CARBOHYDRATESSTRUCTURES AND

FUNCTIONS

Erwin D. Abueva, M.D.

August 2009

INTRODUCTION

Carbohydrates are aldehyde or ketone derivatives of the polyhydric alcohols and their derivatives

In animal cells, this biomolecule serves as an important source of energy for vital activities in the form of glucose and glycogen

Some carbohydrates have specific functions Ribose in the nucleoprotein Galactose in certain lipids Lactose in milk Cellulose in fibers

CLASSIFICATION OF CARBOHYDRATES

Monosaccharides

Disaccharides

Oligosaccharides

Polysaccharides

MONOSACCHARIDES

the simplest unit of carbohydrate molecule which cannot be further hydrolyzed

consist of only one sugar and are usually colorless, water-soluble, crystalline solids

are the building blocks of disaccharides like sucrose (table sugar) and polysaccharides (cellulose and starch)

further classified as either aldoses or ketoses may be subdivided according to the number of

carbon atoms they possess

PROPERTIES OF MONOSACCHARIDES

Asymmetry of CarbohydratesWhen a compound has more

than one asymmetric center, each center is viewed in turn, and a representation is constructed showing the relative positions of the substituents of the various asymmetric centers.

DEFINITION OF TERMS

Stereoisomers: compounds that have the same structural formula but differ in spatial configuration (glucose, galactose, and fructose are isomers)

Enantiomers: isomers that are nonsuperimposable mirror images of each other (D-sugar and L-sugar)

Epimers: two sugars that differ in configuration around 1 specific carbon (galactose and glucose are epimers on carbon 4)

Anomers: isomeric forms of sugar that differ in configuration around carbonyl carbon (C1 in aldose and C2 in ketose)

ISOMERS OR ENANTIOMERS?

GLUCOSE MANNOSE

ISOMERS OR ANOMERS?

GLUCOSE FRUCTOSE

ENANTIOMERS OR EPIMERS?

GLUCOSE GALACTOSE

MONOSACCHARIDES

Number of Carbons

Generic Name Examples

3 Triose Glyceraldehyde, Dihydroxyacetone

4 Tetrose Erythrose

5 Pentose Ribose, Ribulose, Xylulose

6 Hexose Glucose, Galactose, Mannose, Fructose

7 Heptose Sedoheptulose

9 Nonose Neuraminic acid or Sialic acid

MONOSACCHARIDES

HEXOSES Monosaccharide with 6 carbon atoms Classified by functional group, with aldohexoses

having an aldehyde at position 1, and ketohexoses having a ketone at position 2

Glucose is found in fruit juices, in hydrolysis of starch, cane sugar, maltose, and lactose

Mannose is found in plant mannans and gums Fructose are found in fruit juices. It can be converted

to glucose in the liver and so used in the body Galactose is used in the synthesis of lactose in the

mammary gland to make milk.

MONOSACCHARIDES

Principal sugar in blood

Major metabolic fuel Dextrose or grape

sugar Found in fruit juices White crystalline solid Soluble in water

GLUCOSE

MONOSACCHARIDES

Less soluble and less sweet than glucose

In plants, constituent of pectin

GALACTOSE

MONOSACCHARIDES

Levulose or fruit sugar

Sweetest of all sugars

FRUCTOSE

MONOSACCHARIDES

Cyclization of monosaccharides Monosaccharides exist in the open chain (acyclic) form

and in ring form Formation of a hemiacetal (or hemiketal) ring results in

the creation of anomeric carbon at C1 of an aldose and C2 of a ketose

the anomers are designated as alpha- and beta- D-glucose.

These anomeric forms are important in enzymatic reactions.

MONOSACCHARIDES

Alpha-D-glucose Beta-D-glucose

MONOSACCHARIDES

The vast majority of the sugars in humans are D-sugars D-glucose: dextrose or blood sugar; 6-member ring

(stable) D-galactose: stereoisomer of glucose: converted to

glucose during metabolism D-fructose: readily converted to glucose in metabolism

by isomerization; forms a 5-member ring rather than a 6-member ring

D-ribose: constituent of RNA, ATP, and coenzymes 2-deoxyribose-D-ribose: missing OH at #2 position of

ribose (substituted by H); found in DNA; forms a 5-member ring

MONOSACCHARIDES

Representation of sugar conformation: Fischer projection: the carbon is written

vertically, with C1 at the top, and the hydroxyl and hydrogen substituents written to the sides

Haworth projection: Carbon is written farthest to the right, the plane of the ring is flat on the paper, and the hydroxyl and hydrogen groups are project either “above” or “below” the plane drawn

MONOSACCHARIDES

Fischer projection Haworth projection

MONOSACCHARIDES

PENTOSESHas 5 carbon atomsEither has an aldehyde functional group in

position 1 (aldopentoses), or a ketone functional group in position 2 (ketopentoses)

Ribose is an important constituent of RNA, deoxyribose in DNA, in enzymes and in high energy molecules like ATP

Arabinose and Xylose, which are found in wood gums, are important constituents of glycoproteins

MONOSACCHARIDES - PENTOSES

D-Ribose D-Arabinose D-Xylose D-Lyxose

MONOSACCHARIDES

Oxidation-Reduction Reactions Oxidation of the –CH2-OH group at carbon 6 produces a

–uronic acid Example: (1) glucose to glucoronic acid

(2) galactose to galacturonic acid

Reduction of the carbonyl carbon (aldehyde or keto group) produces a new alcohol group. Such compounds are called polyols. Example: (1) glucose is reduced to sorbitol

(2) fructose is reduced to mannitol

(3) ribose is reduce to deoxyribose

MONOSACCHARIDES

MONOSACCHARIDES

Formation of Derivatives A. Glycoside Formation

Any monosaccharide unit can react with a non-carbohydrate unit (aglycone) such as alcohol, glycerol, a sterol or a phenol to give a glycoside

Glycosides are nonreducing sugars and will not react with Tollen’s, Benedict’s nor Fehling’s reagents because the linkage with the noncarbohydrate group is at C1

MONOSACCHARIDES

Formation of Derivatives B. Exhaustive Methylation

Reaction with alcohols affects only the anomeric carbon. The other hydroxyl groups can also be methylated using dimethyl sulfate

MONOSACCHARIDES

Formation of Derivatives C. Formation of Amino Sugars

Most monosaccharides can acquire an amino group at C-2. In addition, these amino groups can be acetylated to form N-acetyl derivatives.

Amino sugars are frequently found in glycoproteins (in cell membranes) and proteoglycans

MONOSACCHARIDES Formation of Derivatives

D. Formation of Sugar SulfatesSome polysaccharides contain sulfates esterified at C-2, C-4, and for C-6. These polysaccharides are found mostly in proteoglycans of the extracellular matrix.

Ex. Chondroitin sulfates, keratan sulaftes, dermatan sulfates