BCH 3000 PRINCIPLES OF BIOCHEMISTRY

Transcription

1 BCH 3000 PRINCIPLES OF BIOCHEMISTRY (Semester /15) 1

2 CARBOHYDRATE Carbohydrates as the main component of life Classification Structure Chemical reactions and biochemical functions of carbohydrate 2

3 CARBOHYDRATE 3

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5 CARBOHYDRATES 'hydrate of carbon', with structural formula (CH 2 O)n Carbohydrate = saccharides = biological molecules The basic building blocks of carbohydrates are the monosaccharides i.e. simplest = monosaccharides, eg. glucose These are linked together in longer chains to form oligosaccharides (2-20 residues - eg. Maltose) and polysaccharides (> 20 residues eg. starch, cellulose, glicogen ) 5

6 CARBOHYDRATES The root sacchar- comes from the Latin saccharum, "sugar". Why saccharide called carbohydrate? Most have general formula (CH 2 O)n Many saccharides a. Modified b. Contains amino groups, sulphates, phosphates, etc. 6

7 Functions of carbohydrates important source of energy for the body - 1g of carbohydrate provides 4.2 kcal of energy o Almost all of the cells use glucose to distribute energy -brain cells ; erythrocytes (red blood cells) are completely dependant on glucose as an energy source. Act as energy storage - glycogen stores act as a readily available energy reserve. A person weighing 70kg has a glycogen reserve of about g, which is about kcal. 7

8 Functions of carbohydrates Carbohydrates find many uses as structural elements eg. Cellulose cell walls in plants, bacteria, exoskeleton of insects, Carbohydrates are utilized as raw materials for several industries. For e.g., paper, plastics, textiles etc. Marker molecules for cell recognition Blood types A,B,O Found in biological molecules e g coenzymes and nucleic acids 8

9 Classification of Carbohydrates 1. Monosaccharide- one sugar residue. Most well known is glucose, C 6 H 12 O 6 2. Oligosaccharide- a few (2-9) sugar residues. Most well known is cane sugar or sucrose, C 12 H 22 O Polysaccharide - many sugar residues. Most common are glycogen, starch and cellulose, from animals, plants and plants. 9

10 Monosaccharides 1. Two classes- aldoses (aldehyde) and ketoses (keto) 2. In our formula, (CH 2 O)n, n is 3 or more 3. Simplest are dihydroxycetone, a ketose where n=3, and glyceraldehyde, an aldose where n=3 - Triose 4. Simple sugars monomeric 5. can have different number of carbon atoms 6. can be combined to form disaccharides and polysaccharides 7. some can have a linear or ring structure 10

11 Monosaccharides Aldose = an aldehyde with two or more hydroxyl groups. Ketose = a ketone with two or more hydroxyl groups Both are trioses = simplest monosaccharides; threecarbon sugars 11

12 Aldehyde Ketone back 12

13 Aldoses and Ketoses 13

14 Monosaccharides Both have the same compositions = TAUTOMERS = (isomer) Tautomers are organic compounds that are interconvertible by a chemical reaction called tautomerization. Can change from one form to the other but takes a very long time Catalysts can speed up the change Why tautomer?? Change from one form (aldehyde) to another (ketone) 14

15 Isomers = are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. Many isomers share similar if not identical properties in most chemical contexts 15

16 ENANTIOMER (Optical isomers) In glycerldehyde chiral carbon = 4 different groups get 2 isomers = enantiomer mirror images which are not superimposable - D and L isomer 16

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20 Enantiomers -if they are mirror images of each other- one is the mirror image of the other, two stereoisomers are enantiomers if they are different but each can be superimposed on the mirror image of the other. 20

21 Stereoisomers means that the two molecules differ in their three-dimensional shapes only but that they have the same structural formulas. This means they have the same exact groups attached in the same way. Only the three-dimensional orientation of these groups are different. enantiomers are stereoisomers 21

