CARBOHYDRATES Dr. Phadnaik Fru Man Glu Carbohydrates Sugar alcohol Mono sacch. Oligosacch. Di sacch. (lactose) Pentoses Disacch. Plant (sucrose, maltose Sources & Trehalose) Animal Sources
polysaccharides small amount (not important) Starch Carbohydrates (glycans) have the following basic composition: I (CH2O)n or H - C - OH I
Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose. Disaccharides - 2 monosaccharides covalently linked. Oligosaccharides - a few monosaccharides covalently linked. Polysaccharides - polymers consisting of chains of monosaccharide or disaccharide units. MONOSACCHARIDES
Aldoses (e.g., glucose) have Ketoses (e.g., fructose) have an aldehyde group at one a keto group, usually at C2. end.
H O C CH2OH H C OH C O
HO C H HO C H
H C OH H C OH
H C OH H C OH CH OH CH2OH 2 D-glucose D-fructose D- D- D- Glucose Mannose Galactose CHO CHO CHO H OH H OH HO H HO H HO H HO H HO H H OH H OH H OH H OH H OH
CH2OH CH OH 2 CH2OH
CH2OH C O
HO C H
H C OH
H C OH
CH2OH D-fructose D VS L DESIGNATION
CHO CHO D & L designations are based on the H C OH HO C H configuration about CH2OH CH2OH the single asymmetric C in D-glyceraldehyde L-glyceraldehyde glyceraldehyde. CHO CHO The lower H C OH HO C H representations are CH OH CH OH Fischer Projections. 2 2 D-glyceraldehyde L-glyceraldehyde SUGAR NOMENCLATURE
O H O H For sugars with more C C than one chiral center, H – C – OH HO – C – H HO – C – H H – C – OH D or L refers to the H – C – OH HO – C – H asymmetric C farthest H – C – OH HO – C – H CH2OH CH2OH from the aldehyde or D-glucose L-glucose keto group. Most naturally occurring sugars are D isomers. D & L sugars are mirror O H O H images of one another. C C They have the same H – C – OH HO – C – H name, e.g., D-glucose HO – C – H H – C – OH & L-glucose. H – C – OH HO – C – H Other stereoisomers H – C – OH HO – C – H have unique names, e.g., glucose, mannose, CH2OH CH2OH galactose, etc. D-glucose L-glucose The number of stereoisomers is 2n, where n is the number of asymmetric centers. The 6-C aldoses have 4 asymmetric centers. Thus there are 16 stereoisomers (8 D-sugars and 8 L-sugars). HEMIACETAL & HEMIKETAL FORMATION
H H An aldehyde can react with C O + R' OH R' O C OH an alcohol to R R form a aldehyde alcohol hemiacetal hemiacetal.
R R A ketone can react with an C O + "R OH "R O C OH alcohol to form R' R' a hemiketal. ketone alcohol hemiketal Monosaccharides have hydroxyl and carbonyl groups in the same molecule and those with five or more carbons exist almost entirely as five- and six- membered cyclic hemiacetals. Anomeric carbon: The new stereocenter created as a result of cyclic hemiacetal formation. Anomers: Carbohydrates that differ in configuration at their anomeric carbons named a and b.
CHO Pentoses and 1 H C OH hexoses can 2 cyclize as the HO C H ketone or 3 D-glucose H C OH (linear form) aldehyde reacts 4 with a distal OH. H C OH 5
Glucose forms an CH2OH intra-molecular 6 6 hemiacetal, as the 6CH2OH CH2OH 5 5 C1 aldehyde & H O H H O OH C5 OH react, to H H form a 6-member 4 OH H 1 4 OH H 1 pyranose ring, OH OH OH H 3 2 3 2 named after H OH H OH pyran. a-D-glucose b-D-glucose These representations of the cyclic sugars are called Haworth projections.
