Carbohydrates Chemistry of Importance PDF

Chemistry of Carbohydrates By Prof. Dr. Abdellah Ali Omar M.B.B.Ch, M.S, M.D Professor of Medical Biochemistry CARBO HYDRATES ( C + H2O ) Hydration of Carbon are Polyhydroxy Aldehydes, Polyhydroxy Ketones, their Polymers & Substances derived f...

Chemistry of Carbohydrates By Prof. Dr. Abdellah Ali Omar M.B.B.Ch, M.S, M.D Professor of Medical Biochemistry CARBO HYDRATES ( C + H2O ) Hydration of Carbon are Polyhydroxy Aldehydes, Polyhydroxy Ketones, their Polymers & Substances derived from them  Poly-OH CHO or Poly-OH C=O, Derivatives or Polymers  Carbohydrates Yields One Unit of H2O for Every Carbon atom 6 CO2 + 6 H2O → C6H12O6 + 6 O2  I (CH2O)n or H - C - OH I  Carbohydrates are Obtained mainly by Eating Plants Carbohydrates Constitute ~65% of the Typical Human Diet  Importance 1- Widely Distributed in living tissues 2- They form an important source of readily available energy 3- They can be stored as a reserve food (Glycogen) Immediate Source of Energy, 4.1 cal / gm (60-70%) Lactose of milk is chief source of energy for babies and Fructose of semen provides energy to sperms Provides bulk to the food, 300 – 400 gms / day Takes part in structure of cell membranes Form compounds like: Glycoproteins, Glycolipids Conversion into fats if taken in Excess Glycosaminoglycans act as lubricant in joints ( ground substance) Classification Based on Hydrolysis (according to Number of Sugar Units) Monosaccharides: Simple sugars, Not Further Hydrolyzed; Ribose, Glucose, Fructose, Galactose, Mannose  Disaccharides: On hydrolysis yield 2 units of Monosaccharides; Lactose, Sucrose, Maltose, Trehalose. Oligosaccharides: Consists of 3-10 units of monosaccharides i.e. Trisaccharides (Malto-triose, raffinose), Tetrasaccharides (stachyose),  Polysaccharides: more than 10 units of Monosaccharides These may be: Homopolysaccharides: Include one type of monosaccharides: Starch, Glycogen, Cellulose, Dextrins, Innulin Heteropolysaccharides:  Heteropolysaccharides Include Different types of Monosaccharides Heparin, Hyaluronic acid, Chondroitin sulfate, Agar, Pectins Miscellaneous: Lipopolysaccharides, Glycolipids, Glycoproteins. Based on Number of Carbon Atoms 1. Trioses 3C. Glyceraldehydes (Aldo), DHA (Keto). 2. Tetroses 4C. Erythrose (Aldo), erythrulose (Keto). 3. Pentoses 5C. Ribose (Aldo), ribulose (Keto). 4. Hexoses 6C. Glucose, Mannose, galactose (Aldo), fructose (Keto). 5. Heptose 7C. sedoheptose, sedoheptulose. 6. Dimmers 2 units of monosacharides e.g. Lactose, sucrose, maltose. 7. Polymers > 10 units Starch, glycogen, inulin,cellulose etc. Based on type of Active Groups 1. Aldehydes: Glucose, galactose, ribose, etc. 2. Ketones : Fructose, ribulose, erythrulose. Based on Reducing Properties 1. Reducing Sugars: monosaccharides and disaccharides except sucrose. 2. Non reducing Sugars sucrose and all polysaccharide Based on General Characteristics Sugars Starches 1. Sweet in taste Tasteless 2. Crystalline form Amorphous 3. Make true Make colloidal solutions solutions 4. Smaller in size Larger in size 5. Low molecular Higher molecular weight weight 6. Reducing Non reducing  Based on Functions 1.Nutritional Monosaccharides, disaccharides, starch. 2.Structural Cellulose, liginins, chitin, agar etc. 3.Anticoagulant: Heparin (mucopolysaccharides). 4.Lubricative: Hyaluronic acid in joints. 5.Energy reservoir: Starch and glycogen (liver & muscles) Based on Digestion 1. Digestible Disaccharides, Starch, Glycogen. 