22 Diastereomers (or diastereoisomers) are stereoisomers that are not enantiomers (nonsuperimposable mirror images of each other). can have different physical properties and different reactivity. pairs of isomers that have opposite configurations at one or more of the chiral centers but are not mirror images of each other cis-trans isomerism is a form of diastereomerism 22

23 Diastereomers 23

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25 QUESTION Which is L isomer? Which is D isomer? Fischer Projections Glyceraldehyde is actually the basis for the L and D nomenclature Solution of D-glyceraldehyde rotates polarized light to the right (dextrorotatory) and L-glyceraldehyde rotates light to the left (levorotatory) 25

26 Plane Polarized Light 26

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30 Fischer Projections Monosaccharides can have multiple chiral centers need some conventions for drawing their structures. For linear chains, the stereochemistry is often represented as a Fischer Projection: 30

31 In a Fischer projection the carbon chain is oriented in the vertical direction, with a conformation that projects the carbon bonds onto a flat plane, and with all horizontal bonds projecting out, in front of the plane When the molecule is oriented with the C1 aldehyde at the top, pointing away from the viewer, this defines a convention where the C2 hydroxyl group will be on the left for L-glyceraldehyde, and on the right for D- glyceraldehyde 31

32 Stereochemistry of Longer Monosaccharides i. For longer monosaccharides, the assignment of the L and D configuration is determined by the configuration of the chiral carbon farthest away from the C1 carbonyl (ie. Highest chiral number) ii. Eg. - glucose, a 6-carbon sugar, the C5 carbon is used. If the C5 hydroxyl group is on the left, the molecule is L- glucose. If the hydroxyl group is on the right, it is D- glucose iii. In a carbon chain with 2 possible configurations for each chiral center, there are a total of 2n stereoisomers for a compound with n chiral carbons 32

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34 Some D- Aldose Isomers 34

35 Some D-Ketose Isomers 35

36 Under natural conditions, only one enantiomer predominates the D-isomer cf. amino acid L-isomer e.g. monosaccharide monosaccharide - = D- monosaccharide 36

37 Diastereoisomer : Diastereomers are stereoisomers that are not enantiomers or mirror images of each other. Diastereomers can have different physical properties and different reactivity. 37

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39 Chiral carbons Tetrose Are they enantiomers? 39

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41 PENTOSE Contains 5 carbon atoms 3 chiral carbons = 2 3 stereoisomers - 4 pairs of enantiomers - untuk aldose = aldopentose Stereochemistry of Longer Monosaccharides BUT ketopentose only 2 chiral carbons 4 isomers only 41

42 Some D- Aldose Isomers 42

43 Aldopentose Ketopentose 43

44 HEXOSE Monasaccharide with 6 carbon atoms Number of isomers high Common hexosee = glucose & fructose, mannose & galactose also abundant all play important biological roles 44

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46 Cyclic structures Cyclic Structures are the common form of monosaccharides with 5 or 6 carbon atoms. O O form when the hydroxyl group on C-5 reacts with the aldehyde group or ketone group. 46

47 Drawing the Cyclic Structure for Glucose STEP 1 : Number the carbon chain and turn clockwise to form a linear open chain. H HO H H H C C C C C O OH H OH OH CH 2 OH H H OH OH OH HOCH 2 C C C C C H 2 H OH O H 47

48 H HO H H H O 1 C C OH C H C OH C OH CH 2 OH H H OH H HOCH 2 C C C C C OH OH H OH O H 48

49 Drawing the Cyclic Structure for Glucose STEP 2: Fold into a hexagon. Bond the C5 O to C1. Place the C6 group above the ring. Write the OH groups on C2 and C4 below the ring. Write the OH group on C3 above the ring. Write a new OH on C1. 4 OH 5 6 CH 2 OH OH 3 2 O OH 1 OH 49