1CH2OH
2 C O
HO C H 3 HOH2C 6 O 1CH2OH H C OH 4 5 H HO 2 H C OH H 5 4 3 OH OH H 6CH2OH D-fructose (linear) a-D-fructofuranose
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. 6 6CH2OH CH2OH 5 5 H O H H O OH H H 4 OH H 1 4 OH H 1 OH OH OH H 3 2 3 2 H OH H OH a-D-glucose b-D-glucose
Cyclization of glucose produces a new asymmetric center at C1. The 2 stereoisomers are called anomers, a & b. Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1: a (OH below the ring) b (OH above the ring). H OH H OH
4 6 H O H O HO 5 HO HO 2 H HO OH 3 H OH 1 H OH H OH H H
a-D-glucopyranose b-D-glucopyranose
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. SUGAR DERIVATIVES
COOH CHO CH OH 2 H C OH H C OH H C OH HO C H HO C H H C OH H C OH H C OH H C OH H C OH H C OH CH2OH CH2OH COOH D-ribitol D-gluconic acid D-glucuronic acid
sugar alcohol - lacks an aldehyde or ketone; e.g., ribitol. sugar acid - the aldehyde at C1, or OH at C6, is oxidized to a carboxylic acid; e.g., gluconic acid, glucuronic acid. SUGAR DERIVATIVES
CH2OH CH2OH
H O H H O H H H OH H OH H OH OH OH O OH
H NH2 H N C CH3 H a-D-glucosamine a-D-N-acetylglucosamine amino sugar - an amino group substitutes for a hydroxyl. An example is glucosamine. The amino group may be acetylated, as in N-acetylglucosamine. AMINO SUGARS
Amino sugar: A sugar that contains an -NH2 group in place of an -OH group. Only three amino sugars are common in nature N-Acetyl-D-glucosamine is a derivative of D-glucosamine.
CHO CHO CHO CHO O 2 H NH2 H2 N H H NH2 H NHCCH3 HO H HO H HO H HO H H OH H OH HO 4 H H OH H OH H OH H OH H OH
CH2 OH CH2 OH CH2 OH CH2 OH D-Glucosamine D-Mannosamine D-Galactosamine N-Acetyl-D- glucosamine
O H O COO H3C C NH R HC OH H H R = HC OH H OH CH2OH OH H N-acetylneuraminate (sialic acid)
N-acetylneuraminate (N-acetylneuraminic acid, also called sialic acid) is often found as a terminal residue of oligosaccharide chains of glycoproteins. Sialic acid imparts negative charge to glycoproteins, because its carboxyl group tends to dissociate a proton at physiological pH, as shown here. Deoxy Sugars. Carbohydrates that are missing a hydroxy group.
1 1 CHO CHO 2 H 2 OH H H HOH C OH HOH2C OH 3 2 O 3 O H OH H H H OH H H 4 4 H OH H H H OH H H H OH OH 5 OH 5 CH2OH CH2OH
D-ribose 2-Deoxy-D-ribose
1 1 CHO CHO 6 6 HO 2 H H HO 2 H H 3 5 3 5 O H OH H O OH H OH H OH CH OH CH3 4 4 2 4 4 H OH H HO 2 H OH H HO 2 5 H 5 H HO H HO 3 HO H HO 3 6 1 6 1 CH2OH OH H CH3 OH H L-Galactose 6-Deoxy-L-Galactose (fucose)
19 HOH2C HOH2C HO HO O O HO O OH HO C OH HN 2 HN CH H 3 H O O N-acetyl-D-glucosamine N-Acetylmuramic acid (GlcNAc or NAG) (MurNAc) REDUCTION TO ALDITOLS, ALDEHYDE ALCOHOL The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of
reducing agents, including NaBH4 and H2/M.