2. Non digestible Cellulose, Agar, Pectins. Carbohydrates are Classified according to Number of Monomers (Sugar Units) Monomeric (One sugar Unit) called “Monosaccharides.” Dimeric (Two sugar Unit) are called “Disaccharides.” Polymeric (More than 10 sugar Unit) are: “Polysaccharides.” Monosaccharides  Monosaccharides Of Medical Importance Trioses Pentoses Hexoses Triose = contain 3 carbon atoms Tetrose = contain 4 carbon atoms Pentose = contain 5 carbon atoms Hexose = contain 6 carbon atoms Heptose = contain 7 carbon atoms  CH 2OH H O O H OH H OH n n CH 2OH CH 2OH aldose ketose  are Carbohydrates Contain One sugar Unit  Can be Classified according to Type of Chemically active group  Aldoses have an Ketoses have a keto aldehyde group at one 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 CH2OH CH2OH D-glucose D-fructose Aldoses Carbohydrates – Aldose Family Ketoses Carbohydrates – Ketose Family 30  O H O H C C H – C – OH HO – C – H HO – C – H H – C – OH H – C – OH HO – C – H H – C – OH HO – C – H CH2OH CH2OH D-glucose L-glucose  Trioses Are Monosaccharides Carbohydrates contain One Sugar Unit composed of 3 carbons Examples: (1) Aldotrise; Glyceraldehyde (2) Ketotriose; Dihydroxyacetone Present in cells as intermediates during Glucose Catabolosm  Pentoses: 5 Carbon Sugars Ribose: RNA Adenosine + Phosphate ----> AMP N. base + Ribose + Phosphate ----> AMP Deoxyribose: DNA Sugars Occurrence Biological Importance D-Ribose Nucleic Acids (RNA,DNA) Form Nucleic Acids Form- Co Enzymes Form Flavoproteins Are intermediates of HMP Shunt D- Ribulose Formed in metabolic processes Is intermediate of HMP Shunt D–Arabinose Gum Arabic, Plum & Cherry gums Constituents of Glycoproteins D- Xylose Wood Gums Constituent of Glycoproteins Proteoglycans Glycosaminoglycans D- Lyxose Heart Muscle Part of Lyxoflavin in human heart muscle L-Xylulose Formed in Uronic Acid Pathway Found in Urine essential pentosuria  Hexoses D sugars are Important in Nutrition D - glucose, D - fructose, D - galactose, D - mannose Sugars Occurrence Biological Importance Basic sugar of body Fruit Juices Provides sweet taste to food D-Glucose Hydrolysis of Starch Maintains blood glucose level Cane Sugar Used by body tissues Maltose & Lactose Main source of energy to brain Hyperglycemia & Glycosuria in DM Can be changed to Glucose Fruit Juices, Honey Can provide energy D-Fructose Hydrolysis of sucrose Compound of HMP shunt Hydrolysis of Inulin Provides energy to sperms Hereditary fructose intolerance leads to fructose accumulation & hypoglycemia Can be changed to Glucose Can provide energy D-Galactose Hydrolysis of Lactose Synthesis in Mammary gland Constituents of Glyco - Proteins Constituents of Glyco - Lipids Can cause Galactosemia & Cataract D-Mannose Hydrolysis of plants mannans Constituents of many glycoproteins Hydrolysis of gums  Glucose Major compound of Metabolism Used by all Tissues for energy Some tissues use ONLY glucose Lens of eye, Nervous tissue, RBC’s, Parts of kidney Highly regulated in the blood because of its importance  Normal Fasting Blood Glucose: 75-105 mg/dl plasma Hypoglycemia: below that Fasting Hypoglycemia Reactive Hypoglycemia Too much insulin moves glucose out of blood Hyperglycemia: above that OH on the Right side of Anomeric carbon atom Alpha sugar () OH on the Left side of Anomeric carbon atom Beta sugar (β) Straight Chain Formula Fisher Fisher Projection Projection Formula Formula  ‫‪Haworth Formula‬‬ ‫‪Haworth‬‬ ‫‪Formula‬‬ ‫ما على الشمال‬ ‫ما على اليمين‬ ‫يكتب فوق‬ ‫يكتب تحت‬ Anomers  Fructose KetoHexose forms a 4 Member Ring (Furan) absorbed by Body but Metabolized to Glucose in Liver before it is Usable by the body Commonly found in Fruit, Honey Hemiacetal & Hemiketals Aldehyde Ketone Fisher Projection Formula  Epimers 2 Monosaccharides are different in OH configuration of One Assymetric carbon atom Glucose & Galactose are Epimers Glucose & Mannose are Epimers Mannose & Galactose