50 Drawing the Cyclic Structure for Glucose STEP 3 : Write the new OH on C1 down for the form. up for the form. CH 2 OH O CH 2 OH O OH OH OH OH -D-Glucose OH OH OH OH -D-Glucose 50

51 Summary of the Formation of Cyclic Glucose 51

52 -D-Glucose and β-d-glucose in Solution When placed in solution, cyclic structures open and close. -D-glucose converts to β-d-glucose and vice versa. at any time, only a small amount of open chain forms. CH2OH CH 2 OH CH 2 OH H O O OH OH OH OH OH OH OH OH -D-glucose D-glucose (open) β-d-glucose (36%) (trace) (64%) O O C H OH OH OH 52

53 Cyclic Structure of Fructose Fructose is a ketohexose. forms a cyclic structure. reacts the OH on C-5 with the C=O on C-2. CH 2 OH C O CH 2 OH O CH 2 OH CH 2 OH O OH HO C H OH OH H C OH OH CH 2 OH H C OH CH 2 OH OH OH D-fructose α-d-fructose -D-fructose 53

54 CYCLICAL STRUCTURE CHO with 5 & 6 carbon atoms normally exist as ring structures Cyclization = interactions between functional groups at C-1 & C-5 hemiacetal (in aldohexose) Or between C-2 dan C-5 hemiketal (in ketohexose) carbonyl carbon becomes new chiral centre = anomeric carbon Cyclic sugars 2 different forms - dan = Anomers 54

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56 Hemiacetal & Hemiketal 56

57 Drawing the Cyclic Structure for Glucose New Chiral centre Glucose 57

58 Drawing the Cyclic Structure for Fructose 1 New Chiral centre Fructose 58

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62 CYCLICAL STRUCTURES CHO 5 carbon = furanose furan CHO 6 carbon = pyranose Pyran Normally CHO > 5 carbon in cyclic form o o Free carbonyl group can anomer Can change from one form to another 62

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65 Furanose 65

66 pyranose 66

67 Which isomer used for reaction? Certain reactions any isomer Others only one anomer eg. RNA & DNA requires -D-ribose & - D-deoksiribose Fischer Projection usefull to explain `stereochemsitry sugars, But does not give true picture of overall shape accurately. use HAWORTH PROJECTION FORMULA 67

68 Fischer Projection Formula Haworth Projection Formula or? 68

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71 Important Simple Monosaccharides 1. Glucose 2. Mannose 3. Galactose 4. Fructose 5. Ribose 71

72 RNA - only ribofuranose Dinding sel (polysaccharide) - pyranose KETOPENTOSE almost all in cyclic form, but only furanose eg. a -D-ribulose 72

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74 REACTIONS OF MONOSACCHARIDES 1. Mutarotation. 2. Oxidation to CO 2 + H 2 O Reactions due to aldehyde group 3. Reducing sugars. 4. Reduction to polyols. Reactions due to alcohol group 5. Esterification. 6. Formation of acetals, also called glycosides 74

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76 REACTIONS OF MONOSACCHARIDES 1. Oxidation & Reduction Important in biochemistry provides energy when CHO completely oxidised Photosynthesis reversible process when CO 2 & H 2 O reduced Oxidation reaction can be used to detect the presence of carbohydrates eg. Aldehyde [O] carboxyl basis for test for aldose When aldehyde is oxidised, the oxidising agent is reduced 76

77 Because of his property, they are called reducing agents. Ketose is also a reducing agent Y? Ketoses can also be reducing sugars because they can isomerise (a tautomerisation) to aldoses via an enediol: 77

78 REDUCING SUGARS Sugars that contain aldehyde groups that are oxidised to carboxylic acids are classified as reducing sugars. They are classified as reducing sugars since they reduce the Cu 2+ to Cu + which forms as a red precipitate, copper (I) oxide. 78

