basics of carbohydrate structure

8/12/2019 Basics of Carbohydrate Structure

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Carbohydrates, the primary products of photosynthesis, are abundant in nature. As anenergy source, carbohydrates comprise 50% of the average daily caloric intake for Americans.Carbohydrates also serve as stored energy reserves, in the form of glycogen in animals and starchin plants; glucose can be released from these storage forms for immediate use as fuel.Carbohydrates are important in tissue structure components (as polysaccharides, glycoproteinsand glycolipids) in cell membranes and in intercellular materials. They also provide precursorsfor biosynthesis of proteins, nucleic acids and lipids and other biochemical materials.

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this triose. They are named D- and L-glyceraldehyde (or, R- and S-glyceraldehyde), and arerepresented in Figure 2.

Dihydroxyacetone has no asymmetric carbon atom, but longer ketoses do have one ormore. As the carbon skeleton is elongated, larger numbers of isomeric monosaccharides are

possible. Figures 3 and 4 show how physiologically important sugars are structurally related toD-glyceraldehyde or dihydroxyacetone.

Figure 3 shows structures of only physiologically important sugars. That is, only three ofthe 4 aldopentoses in the D-series are presented, and only 3 of the 8 aldohexoses. The arrowsshow relationships between sugars of a given size and the structures formed by adding one moreH-CO-H or HO-C-H to the carbon chain. For example, the 3,4,5 and 6-carbons of the hexosesD-glucose and D-mannose are identical to the 2,3,4, and 5-carbons of the pentose D-arabinose. -and all of them have CHO at the 1-carbon. Going from pentose to hexose, one C has been added,

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at the 2-position. D-glucose and D-mannose differ only in the conformation of substituents atthat 2-position: H-C-OH in glucose and HO-C-H in mannose.

Once again, only 1 of the 4 possible ketohexoses is shown. Note that hexoses have one

less asymmetric carbon than aldoses of the same length, so there are fewer ketose isomers thanaldose isomers for a given length.

+3 &GC;=D= >AD ADCH?BID


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opposite end, bearing a hydroxyl group, is the "non-reducing end". The ability of a sugar toreduce cupric ion is the basis of a chemical assay for monosaccharides or terminal residues ofcarbohydrate chains.

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The maximum number of stereoisomers of a ketose or aldose of given carbon chainlength can be predicted by the formula

2n = number of stereoisomers

where n= the number of asymmetric carbon atoms. Ketoses have one less asymmetric carbonthan do aldoses of the same length. Hence the number of aldohexose stereoisomers is 16. Ofthese, 8 are D-isomers and 8 are L-isomers. The number of ketohexose stereoisomers is 8 (4 D-isomers and 4 L-isomers).

#3 /HI>A= ;??HA E>B= ?G;=DC ABM@DA M@>< ;FD< ?@>BG S;AE>MB;< occurs in ring formation of ketoses, as follows:

Ketohexoses form 5-membered rings, which are called SHA>CCBMB;G >=JEEDMAB? ?>AK;


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With the formation of a closed ring, another asymmetric carbon is formed at carbon-1 inaldoses or carbon 2 in ketoses, as the substituents of this carbon in the ring can occupy twodifferent positions. These closed rings are termed and isomers, as in -D- and -D-glucopyranoses ( -D- and -D-glucose). The and isomers are >T;AM@ FA;LD?MB;< ;S9.9IGH?;=D


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The actual shape of the pyranose ring is a "boat" form or a "chair" form, since the 6-membered ring cannot be planar. The chair form is more stable, having less steric interferenceamong the substituents on the ring carbons. The substituents are more precisely labeled as axial(a) or equatorial (e) in orientation rather than as upward or downward as in the Haworth

projection.

Aldopentoses and ketohexoses in the 5-membered furanose ring form are represented by thefollowing Haworth projections:

Furanoses also have and anomers; again, the anomer has the anomeric hydroxyl group onthe same side of the ring as the highest numbered carbon (carbon 5 in aldopentoses and carbon 6in ketohexoses). The 5-member ring cannot be perfectly planar. Puckering occurs, with carbon 2and carbon 3 out of the plane formed by the other carbons and the oxygen of the ring. The carbon2 and carbon 3 atoms are out of the plane in opposite directions.

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&3 /HI>A >G?;@;G=. The carbonyl group of monosaccharides may be reduced to ahydroxyl group.

P3 6GJ?DA;G, formed by reduction of D-glyceraldehyde, is the backbone of many lipids.

O3 .9=;AKBM;G, from the reduction of either glucose or fructose, is an intermediate in the synthesisof fructose from glucose, especially in the prostate gland.

V3 0J;B


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,3 /HI>A >?BC= :

P3 'A;?BC= result from the oxidation of the non-reducing terminalcarbon to COOH, as in .9IGH?HA;?BC, which occurs in polymersin intercellular material such as joint fluid and vitreous fluid of the

eye. Glucuronic acid also plays an important role in the metabolismand excretion of certain insoluble metabolites and drugs. For example,liver enzymes attach glucuronic acid to bilirubin, a product of the

breakdown of heme, making the bilirubin water- soluble andfacilitating its excretion into bile.

O3 &GC;?BC= result from the oxidation of the aldehyde of aldoses, as in .9IGH?;?BC. The phosphate ester of D-gluconic acid is an importantintermediate in the formation of pentoses from glucose.

V3 &=?;AKB? >?BC N5BM>EB< +R is a lactone of a hexonic acid. It is anessential nutrient for humans and guinea pigs. Ascorbic acid participatesin oxidation- reduction reactions, and in the addition of hydroxyl groups to

proline and lysine in the formation of collagen.

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*BI3 P_3 .9IGH?;?BC

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+3 /HI>A F@;=F@>MD=3The first step in the metabolism of a monosaccharide is its activation bytransfer of a phosphate group from ATP to the sugar to form a sugar-phosphate ester. Manyintermediates of carbohydrate metabolism are phosphorylated sugar derivatives.

.3 .D;WJ=HI>A= .These sugars lack one hydroxyl group. Most abundant is O9CD;WJABK;=D, which is the pentosemoiety in DNA. Other examples are SH?;=D and A@>EA=

*BI3 OO3 &EBA=

*B 3 OQ3 /H >A @;= @>MD D=MDA=

basics of carbohydrate structure

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