CHO CH2 OH H OH H OH CH2 OH HO O HO H HO H HO NaBH4 OH H OH H OH OH H OH H OH
b-D-Glucopyranose CH2 OH CH2 OH D-Glucose D-Glucitol (D-Sorbitol) An alditol OTHER ALDITOLS
Other alditols common in the biological world are:
CH2 OH
HO H CH2 OH
CH2 OH HO H H OH H OH H OH HO H H OH H OH H OH
CH2 OH CH2 OH CH2 OH Erythritol D-Mannitol Xylitol EXAMPLE: ALKYLATION OF METHYL A-D- GLUCOPYRANOSIDE
HOCH2 O HO + 4CH3I HO OH OCH3
Ag2O, CH3OH EXAMPLE: ALKYLATION OF METHYL A-D- GLUCOPYRANOSIDE
HOCH2 O HO + 4CH3I HO OH OCH3
Ag2O, CH3OH
CH3OCH2 O CH3O (97%) CH3O
CH3O OCH3 Alkylation of the OH Groups OXIDATIONS Oxidation can be done in several ways. + Tollens reagent (Ag (NH3)2 or Benedict’s solution (Cu2+ tartrate complex). Not synthetically useful due to side reactions. Bromine water oxidizes aldoses (not ketoses) to monocarboxylic acids (Aldonic Acids). Nitric Acid oxidizes aldoses to dicarboxylic acids (Aldaric acids). Enzyme catalyzed oxidation of terminal OH to carboxylic acid (Uronic Acid) Periodic Acid oxidizes and breaks C C bonds. Later for that. REDUCING SUGARS
Sugars with aldehyde (or ketone group) in solution. The group can be oxidized and is detected with Tollens or Benedicts solution. Ketone groups converted to aldehyde via tautomeric shifts PROBLEM WITH TOLLENS 2-Ketoses are also oxidized to aldonic acids in basic solution (Tollens).
CHOH CH2 OH CHO COOH C-OH C= O (1) (2) CHOH (3) CHOH ( CHOH) ( CHOH) n n ( CHOH) n ( CHOH) n CH2 OH CH2 OH CH2 OH CH2 OH A 2-ketose An enediol An aldose An aldonic acid
Oxidation Ketose to aldose conversion via keto enol tautomerism Reducing sugar OXIDATION TO CARBOXYLIC ACIDS OXIDATION BY PERIODIC ACID, HIO4 OR H5IO6
Periodic acid cleaves the C-C bond of a glycol.
OH C OH HO O - 2 H O + I 2 OH C OH HO OH
A 1,2-diol Periodic acid
OH C O O C O I + H3 IO4 C O OH C O OH Iodic acid
A cyclic periodic ester OXIDATION BY HIO4 It also cleaves a-hydroxyketones
H H H H C OH H C OH H C O + H5 IO6 H2 O C O HO C OH HO C O andR a-hydroxyaldehydes. R R a-Hydroxyketone Hydrated intermediate
O OH OH H C H C OH H C O + H O H5 IO6 H C OH 2 H C OH H C O R R R a-Hydroxyaldehyde Hydrated intermediate EXAMPLES. IDENTIFY EACH OF THE GLUCOSE DERIVATIVES.
A + 4 HIO4 yielded 3 HCO2H + HCHO + OHC-CO2H
B + 5 HIO4 yielded 4 HCO2H + 2 HCHO
C + 3 HIO4 yielded 2 HCO2H + 2 OHC-CO2H
CO2H Analysis of A: 4 moles of periodic acid used. 4 bonds broken. H OH
Products: HO H Formic acid from –CHOH- or CHO-. H OH Formaldehyde from CH2OH- OHC-CO2H from –CHOH-CO2H H OH
CH2OH ANOTHER EXAMPLE
Oxidation of methyl b-D-glucoside consumes two
moles of HIO4 and produces one mole of formic acid, which indicates three adjacent C-OH groups.
periodic acid cleavage at these two bonds 2 HIO4 H
CH OH CH2 OH 2 C O HO O 2 HIO5 O HO C O OH OCH OCH3 C O 3 OH H Methyl b-D-glucopyranoside H NITRIC ACID OXIDATION
CH O CO2H
H OH H OH HNO HO H 3 HO H OH OH H 60°C H H OH H OH
CH2OH CO2H
D-Glucose D-Glucaric acid (41%) URONIC ACIDS
Uronic acids contain both an aldehyde and a terminal
CO2H function.
