Carbohydrates are polyhydroxyderivatives of aldehydes or ketones have the general molecular formula CH2O

Roles of Carbohydrates Storage of metabolic fuel Structural framework of nucleic acids Structural elements in the cell walls of bacteria and plants Linked to proteins and lipids which are involved in cell recognition

Monosaccharides The basic units in carbohydrates Could either be an aldose or ketose Typically contain 3 to nine carbons

Monosaccharides are classified by their number of carbon atoms

EXERCISE Draw the possible structure(s) of 1. aldotetrose 2. ketotetrose

Monosaccharides Stereoisomers Carbohydrates contain many chiral carbons

OPTICAL ISOMERS Stereosiomers can be distinguished using plane of polarized light

Naming stereoisomers Based on the structure of L- and D-glyceraldehyde

D- sugars are the most common carbohydrates. D-refers to the right hand orientation of the OH on the chiral carbon farthest from the carbonyl carbon L-sugars: L refers to the left hand orientation of the OH on the chiral carbon farthest from the carbonyl carbon

Glucose The most common sugar Known as dextrose Also known as blood sugar

Galactose One of the monosaccharides in the disaccharide lactose Found in plant gums and pectins polysaccharides An epimer of glucose at C4 Converted to glucose during metabolism

Fructose One monosaccharide in the disaccharide sucrose (table sugar). Called levulose or fruit sugar Found in honey and fruits Sweeter than sucrose and glucose Commercially prepared as high- fructose sugars from corn starch for sweetness An epimer of glucose at C4 Readily converted to glucose in metabolism by isomerization

Reactions of Monosaccharides Oxidation

Oxidation using a mild oxidizing agent

Oxidation using a strong oxidizing agent

Oxidation to Uronic Acids

Reduction to alditol

Reaction with dilute base

Reaction with a concentrated base-fragmentation

Alcohols react readily with aldehydes to form hemiacetals

ketones react readily with alcohols to form hemiketals

Fructose forms either  a 6-member pyranose ring, by reaction of the C2 keto group with the OH on C6, or  a 5-member furanose ring, by reaction of the C2 keto group with the OH on C5.

The formation of hemiketals and hemiacetals results in an asymmetric carbon atom. Isomers that differ only in their configuration about the new asymmetric carbon are called anomers, the new assymetric carbon is called anomeric carbon. a-anomer has the hydroxyl group at the right side of the anomeric carbon  -anomer has the hydroxyl group at the left side of the anomeric carbon

Mutarotation: Mutarotation: the change in specific rotation that accompanies the equilibration of  - and  - anomers in aqueous solution example: when either  -D-glucose or  -D-glucose is dissolved in water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms

EXERCISE   D-Mannose exists in aqueous solution as a mixture of  and  forms. Draw Haworth projections.

Because of the tetrahedral nature of carbon bonds, pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar. The representation above reflects the chair configuration of the glucopyranose ring more accurately than the Haworth projection.

chair conformation For pyranoses, the six-membered ring is more accurately represented as a chair conformation

EXERCISE Draw the chair conformations for  -D mannopyranose and  -D-mannopyranose

Formation of Glycosides Treatment of a monosaccharide, all of which exist almost exclusively in a cyclic hemiacetal form, with an alcohol gives an acetal

Disaccharides

Maltose –present in malt, the juice from sprouted barley and other cereal grains –maltose consists of two units of D- glucopyranose joined by an  -1,4- glycosidic bond –maltose is a reducing sugar

Lactose –lactose is the principal sugar present in milk; it makes up about 5 to 8 percent of human milk and 4 to 6 percent of cow's milk –it consists of D-galactopyranose bonded by a  -1,4-glycosidic bond to carbon 4 of D- glucopyranose –lactose is a reducing sugar

Sucrose –is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets –sucrose is a nonreducing sugar

