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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 Name Formula Triose Tetrose Pentose C3 H6 O3 C4 H8 O4 Hexose Heptose Octose C5 H1 0 O 5 C6 H1 2 O 6 C7 H1 4 O 7 C8 H1 6 O 8 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 O H C H – C – OH HO – C – H H – C – OH H – C – OH CH2OH D-glucose O H C HO – C – H H – C – OH HO – C – H HO – C – H CH2OH L-glucose 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 H O C H C OH HO C H H C OH H C OH CH2OH D-glucose 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). CH2OH • Called levulose or fruit sugar C O • Found in honey and fruits HO C H • Sweeter than sucrose and glucose H C OH • Commercially prepared as highH C OH fructose sugars from corn starch for sweetness CH2OH • An epimer of glucose at C4 D-fructose • 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 CHO enzymeH OH catalyzed HO H oxidation H OH H OH CH2 OH D -Glu cose H HO H H CHO OH H OH OH COOH COOH HO HO D -Glucu ronic acid (a u ronic acid ) O OH OH 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 CH2OH 1 HO H H 2C O C H C OH C OH 3 4 5 6 HOH2C 6 CH2OH D-fructose (linear) H 5 H 1 CH2OH O 4 OH HO 2 3 OH H -D-fructofuranose 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: 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 HO HO CH2 OH O OH OH -D -Glucopyranose [] D 2 5 = + 18.7° HO HO CH2 OH OH O HO C H Open-chain form HO HO CH2 OH O HO OH -D -Glucopyranose [] D 2 5 = +112° 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. For pyranoses, the six-membered ring is more accurately represented as a chair conformation HO HO CH2 OH O anomeric carbon OH() OH -D -Glu copyran os e ( - D -Glucos e) HO HO CH2 OH OH O C OH H D -Glucos e HO HO CH2 OH O HO OH( ) - D -Glu copyran os e ( - D -Glucose) 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 anomeric carbon CH2 OH O OH H + H H + CH3 OH OH H -H2 O HO H glycos idic H OH CH2 OH bond -D -Glu copyran os e O OCH3 H (-D -Glu cose) H + OH H H HO CH2 OH OH H H OH H HO OCH3 H OH H OH Methyl -D -glu copyran os ide Methyl -D -glu copyran os ide (Methyl -D -glu coside) (Methyl -D -glucos ide) Disaccharides Maltose – present in malt, the juice from sprouted barley and other cereal grains – maltose consists of two units of Dglucopyranose joined by an -1,4glycosidic 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 Dglucopyranose – 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)- -Dglucopyranoside Polysaccharides Glycogen • is the energy-reserve carbohydrate for animals • glycogen is a branched polysaccharide of approximately 106 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 Dglucose 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 waterinsoluble 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 Nacetyl-muraminic acid (NAM) and Nacetylglucosamine (NAG) joined by -1,4glycosidic bonds. CH2 OH O O O H3 C CH C= O NH NH O= C CH2 OH O O HO O N O= C CH3 CH3 L A la D Gln O L Ly s ( CH2 ) 4 N H- C-( Gly ) 5 -N H-- -- - D A la C= O O N H-( Gly) 5 C- - -- To tetrapeptide side chains The NAM-NAG polysaccharide is in turn cross-linked by small peptides • 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 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 Nacetylgalactosamine 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 Nacetylglucosamine and galactose • Ester sulfate on C6 of GalNAc HEPARIN • Glucosamine and d-glucoronic acid or Liduronic 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 wholeblood anticoagulant and blood lipidclearing 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