CH O
H OH HO2C O HO H HO H OH HO OH H OH OH
CO2H D-Glucuronic acid OSAZONES, EPIMERS
O CHO CH=NNHPh 3 PhNHNH2 PhCHO
CHOH C=NNHPh O
osazone aldose osone USE OF OSAZONE IN STRUCTURE DETERMINATION Fischer found that (+) glucose and (+) mannose yielded the same osazone indicating that they differed only at the C2 configuration. Hence, if we know the configuration of (+) glucose we immediately have that of (+) mannose. Stereoisomers that differ in configuration at only one stereogenic center are called epimers. D-glucose and D- mannose are epimers.
NNHPH CHO CHO HC
HO H H OH PhHNN
HO H HO H HO H
H OH H OH H OH
H OH H OH H OH
CH2OH CH2OH CH2OH
D glucose D mannose CONFORMATIONAL FORMULAS; B TO A CONVERSION For pyranoses, the six-membered ring is more accurately represented as a chair conformation.
OH OH H H Open chain form H O H OH HO HO HO OH HO O H OH H OH H H H H b-D-Glucopyranose rotate about (b-D-Glucose) C-1 to C-2 bond OH H OH H H OH H O HO HO HO H HO H H OH H OH H O H OH a-D-Glucopyranose (a-D-Glucose) CONFORMATIONAL FORMULAS The orientations of groups on carbons 1-5 in the Haworth and chair projections of b-D-glucopyranose are up-down-up-down-up.
6 CH OH 2 6 5 O H OH(b) 4 CH2 OH H HO O 4 OH H 1 HO 5 HO H 3 2 OH( b) 3 2 OH 1 H OH b-D-Glucopyranose b-D-Glucopyranose (Haworth projection) (chair conformation) GLYCOSIDIC BONDS The anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond:
R-OH + HO-R' R-O-R' + H2O E.g., methanol reacts with the anomeric OH on glucose to form methyl glucoside (methyl- glucopyranose).
H OH H OH
H O H2O H O HO HO HO H + CH3-OH HO H H OH H OH
H OH H OCH3 a-D-glucopyranose methanol methyl-a-D-glucopyranose GLYCOSIDES, ANOMERIC OH BECOMES OR, ACETAL FORMATION. Glycoside: A carbohydrate in which the -OH of the anomeric carbon is replaced by -OR. methyl b-D-glucopyranoside (methyl b-D-glucoside)
glycosidic bond CH2 OH CH2 OH CH2 OH O OH O OCH O H H + H 3 H H H H H + CH OH + OH H 3 OH H OH H -H2 O HO H HO H HO OCH3 H OH H OH H OH b-D-Glucopyranose Methyl b-D-gluco- Methyl a-D-gluco- (b-D-Glucose) pyranoside pyranoside ( Methyl b-D-glucoside) (Methyl a-D-glucoside) Disaccharide
A disaccharide is formed when a dehydration reaction joins two monosaccharides This covalent bond is called a glycosidic bond Lactose = Glu + Gal Maltose = Glu + Glu Sucrose = Glu + Fru 6 CH2OH 6 CH2OH 5 5 Disaccharides: H O H H O H H H Maltose, a cleavage 4 OH H 1 4 OH H 1 OH O OH product of starch 3 2 3 2 (e.g., amylose), is a H OH maltose H OH disaccharide with an a(1 4) glycosidic 6CH2OH 6 CH2OH 5 5 link between C1 - C4 H O H O OH H H OH of 2 glucoses. 4 OH H 1 O 4 OH H 1 OH H H It is the a anomer 3 2 3 2 H OH H OH (C1 O points down). cellobiose Cellobiose, a product of cellulose breakdown, is the otherwise equivalent b anomer (O on C1 points up). The b(1 4) glycosidic linkage is represented as a zig- zag, but one glucose is actually flipped over relative to the other. Other disaccharides include:
Sucrose, common table sugar, has a glycosidic bond linking the anomeric hydroxyls of glucose & fructose. Because the configuration at the anomeric C of glucose is a (O points down from ring), the linkage is a(12). The full name of sucrose is a-D-glucopyranosyl- (12)-b-D-fructopyranose.)