EXERCISE Draw the chair conformation of 1. methyl  - and  -D-mannopyranoside 2.  -D-mannopyranosyl-(1-4)-  -D- glucopyranoside

Polysaccharides

Glycogen is the energy-reserve carbohydrate for animals glycogen is a branched polysaccharide of approximately 10 6 glucose units joined by  -1,4- and  -1,6-glycosidic bonds the total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle

Starch starch can be separated into amylose and amylopectin amylose is composed of unbranched chains of up to 4000 D-glucose units joined by  -1,4-glycosidic bonds amylopectin contains chains up to 10,000 D- glucose units also joined by  -1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by  -1,6-glycosidic bonds

Cellulose is a linear polysaccharide of D-glucose units joined by  -1,4-glycosidic bonds it has an average molecular weight of 400,000 g/mol, corresponding to approximately 2200 glucose units per molecule cellulose molecules act like stiff rods and align themselves side by side into well-organized water- insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds this arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength it is also the reason why cellulose is insoluble in water

Bacterial Cell Walls The bacterial cell wall is a unique structure which surrounds the cell membrane. Maintaining the cell's characteristic shape Countering the effects of osmotic pressure Consist of many layers of peptidoglycan connected by amino acid bridges

Peptidoglycan is composed of an alternating sequence of N- acetyl-muraminic acid (NAM) and N- acetylglucosamine (NAG) joined by  -1,4- glycosidic bonds.

in Staphylococcus aureus, the cross link is a tetrapeptide this tetrapeptide is unusual in that it contains two amino acids of the D-series, namely D-Ala and D-Gln each tetrapeptide is cross linked to an adjacent tetrapeptide by a pentapeptide of five glycine units The NAM-NAG polysaccharide is in turn cross-linked by small peptides

PROTEOGLYCANS

Glycosaminoglycans (GAGs) are anionic polyanionic polysaccharides chains made of repeating disaccharide units play important roles in the structure and function of connective tissues polysaccharides based on a repeating disaccharide where one of the monomers is an amino sugar and the other has a negative charge due to a sulfate or carboxylate group Glycosaminoglycans tend to be negatively charged, because of the prevalence of acidic groups. GAGs are usually attached to proteins to form proteoglycans

Common GAGs heparin: natural anticoagulant hyaluronic acid: a component of the vitreous humor of the eye and the lubricating fluid of joints :chondroitin sulfate and keratan sulfate: components of connective tissue

Chondroitin 6-sulfate The repeating disaccharide units of N- acetylgalactosamine ang glucoronic acid can be sulfated in either 4 or 6 position of GalNAc They are prominent components of cartilage tendons, ligaments and aorta Palate is made from chondroitin sulfate

Keratan Sulfate Repeating disaccharide unit of N- acetylglucosamine and galactose Ester sulfate on C6 of GalNAc

HEPARIN Glucosamine and d-glucoronic acid or L- iduronic acid form the characteristic disaccharide unit In addition to N-sulfate or O-sulfate on C6 of glucosamine, heparin can also contain sulfate on C3 of the hexosamine and C2 of the uronic acid Functions predominantly as anticoagulant and lipid clearing agent

Dermatan Sulfate Predominant uronic acid is L-iduronic acid Antothrombotic like heparin but in contrast, shows only minimal whole- blood anticoagulant and blood lipid- clearing activities Found in skin

Hyaluronic acid Found in synovial fluid of the joints Forms a viscous solution that is an excellent lubricant

Specific enzymes are responsible for oligosaccharide assembly Glycosyltransferase-catalyzes the synthesis of oligosaccharides through the formation of glycosidc bond

Glycoproteins Components of the cell membrane, where they play variety of roles in cell adhesion

Protein glycosylation takes part in the lumen of the endoplasmic reticulum and in the golgi complex

LECTINS Proteins that bind specific carbohydrate structures Are ubiquitous, being found in plants, animals and microorganisms

C-type (for Calcium requiring)-found in animals