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Chapter 7

CHAPTER 7.doc

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Department
Chemistry
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CHY 204
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Mario Estable

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CHAPTER 7: CARBOHYDRATES & GLYCOBIOLOGY -are carbon chains & most abundant biomolecules. Glycoconjugates are complex cho polymers covalently attached to pro or lipids & act as signals that determine intercellular location/metabolic fate of these hybrid molecules. Carbohydrates are polyhydroxyl aldehydes/ketones, ie. Yields compounds upon hydrolysis: CO2+H2OCHO (Photosynthesis); CHOENERGY (Oxidation). There are 4 major size classes of cho: monosacchs (simple sugars consist of a singly polyhydroxy aldehyde or ketone unit; if has 4/more C tend to have cyclic structures), disacchs (2 subunits of monosachhs), oligosacchs (short cahins of monosachhs units joined by glycosidic bonds, <20 subunits; in cells, most oligosacchs of 3/more units don’t occur as free entities but joined to nonsugar molec in glycoconjugates—cho+pro/lipid) & polysacchs (sugar polymers of >20 subunits. 7.1 Monosaccharides & Disaccharides -The simplest of cho, monosacchs are either aldehyde or ketones w/ 2 more hydroxyl groups (aldoses & ketoses). Many of the C atoms to which the –OH groups are attached are chiral centers which give rise to many sugar stereoisomers found in nature. -2 families of monosacchs are aldoses & ketoses: monosacchs are colourless, crystalline, solids that are freely soluble in H2O but insoluble in nonpolar solvents; the backbone of common monosacchs are unbranched chains in which all C atoms are linked by single bonds. If the carbonyl group (C=O) is at the end of the C chain, the monosacch is an aldose vs. if it is in any other posn, the monosacch is a ketose. The simplest ones are trioses (2-3 C), tetroses (4C), pentoses (5C), hexoses (6C) & heptoses (7C). Name of monosacchs Example Trioses (2-3 C) -Glyceraldehyde, an aldotriose has a chiracl C thus optically active -Dihydroxyacetone, a ketotriose, is the only monosacch w/o a chiral C Pentoses (5 C) -D-ribose, aldopentose, has 3 chiral C -2-deoxy-D-ribose, an aldopentose has 2 chiral C *both are components of nucleotides & nucleic acids Hexoses (6 C) -D-glucose, an aldohexose, has 4 chiral C -D-fructose, a ketohexoses has 3 chiral C *Both are the most abundant monosacchs in nature. -Monosacchs have asymmetric centers: all the monosacchs except dihydroxyacetone contain 1/more chiral C & thus occur in optically active isometric forms. The simplest aldose, glyceraldehyde, contains 1 chiral C & therefore has 2 different optical isomers/enantiomers (Rule: 2^n=# of stereoisomers, eg. 1 chiral C, n=1, 2^1=2 stereoisomers). -Fischer projection formulas are used to represent 3D sugar structures on paper; horizontal bonds project out of the plane toward the reader & vertical bonds project behind the plane, away from the reader. The steroisomers of monosacchs of each C chain leght can be divided in 2 groups that differ in the config about the chiral center most distant from the carbonyl C; those w/ configuration at this ref C same as that of D- glyceraldehyde are designated as D-isomers (-OH is on the right & C=O on top) & those w/ same config as L-glyceraldehyde are designated as L-isomers (-OH is on the left & C=O on top). Of the 16 possible aldohexoses, 8 are D forms & 8 are L forms; most of the hexoses of living organisms are D forms. The aldoses are designated by putting –ose at the end vs. ketoses are also designated by putting –ul- into the name of corresponding aldose. 2 sugars that differ only in the config around 1 C are called epimers, eg. D- glucose, D-galactose & D-mannose (epimers apply only to monosacchs). Some sugars occur naturally in their L form, eg. L-arabinose & some sugar derivatives common in glycoconjugates. -The common monosacchs have cyclic structures: in aq soln. aldotetroses & all monosacchs w/ 5/more C atoms in the backbone occur predominantly as cyclic/ring structures in which the C=O has formed a covalent bond w/ O2 of a –OH group along the chain. The formation of these structures is a result of a general rxn b/w alcohols & aldehydes/ketones to form derivatives canded hemiacetal/hemiketals which contain an nd additional chilral C. Substitution of a 2 alcohol produces acetal/ketal & when the 2 alcohol is part of another sugar molecule, a glycosidic bond forms. *anomers are special epimers & in cyclic forms of monsacchs, anomers differ at hemiacetal/ketal C. -Eg. D-glucose exists in soln as an intramolecular hemiacetal in which the free – OH at C5 has reacted w/ aldehydic C1 making C1 asymettric & producing 2 stereoisomers designated as alpha & beta. Alpha indicates that the –OH group at the anomeric center is on the same side as the –OH attached at the farthest chiral center vs. beta indicates that these –OH groups are on opp sides. 6-membered ring compounds are called pyranoses, ie. Alpha-D-glucosepyranose & beta-D-glucosepyranose. Aldoses that exist in cyclic forms of 5-membered rings are called furanoses but aldopyranose ring is more stable than aldofuranose ring & predominates in aldohexose & aldopentose solns. Isometric forms of monossachs that differ only in their config at hemiacetal/hemiketal C atom are called anomers; hemiacetal or carbonyl C atom is called anomeric C. the alpha & beta anomers of D-glucose interconvert in aq soln via mutarotation (1/3 alpha-D-glucose, 2/3 beta-D-glucose & small amts of linear 5 membered ring glucofuranose forms). Ketohexoses also occur in alpha & beta anomeric forms where in which the hydroxyl group at C5/C6 reacts w/ the keto group at C2 forming a furanose/pyranose ring containing a hemiketal linkage. b -Haworth perspective formulas: used to show stereochem of ring forms of monosacchs that are not planar but tends to assume either of 2 chair conformations. 2 conformations of a molec are interconvertible w/o breakage of covalent bonds vs. 2 config can be interconverted only by breaking a covalent bond. Generally, substituents in equatorial posns are less sterically hindered by neighbouring susttituents & conformers w/ bulky substituents in equatorial posns are favoured. The 1 chair form is the preferred conformation of alpha-D-glucopyranose. -Organisms contain a variety of hexose derivatives: there are a num of sugar derivatives in which a –OH group in the parent compound is replaced w/ another substituent or a C atom is oxidized to a carboxyl group. N-acetyl-beta-D-glucosamine is a derivative of many structural polymers, eg. Bacterial cell wall. -Dissacharides contain a glycosidic bond: disacchs are 2 monosacchs linked together by O-glycosidic bond which is formed when a –OH group of 1 sugar reacts w/ the anomeric C of the other which results in formation of acetal from hemiacetal (eg. Glucopyranose) nd & an alcohol (-OH of 2 sugar) & resulting compound is a glycoside. (--OH of 1 monosacch + Anomeric C of another) Glycosidic bonds are readily hydrolyzed by acids but resist cleavage by base therefore disacchs can be hydrolyzed to yield their free monosacch components by boiling w/ dilute acid. The oxidation of a sugar by Cu2+ (rxn that defines a reducing sugar) occurs only w/ the linear form which exists in equilibrium w/ the cyclic form; when the anomeric C is involved in the glycosidic bond, the sugar residue can’t take the linear form & thus becomes a nonreducing sugar. The end of a chain w/ a free anomeric C (not involved in glycosidic bond) is the redu
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