Lactose, milk sugar, is composed of galactose & glucose, with b(14) linkage from the anomeric OH of galactose. Its full name is b-D- galactopyranosyl-(1 4)-a-D-glucopyranose POLYSACCHARIDES
Polysaccharides, the polymers of sugars, have storage and structural roles The structure and function of a polysaccharide are determined by its sugar monomers and the positions of glycosidic linkages STORAGE POLYSACCHARIDES
Starch, a storage polysaccharide of plants, consists entirely of glucose monomers Plants store surplus starch as granules within chloroplasts and other plastids Glycogen is a storage polysaccharide in animals Humans and other vertebrates store glycogen mainly in liver and muscle cells 6 CH OH CH2OH CH2OH CH2OH CH2OH 2 O H O H H 5 O H H O H H O H H H H H H H H OH H 1 4 OH H 1 OH H OH H OH H O O O O OH OH 3 2 H OH H OH H OH H OH H OH amylose Polysaccharides: Plants store glucose as amylose or amylopectin, glucose polymers collectively called starch. Glucose storage in polymeric form minimizes osmotic effects. Amylose is a glucose polymer with a(14) linkages. The end of the polysaccharide with an anomeric C1 not involved in a glycosidic bond is called the reducing end. CH2OH CH2OH H O H H O H amylopectin H H OH H OH H 1 O OH O H OH H OH
CH2OH CH2OH 6CH2 CH2OH CH2OH H O H H O H H 5 O H H O H H O H H H H H H 1 4 OH H OH H 4 OH H OH H OH H O O O O OH OH 3 2 H OH H OH H OH H OH H OH
Amylopectin is a glucose polymer with mainly a(14) linkages, but it also has branches formed by a(16) linkages. (Branched sugars) Branches are generally longer than shown above. The branches produce a compact structure & provide multiple chain ends at which enzymatic cleavage can occur. CH2OH CH2OH glycogen H O H H O H H H OH H OH H 1 O OH O H OH H OH
CH2OH CH2OH 6CH2 CH2OH CH2OH H O H H O H H 5 O H H O H H O H H H H H H 1 4 OH H OH H 4 OH H OH H OH H O O O O OH OH 3 2 H OH H OH H OH H OH H OH
Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin. But glycogen has more a(16) branches. The highly branched structure permits rapid glucose release from glycogen stores, e.g., in muscle during exercise. The ability to rapidly mobilize glucose is more essential to animals than to plants. 6 CH OH CH2OH CH2OH CH2OH CH2OH 2 5 O H O H O H O H O H OH H H H H H OH H 1 O 4 OH H 1 O OH H O OH H O OH H H OH H H H H 3 2 H OH H OH H OH H OH H OH cellulose Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with b(14) linkages. Every other glucose is flipped over, due to b linkages. This promotes intra-chain and inter-chain H-bonds and
van der Waals interactions, that cause cellulose chains to be straight & rigid, and pack with a crystalline arrangement in thick bundles - microfibrils. Schematic of arrangement of cellulose chains in a microfibril. 6 CH OH CH2OH CH2OH CH2OH CH2OH 2 5 O H O H O H O H O H OH H H H H H OH H 1 O 4 OH H 1 O OH H O OH H O OH H H OH H H H H 3 2 H OH H OH H OH H OH H OH cellulose