are Not Epimers Anomers: Formation of the cyclic structures occurs as a distant ‫ما على اليمين‬ hydroxyl group attacks the carbonyl carbon ‫تحت‬ (the “anomeric” ‫يكتب‬atom). carbon “α” “anomers” “β” ‫ما على الشمال‬ 49 ‫يكتب فوق‬  Enantiomere: are Sterioisomers = Mirror image OH on Right side in SubTerminal C. D form OH on Left side in SubTerminal C. L form L form. 4 carbons 5 carbons included included ‫ما على الشمال يكتب فوق‬ ‫ما على اليمين يكتب تحت‬  Reducing Sugars Sugars containing Free Aldehyde or Ketone group can Reduce other reagents e.g. Benedict's reagent & Fehling's solution Cupric Cuprous Stereoisomerization Asymmetric carbon IS Carbon atom attached to 4 different groups n = number of asymmetric carbons 2 n = number of stereoisomers possible For glucose, how many stereoisomers are possible? D & L Isomers Based on the Configuration of the OH & H attached to Subterminal Carbon Atom  d (+) = dextarotatory rotates Plain Polarized Light to the Right most Glucose is D l (-) = levarotatory rotates Plain Polarized Light to the Left When D (+) & l (-) present in equal amounts: No Optical Activity Asymmetric Carbon Atom: It is the carbon which attached to 4 different groups or atoms like these four in the middle of glucose Any substance contain 1 or more asymmetric C atom Shows 2 Properties: 1 - Optical Activity 2 - Optical Isomerism Optical Activity: is the Ability of the Molecules to Rotate the Plain Polarized Light Either to the right or to the left Mutarotation: It is Gradual Change of Specific Rotation of any Optically active substance having free Aldehyde or Ketone group  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 -D-glucopyranose -D-glucopyranose  Sugar Derivatives: Reduction of Monosaccharides Give Alcohol - Glucose give Sorbitol - Galactose give Galacticol (Dulicitol) - Mannose give Mannitol - Fructose give mixture of Mannitol & Sorbitol - Ribose give Ribitol - Inositol give Cyclitol 2- Oxidation of Glucose Give Acid -Gluconic (Aldonic) at C 1 -Glucuronic (Uronic) at C 6 - Gluccaric (Aldaric) at C 1, 6  3 - Reaction with phosphoric acid the OH groups in a saccharide can react with various acids to give esters. Phosphate esters of saccharides are often important. Sugar Phosphates Intermediates in Metabolic Pathways Why Phosphorylated? they can’t pass through cell membranes so metabolically active molecules stay inside cells 4- Amino sugar Sugar contains an amine group in place of a OH group (NO. 2) Glucosamine Galactosamine mannosamine  Derivatives of Amine containing Sugars, e.g. N-acetyl-Glucosamine & Sialic acid,  5 - Deoxy sugars: the most important One is 2-deoxy-D-ribose Pentose sugar found in Deoxyribonucleic acid (DNA); Oxygen atom at position 2 has been Removed  L-Fucose ( 6-Deoxy-L-Galactose ) Most Common; present in the Carbohydrate portion of Blood-group substances & in RBCs membranes  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 -D-glucosamine -D-N-acetylglucosamine  N-acetyl Neuraminic Acid (NANA)  Neuraminic acid is a 9-carbon Monosaccharide It is condensation product of Pyruvic acid & D-mannosamine  Neuraminic acid does not occur naturally, but its derivatives are found in animal tissues & in bacteria, especially in Glycoproteins & Gangliosides  N- or O-substituted derivatives of Neuraminic acid are collectively known as Sialic acids, the predominant one being N-acetyl-Neuraminic acid  O H D-Mannosamine + Pyruvic acid O  H3C C NH COO R HC OH H H R= HC OH H OH CH2OH OH H N-acetylneuraminate (sialic acid)  Sialic acid is often found as a Terminal Residue of Oligosaccharide chains of Glycoproteins Sialic acid Give Negative Charge to GlycoProteins, Due to its carboxyl group D-Mannosamine + Pyruvic acid