CARBOHYDRATES STRUCTURES AND FUNCTIONS Erwin D. Abueva, M.D. August 2009

I NTRODUCTION 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

C LASSIFICATION OF C ARBOHYDRATES Monosaccharides Disaccharides Oligosaccharides Polysaccharides

M ONOSACCHARIDES 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 Carbohydrates When 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.

D EFINITION OF T ERMS 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)

I SOMERS OR E NANTIOMERS ? GLUCOSEMANNOSE

I SOMERS OR A NOMERS ? GLUCOSEFRUCTOSE

E NANTIOMERS OR E PIMERS ? GLUCOSEGALACTOSE

M ONOSACCHARIDES Number of Carbons Generic NameExamples 3TrioseGlyceraldehyde, Dihydroxyacetone 4TetroseErythrose 5PentoseRibose, Ribulose, Xylulose 6HexoseGlucose, Galactose, Mannose, Fructose 7HeptoseSedoheptulose 9NonoseNeuraminic acid or Sialic acid

M ONOSACCHARIDES 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.

M ONOSACCHARIDES Principal sugar in blood Major metabolic fuel Dextrose or grape sugar Found in fruit juices White crystalline solid Soluble in water GLUCOSE

M ONOSACCHARIDES Less soluble and less sweet than glucose In plants, constituent of pectin GALACTOSE

M ONOSACCHARIDES Levulose or fruit sugar Sweetest of all sugars FRUCTOSE

M ONOSACCHARIDES 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.

M ONOSACCHARIDES Alpha-D-glucoseBeta-D-glucose

M ONOSACCHARIDES 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

M ONOSACCHARIDES 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

M ONOSACCHARIDES Fischer projectionHaworth projection

M ONOSACCHARIDES PENTOSES Has 5 carbon atoms Either 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

M ONOSACCHARIDES - PENTOSES D-RiboseD-ArabinoseD-XyloseD-Lyxose

M ONOSACCHARIDES Oxidation-Reduction Reactions Oxidation of the –CH 2 -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

M ONOSACCHARIDES

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 Sulfates Some 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