Multisubunit Cellulose Synthase complexes in the plasma membrane spin out from the cell surface microfibrils consisting of 36 parallel, interacting cellulose chains. These microfibrils are very strong. The role of cellulose is to impart strength and rigidity to plant cell walls, which can withstand high hydrostatic pressure gradients. Osmotic swelling is prevented. Explore and compare structures of amylose & cellulose using Chime. CH OH D-glucuronate 6 2 O COO H 5 6 H 1 O O 4 H H 5 H OH H 4 OH H 1 3 2 H H NHCOCH3 3 2 O H OH N-acetyl-D-glucosamine hyaluronate Glycosaminoglycans (mucopolysaccharides) are linear polymers of repeating disaccharides. The constituent monosaccharides tend to be modified, with acidic groups, amino groups, sulfated hydroxyl and amino groups, etc. Glycosaminoglycans tend to be negatively charged, because of the prevalence of acidic groups. CH OH D-glucuronate 6 2 O COO H 5 6 H 1 O O 4 H H 5 H OH H 4 OH H 1 3 2 H H NHCOCH3 3 2 O H OH N-acetyl-D-glucosamine hyaluronate
Hyaluronate (hyaluronan) is a glycosaminoglycan with a repeating disaccharide consisting of 2 glucose derivatives, glucuronate (glucuronic acid) & N-acetyl- glucosamine. The glycosidic linkages are b(13) & b(14).
heparan sulfate core glycosaminoglycan protein
transmembrane a-helix cytosol
Proteoglycans are glycosaminoglycans that are covalently linked to serine residues of specific core proteins. The glycosaminoglycan chain is synthesized by sequential addition of sugar residues to the core protein. Some proteoglycans of the extracellular matrix bind non-covalently to hyaluronate via protein domains called link modules. E.g.: • Multiple copies of the aggrecan proteoglycan associate with hyaluronate in cartilage to form large complexes. • Versican, another proteoglycan, binds hyaluronate in the extracellular matrix of loose connective tissues.
CH OH D-glucuronate 6 2 O COO H 5 Websites on: 6 H 1 O O 4 H Aggrecan H 5 H OH H 4 OH H 1 3 2 Aggrecan & H NHCOCH3 H O versican. 3 2 O H OH N-acetyl-D-glucosamine hyaluronate iduronate-2-sulfate N-sulfo-glucosamine-6-sulfate H CH2OSO3
H O H O H COO H OH H O OH H H O
H OSO3 H NHSO3 heparin or heparan sulfate - examples of residues
Heparan sulfate is initially synthesized on a membrane-embedded core protein as a polymer of alternating N-acetylglucosamine and glucuronate residues. Later, in segments of the polymer, glucuronate residues may be converted to the sulfated sugar iduronic acid, while N-acetylglucosamine residues may be deacetylated and/or sulfated. PDB 1RID Heparin, a soluble glycosaminoglycan found in granules of mast cells, has a structure similar to that of heparan sulfates, but is more highly sulfated. When released into the blood, it inhibits clot formation by interacting with the protein antithrombin. heparin: (IDS-SGN) Heparin has an extended helical 5 conformation. C O N S
Charge repulsion by the many negatively charged groups may contribute to this conformation. Heparin shown has 10 residues, alternating IDS (iduronate- 2-sulfate) & SGN (N-sulfo-glucosamine-6-sulfate).
heparan sulfate core glycosaminoglycan Some cell surface heparan protein sulfate glycosaminoglycans remain covalently linked to transmembrane core proteins embedded in a-helix the plasma membrane. cytosol
The core protein of a syndecan heparan sulfate proteoglycan includes a single transmembrane a-helix, as in the simplified diagram above.