Disaccharides Disaccharides Sucrose = Glucose + Fructose Lactose = Glucose + Galactose Maltose = Glucose + Glucose Isomaltose = 1 – 6 Glucose Dimer Cellobiose = beta 1-4 Glucose Dimer Trehalose = 1 – 1 Glucose Dimer Sugar Occurrence Biological Importance Milk Provides energy to infants Lactose May occur in urine Helps in absorption of Ca & P in pregnancy Lactase deficiency causes lactose intolerance Provides sweet taste to food Non Reducing sugar is its property Sucrose Cane and beat sugar Provides energy to the tissues Surghum, Pineapple In sucrase deficiency, malabsorption leads to Carrot roots diarrhea Maltose Hydrolysis of Starch Converted to Glucose by Maltase Germinating cereals Provides energy in body and malt  Maltose Found at Seed stage of Plant life Cycle Starch in Seed is broken down during Germination Found when starch is digested to smaller molecules Also found in beer   1-4 Link Glucose + Glucose Glycosidic Bonds:  Sucrose: Widely Distributed Disaccharides Table sugar: sources are Sugar Cane & Sugar Beets Contain fructose and fructose is very sweaty Non-reducing sugar: NOT CONTAIN Free Aldehyde or Ketone group  Sucrose: called Invert Sugar Sucrose: + 66.5 When hydrolyzed: Optical Rotation Changes Glucose: + 52.7 Fructose: - 92.4 Net Optical Activity: - 40 There is Invert from Positive Rotation to Negative Rotation  Lactose: (Milk Sugar) Principle CHO of Cow’s Milk Lactase: enzyme that digests Lactose in GI tract Lactose -----> Glucose & Galactose Lactose Intolerance or Lactase Deficiency the person is unable to digest Lactose because can’t produce Lactase Galactose + Glucose B 1-4 Link Glucose + Fructose 1 – 2 Link Relative Sugar Sweetness Scale Lactose 0.16 Galactose 0.32 Maltose 0.33 Glucose 0.74 Sucrose 1.00 Invert Sugar 1.25 Fructose 1.73 Sodium cyclamate 30 Aspartame 180 Saccharin 450 Sucralose 600  6 CH2OH Glucose + Glucose 6 CH2OH H 5 O 1 – 4 Link H H 5 O H H H 1 4 1 4 OH H OH H OH O OH 3 2 3 2 H OH H OH maltose 6 CH2OH Glucose + Glucose 6 CH2OH  1 – 4 Link 5 O 5 H H O OH H H 4 1 O 4 1 OH H OH H OH H H 3 2 3 2 H OH H OH cellobiose  Lactose (Milk Sugar) is Composed of Galactose & Glucose, with (14) linkage from the Anomeric Carbon of Galactose Its Full Name is -D-galactoPyranosyl-(1 4)--D-glucoPyranose sucrose cellobiose trehalose maltose  Polysaccharides Homopolysaccharides Heteropolysaccharides Carbohydrates Homopolysaccharides (Homoglycans) Starch Glycogen Dextrins Cellulose Inulin Chitin  Starch: 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.  Amylopectin: is a Glucose Polymer with Mainly (14) Linkages, but it also has Branches formed by (16) Linkages. Branches are generally longer The branches produce a compact structure & provide multiple chain ends at which enzymatic cleavage can occur  Hydrolysis of starch Course of Reaction Hydrolysis with I2 Starch Blue Soluble starch Blue Amylodextrin Purple Erythrodextrin Red Achrodextrin Color less Maltose --------  Hydrolysis by Enzyme Amylase ends at Maltose Starch Amylase Maltose Maltase Glucose + Dextrins Starch Acid Hydrolysis Glucose The most common polysaccharide in animal cells is glycogen Glycogen is a polymer of glucose, containing both α(1→4) & α(1→6) linkages (at branching points) Branching occurs about every 10 glucose residues STARCH & GLYCOGEN Common features: 1. Both are homopolysaccharides. 2. Both are polymers of glucose. 3. Both are tasteless & large in size. 4. Both give no aldo & keto reactions. 5. Both are hydrolyzed by amylase.  