaccharide Linkage is between anomeric carbons of contributing monosaccharides, which are glucose and fructose 09/16/2010 Biochem: Carbohydrates II p. 31 of 48 Other disaccharides Maltose Cellobiose a-glc-glc with a-glycosidic bond from left-hand glc Produced in brewing, malted milk, etc. b-glc-glc Breakdown product from cellulose Lactose: b-gal-glc Milk sugar Lactose intolerance caused by absence of enzyme capable of hydrolyzing this glycoside 09/16/2010 Biochem: Carbohydrates II p. 32 of 48 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 09/16/2010 Biochem: Carbohydrates II p. 33 of 48 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. 09/16/2010 Biochem: Carbohydrates II p. 34 of 48 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 09/16/2010 Biochem: Carbohydrates II p. 35 of 48 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 09/16/2010 Biochem: Carbohydrates II p. 36 of 48 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 09/16/2010 Biochem: Carbohydrates II p. 37 of 48 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 Olinked glycosides 09/16/2010 Biochem: Carbohydrates II p. 38 of 48 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 09/16/2010 Biochem: Carbohydrates II Diagram courtesy of World of Molecules p. 39 of 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 09/16/2010 Biochem: Carbohydrates II p. 40 of 48 Categories of polysaccharides Storage homoglycans (all Glc) Structural homoglycans Starch: amylose (a(14)Glc) , amylopectin Glycogen Cellulose (b(14)Glc) Chitin (b(14)GlcNac) Heteroglycans Glycosaminoglycans (disacch.units) Hyaluronic acid (GlcUA,GlcNAc)(b(1 3,4)) 09/16/2010 Biochem: Carbohydrates II p. 41 of 48 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 09/16/2010 Biochem: Carbohydrates II p. 42 of 48 Amylose Unbranched, a-14 linkages Typically 100-1000 residues Not soluble but can form hydrated micelles and may be helical Amylases hydrolyze a-14 linkages Diagram courtesy Langara College 09/16/2010 Biochem: Carbohydrates II p. 43 of 48 Amylopectin Mostly a-14 linkages; 4% a-16 Each sidechain has 15-25 glucose moieties a-16 linkages broken down by debranching enzymes 300-6000 total glucose units per amylopectin molecule One reducing end, many nonreducing ends 09/16/2010 Biochem: Carbohydrates II p. 44 of 48 Glycogen Principal storage form of glucose in human liver; some in muscle Branched (a-14 + a few a-16) More branches (~10%) Larger than starch: 50000 glucose One reducing end, many nonreducing ends Broken down to G-1-P units Built up from G-6-P G-1-P UDP-Glucose units 09/16/2010 Biochem: Carbohydrates II p. 45 of 48 Glycogen structure 09/16/2010 Biochem: Carbohydrates II p. 46 of 48 Structural polysaccharides I Insoluble compounds designed to provide strength and rigidity Cellulose: glucose b-14 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-14 linkages: exoskeletons, cell walls (fig. 7.26) 09/16/2010 Biochem: Carbohydrates II p. 47 of 48 Structural polysaccharides II Alginates: poly(b-D-mannuronate), poly(a-L-guluronate), linked 14 Agarose: alternating D-gal, 3,6-anhydro-L-gal, with 6-methyl-D-gal side chains Cellulose-like structure when free Complexed to metal ions: 3-fold helix (“egg-carton”) Forms gels that hold huge amounts of H2O Can be processed to use in the lab for gel exclusion chromatography Glycosaminoglycans: see next section 09/16/2010 Biochem: Carbohydrates II p. 48 of 48