The core protein of a glypican heparan sulfate proteoglycan is attached to the outer surface of the plasma membrane via covalent linkage to a modified phosphatidylinositol lipid. Proteins involved in signaling & adhesion at the cell surface recognize & bind heparan sulfate chains. E.g., binding of some growth factors (small proteins) to cell surface receptors is enhanced by their binding also to heparan sulfates. Regulated cell surface Sulf enzymes may remove sulfate groups at particular locations on heparan sulfate chains to alter affinity for signal iduronate-2-sulfate N-sulfo-glucosamine-6-sulfate proteins, e.g., H CH2OSO3 growth factors. H O H O H COO H OH H O OH H Diagram H O by Kirkpatrick & Selleck. H OSO3 H NHSO3 heparin or heparan sulfate - examples of residues
CH2OH C O Oligosaccharides H O O CH2 CH H NH serine that are covalently OH H residue attached to proteins OH O H or to membrane H HN C CH lipids may be linear 3 or branched chains. b-D-N-acetylglucosamine
O-linked oligosaccharide chains of glycoproteins vary in complexity. They link to a protein via a glycosidic bond between a sugar residue & a serine or threonine OH. O-linked oligosaccharides have roles in recognition, interaction, and enzyme regulation.
CH2OH C O
H O O CH2 CH H NH serine OH H residue OH O H
H HN C CH3 b-D-N-acetylglucosamine
N-acetylglucosamine (GlcNAc) is a common O-linked glycosylation of protein serine or threonine residues. Many cellular proteins, including enzymes & transcription factors, are regulated by reversible GlcNAc attachment. Often attachment of GlcNAc to a protein OH alternates with phosphorylation, with these 2 modifications having opposite regulatory effects (stimulation or inhibition).
CH2OH O HN
H O HN C CH2 CH Asn H C O OH H HN OH H HC R X H HN C CH3 C O O N-acetylglucosamine HN Initial sugar in N-linked HC R Ser or Thr glycoprotein oligosaccharide C O
N-linked oligosaccharides of glycoproteins tend to be complex and branched. First N-acetylglucosamine is linked to a protein via the side-chain N of an asparagine residue in a particular 3-amino acid sequence. NAN NAN NAN
Gal Gal Gal
NAG NAG NAG
Man Man N-linked oligosaccharide
Man Key: NAN = N-acetylneuraminate NAG Gal = galactose NAG = N-acetylglucosamine NAG Fuc Man = mannose Asn Fuc = fucose
Additional monosaccharides are added, and the N- linked oligosaccharide chain is modified by removal and addition of residues, to yield a characteristic branched structure. Many proteins secreted by cells have attached N- linked oligosaccharide chains. Genetic diseases have been attributed to deficiency of particular enzymes involved in synthesizing or modifying oligosaccharide chains of these glycoproteins. Such diseases, and gene knockout studies in mice, have been used to define pathways of modification of oligosaccharide chains of glycoproteins and glycolipids. Carbohydrate chains of plasma membrane glycoproteins and glycolipids usually face the outside of the cell. They have roles in cell-cell interaction and signaling, and in forming a protective layer on the surface of some cells. Lectins are glycoproteins that recognize and bind to specific oligosaccharides. Concanavalin A & wheat germ agglutinin are plant lectins that have been useful research tools. The C-type lectin-like domain is a Ca++-binding carbohydrate recognition domain in many animal lectins. Recognition/binding of CHO moieties of glycoproteins, glycolipids & proteoglycans by animal lectins is a factor in: • cell-cell recognition • adhesion of cells to the extracellular matrix • interaction of cells with chemokines and growth factors • recognition of disease-causing microorganisms • initiation and control of inflammation. Examples of animal lectins: Mannan-binding lectin (MBL) is a glycoprotein found in blood plasma. It binds cell surface carbohydrates of disease- causing microorganisms & promotes phagocytosis of these organisms as part of the immune response. selectin Selectins are integral proteins lectin domain of mammalian cell plasma membranes with roles in cell-cell recognition & binding. outside The C-type lectin-like domain transmembrane is at the end of a multi-domain a-helix cytosol extracellular segment extending cytoskeleton binding domain out from the cell surface. A cleavage site just outside the transmembrane a- helix provides a mechanism for regulated release of some lectins from the cell surface. A cytosolic domain participates in regulated interaction with the actin cytoskeleton. DISACCHARIDES
Sucrose, table sugar (1-α-D-glucopyranosyl-β-D- fructofuranoside) Maltose, from barley (4-α –D-glucopyranosyl α or β D- glucopyranose) Lactose, milk sugar (4- β-D-Galactopyranosyl-D-glucopyranose) SUCROSE, TABLE SUGAR
Table sugar, obtained from the juice of sugar cane and sugar beet.