Starch Glycogen 1. In plant kingdom Occurs in animal kingdom 2. Less branched Highly branched (tree like) 3. Has amylose, amylopectine -- Nil -- 4. Not stored as such in body Stored in body 5. No inherited disorder Glycogen storage diseases 6. Iodine test gives blue color Iodine test gives red color  Cellulose It is a Homopolysaccharides. It is a Polymers of Glucose. It is Long non-Branched chain of glucose units linked by ß 1-4 glycosidic bond. Source: It is the chief constituent of the framework of plant e.g. leaved vegetables, fruit and wood. Properties:  not digested  insoluble in water  source of energy in herbivores  Heteropolysaccharides ( Hetroglycans )) Glycosaminoglycans Mucilages Glycoproteins Heteropolysaccharides When CHO content is > 4% = Mucoproteins When CHO content is < 4% = Glycoproteins Mucoproteins also called Proteoglycans  Glycosaminoglycans These are Long Unbranched Hetero-Polysaccharides formed of Repeating Disaccharides. Contains: 1 - Amino Sugars (glucosamine & galactosamine) in which the amino group is acetylated 2 - Acidic Sugars (glucuronic & L-iduronic acids)  Chemistry & Functions Proteoglycans are linked to Glycosaminoglycans (GAGs) CHO contents is greater (95%) as compared to Glycoproteins Constituent of E.C. matrix & ground substance Acts as barrier in tissues: Metabolites pass but resist penetration of bacteria and infective agents Acts as lubricant and shock absorbent in joints Role as anticoagulant in vitro and vivo – heparin Role as coenzymes – heparin in LPL (clearing factor) As receptor of cell and participate in cell adhesions Role in compressibility of cartilage in wt. bearing Dermatan sulfate in sclera maintains shape of eye Keratan sulfate maintains corneal transparency Glycosaminoglycans 1 - Hyaluronic Acid 2 - Chondritin Sulfate 3 - Keratan sulfate. 4 - Dermatan sulfate 5 - Heparin 6 - Heparan sulfate  1- Hyaluronic Acid Structure: Contains ß glucuronic acid & ß N-acetylglucosamine linked by ß 1-3 glycosidic bond It is the only GAG which contain no sulfate group & not covalently bound to proteins Site: Synovial fluid, Vitrous body of the eye Embryonic tissue, Cartilage & CT Function of Hyaluronic Acid: 1- lubricant and shock absorbent in joints. 2- Permit cell migration during wound repair & morphogenesis. 3- Role in compressibility of cartilage in wt. bearing Role in diseases: increase by tumour cells, so help metastasis  CH2OH D-glucuronate 6  H 5 O 6COO H 4 1 O O H H 5 H OH H 4 H 1 3 2 OH H H NHCOCH3 3 2 O H OH N-acetyl-D-glucosamine hyaluronate 2- Chondritin Sulfate Structure: Contains ß glucuronic A. & ß N-acetylgalactosamine with sulfate at C4 or 6 linked by ß 1-3 glycosidic bond Chondritin Sulfate (Cont) Site: Bone, tendons , ligaments & Cartilage & CT. Skin, cornea, umbalical cord & aorta. Function: 1- Maintain the shape of skeletal system 2- Bind collagen &hold fibers in strong network 3- Role in compressibility of cartilage in wt. bearing 3- keratan sulfate Structure: Contains ß Galactose with occasional sulfate at C6 & ß N-acetylglucosamine with sulfate at C6 linked by ß 1-4 glycosidic bond keratan sulfate (Cont) Site: Cartilage Cornea Function: Maintains Corneal Transparency  4- Dermatan sulfate Structure: Contains a L- iduronic A &/or glucuronic A linked to ß N-acetylglucosamine with sulfate at C4 linked by a 1-3 glycosidic bond Dermatan sulfate (Cont) Site: Skin, blood vessels, heart valves. Cornea & sclera. Function: 1- Maintains Corneal Transparency 2- Maintain the Overall shape of the Eye  5- Heparin Structure: Contains - ß glucuronic A. with sulfate at C2 & - glucosamine with sulfate at C3 & C6 (Few are Acetylated) Linked by ß 1-4 glycosidic bond  Heparin (Cont) Site: Present in mast cells (located along blood vessels of liver, lung, heart, Skin, kidney & spleen) Function: 1-Role as anticoagulant in vitro & vivo 2-Role as coenzymes for LPL (clearing factor)  6- Heparan Sulfate Structure: The same of heparin but with more acetylation of glucosamine & fewer sulfate. Function: 1- As receptor of cell. 2- participate in cell adhesions & cell cell interaction. 3- Present in cell membrane of the kidney plays important role in determining the charge selectiveness of glomerular filteration.  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 & glucuronate residues  Heparan sulfate Later, 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  Some cell surfaces contain Heparan Sulfate Glycosaminoglycans remain covalently linked to core proteins embedded in the plasma membrane heparan sulfate core glycosaminoglycan protein transmembrane -helix cytosol Heteropolysaccharides Glycoproteins These are mucoproteins. CHO are less as compared to proteoglycans (95%) Occurs in fluids and tissues and cell membranes. Proteins with Sialic acids, neuraminic acids.  Glycoproteins Consist of : 1- Protein Core 2- CHO chain (may be branched or non branched). It is formed of A - Hexose : galactose & mannose B - Pentose: arabinose & xylose. C - Methylpentose: L- fucose D - Acetylhexosamine: N-acetlglucosamine & N-acetlgalactosamine. E - Sialic acid F - no uronic A nor sulfate group  Glycoproteins (Cont) Function: 1 - Component of ECM. 2 - Component of mucin of GIT & UT 3 - Component of cell membrane as: a- Blood group antigen (A, B, AB). b- Receptors c- Plasma proteins except albumin. e- Enzymes & hormones. f- Glycophorin in human red cell.  Glycoproteins Protoglycan CHO part is heterooligosaccharide unit CHO part is (< 4%) GAGs (up to 95%) CHO chain (may be branched or CHO chain is long unbranched non branched). It is formed of heteropolysaccharides formed of A-Hexose : galactose & mannose repeating disaccharides. B -Pentose: arabinose &xylose. Contains: C- Methylpentose. 1- amino sugars (glucosamine & galactosamine) in which D- Acetylhexosamine: N-acetlglucosamine & the amino group is acetylated N-acetlgalactosamine. 2- acidic sugars ( glucuronic, E- Sialic A & L-iduronic acids) F- no uronic acid nor sulfate group  Glycoproteins Protoglycan CHO part is CHO part is heterooligosaccharide unit GAGs (up to 95%) (< 4%) Contain no sulfate Contain sulfate Function: Serve as ground a- Component of ECM. b- Component of mucin of GIT substance & c- Component of cell associated with membrane as: structure element of blood group antigen, tissue as bone receptors, plasma proteins except albumin, cartilage & elastin. enzymes & hormones. Heteropolysaccharides Mucilages Present mainly in plants Form gels and have adhesive properties Maybe used in Pharmaceuticals Culture media Usually not digestible. Acts as a laxative, increases intestinal peristalsis. Important examples are: Agar Vegetable Gums Pectins Hemicellulose  heparan sulfate core glycosaminoglycan protein transmembrane -helix cytosol Proteoglycans are GAGs 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  CH2OH C O H O O CH2 CH H NH serine OH H residue OH O H H HN C CH3 -D-N-acetylglucosamine O-Linked Glycosylation 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 Fuc = fucose Asn N-Linked Glycosylation Carbohydrate chains of Plasma membrane Glycoproteins & Glycolipids usually face the outside of the cell They have roles in Cell-Cell Interaction and Signaling & in forming a protective layer on the surface of some cells  Lectins are Glyco-Proteins that Recognize and Bind to Specific Oligosaccharides Examples of animal lectins: Mannan-Binding Lectin (MBL) 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 lectin domain outside transmembrane -helix cytosol cytoskeleton binding domain  Selectins are Integral Proteins of Mammalian Cell Plasma Membranes with roles in Cell-Cell Recognition & Binding  End Of Show 150 25/01/1438