a-1, b-2 bond LACTOSE
The principle sugar present in milk, 5 – 10%. MALTOSE
From malt, the juice of sprouted barley and other cereal grains. DETERMINATION OF STRUCTURE
General steps involved can be summarized as follows. 1. Determination whether the disaccharide is reducing or not. 2. Acid hydrolysis of disaccharide yields two monosaccharide's which are then identified. 3. The determination of glycosidic bond , α or β often rest on enzyme with known specificities. e.g yeast maltase attacks only α –glycosides while almond emulsion is specific for β - glucosides 4. Next step is to ascertain which hydroxyl group of the molecule acting as alcohol is involved in formation of glycosidic linkage. This is done by completely methylating disaccharide. Methylated disaccharide is then hydrolyzed to yield methyleted monosaccharide's which are then identified by oxidation method
An alternative method for determining the position of glycosidic linkage is periodate oxidation. 5. Finally the structure is established by synthesis
STRUCTURE DETERMINATIN OF (+) MALTOSE
Experimental Facts
C12H22O11 Positive for Tollens and Fehlings solution, reducing sugar Reacts with phenylhydrazine to yield osazone,
C12H20O9(NNHC6H5)2 Oxidizes by bromine water to monocaboxylic acid. Exists in two forms which undergo mutarotation. Consistent with two aldoses linked together with one hemiacetal group. MORE DATA….. Maltose undergoes hydrolysis with aq. acid or maltase to yield two D (+) glucose units. Two glucose units joined together: glucose – acetal linkage (glucoside) – glucose – hemiacetal. Maltase hydrolysis is characteristic of a glucosides. Conclude something like HOW TO PROCEED….. Label the rings and label the free OH groups.
Next Slide HYDROLYSIS PRODUCTS
Point of attachment to the other glucose Used in unit. hemiacetal link.
Not the reducing This glucose derivative was aldohexose unit (not the carboxylic acid). the “free” CHO unit, the reducing sugar. Structure on next slide. MALTOSE Dehydration 1–4 reaction in the glycosidic synthesis of maltose linkage
Glucose Glucose Maltose
Dehydration 1–2 reaction in the glycosidic synthesis of sucrose linkage
Glucose Fructose Sucrose STARCH
Starch is used for energy storage in plants It can be separated into two fractions; amylose and amylopectin; each on complete hydrolysis gives only D- glucose. Amylose: A polysaccharide composed of continuous, unbranched chains of up to 4000 D-glucose units joined by a-1,4-glycosidic bonds. Amylopectin: A highly branched polymer of D-glucose; chains consist of 24-30 units of D-glucose joined by a- 1,4-glycosidic bonds and branches created by a-1,6- glycosidic bonds. a Glucose b Glucose
a and b glucose ring structures
Starch: 1–4 linkage of a glucose monomers.
Cellulose: 1–4 linkage of b glucose monomers. GLYCOGEN
Glycogen is the reserve carbohydrate for animals. Like amylopectin, glycogen is a nonlinear polymer of D-glucose units joined by a-1,4- and a-1,6-glycosidic bonds bonds. The total amount of glycogen in the body of a well- nourished adult is about 350 g (about 3/4 of a pound) divided almost equally between liver and muscle. CELLULOSE
Cellulose: A linear polymer of D-glucose units joined by b-1,4-glycosidic bonds. It has an average molecular weight of 400,000 g/mol, corresponding to approximately 2800 D-glucose units per molecule. Both rayon and acetate rayon are made from chemically modified cellulose.