79 Common test reagents are : Benedicts reagent (CuSO 4 / citrate) Fehlings reagent (CuSO 4 / tartrate) hemi-acetal Remember that aldehydes (and hence aldoses) are readily oxidised In order for oxidation to occur, the cyclic form must first ring-open to give the reactive aldehyde (?) So any sugar that contains a hemi-acetal will be a reducing sugar. But glycosides which are acetals are not reducing sugars. 79

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81 Another example of reagent for reducing sugars Tollen s Reagent Use silver ammonium complex Ag (NH 3 ) 2+ as oxidising agent mirror precipitate on the walls of test tube RCHO + 2Ag(NH 3 ) 2+ + OH - RCOO Ag + + 3NH 3 + NH 4+ + H 2 O Other methods use enzyme glucose oxidase to detect glucose 81

82 ESTERIFICATION REACTION Hydroxyl group (OH) in CHO reacts with acids ester E.g. Phosphate ester intermediate in the breakdown of CHO energy Phosphate ester is normally formed when the phosphate from ATP reacts with sugars phosphorylated sugar important in the metabolism of CHO 82

83 MONASACCHARIDE DERIVATIVES Monosaccharides have several OH groups can bind/exchange with other groups modify the original structures A. Phosphate esters Phosphate esters found in many metabolsic pathways - ATP, ADP, etc B. Acid & lactones Oxidation of monosaccharides produces 1. Aldonic acids e.g. aldose reacting with alkaline solution of Cu + 2. Lactone & uronic acids - [O] with enzim 83

84 Mannose 84

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86 C. Alditols Reduction of C=O polyhydroxy compounds = eg D- mannitol & D-glucitol Ribitol or Adonitol is a crystalline pentose alcohol (C 5 H 12 O 5 ) formed by the reduction of ribose. It occurs naturally in the plant Adonis vernalis 86

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88 Mannitol or 1,2,3,4,5,6-hexanehexol (C 6 H 8 (OH) 6 ) is a vasodilator which is used mainly to reduce pressure in the cranium, Chemically, mannitol is an alcohol and a sugar, or a polyol; it is similar to xylitol or sorbitol. Mannitol is also used as a sweetener for people with diabetes. 88

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90 D.Amino sugars 2 derivatives of amino sugars in polysaccharides glucosamine & galactosamine Exchange of OH with NH 2 90

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92 E. Glycoside glycosides are certain molecules in which a sugar part is bound to some other part Bond = glycosidic bond Formed when H 2 O is removed from OH of saccharide & other compounds containing OH - Found in plants/animals e.g. - Salicin, a glycoside related to Aspirin NB: OH - must be attached to anomeric carbon 92

93 Salicin, a glycoside related to Aspirin 93

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95 OH - from sugar with another OH - ether bond OH - - must be anomeric bond= glycosidic bond product = glycoside furanose = furanoside, pyranose = pyranoside. 95

96 OLIGOSACCHARIDES Glycosidic bonds between monosaccharides = basis for the formation of oligosaccharides and polysaccharides Bonds = between a anomer or b anomer and another OH - of another sugar lots of combinations - OH - must be numbered to differentiate notation for glycosidic bond which anomeric atom involved e.g.. (1 4), (1 6), (1 1) 96

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98 OLIGOSACCHARIDES Formed when two monosaccharides are joined together by glycosidic bond Plays important roles in living organisms Simplest and most important oligosaccharide = disaccharide Examples of disaccharide = sucrose, lactose, maltose 98

99 4 important characteristics to differentiate disaccharides 1. monomer found and their configuration 2. which carbon involved in the bond 3. the arrangement of the monomers 4. the anomeric configuration of the hydroxyl group 99

100 Chemical characteristics of poly- & oligosaccharides formed will depend on 1. Chemical characteristics of monosaccahrides 2. The type of glycosidic bonds formed (i.e. which anonmer; which carbon atom etc ) e.g. The differences between celluloses and starch is because of the difference in the glycosidic bond formed between glucose Because of the variation in the glycosidic bond, can get various types of polymers branched or linear 100

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103 Bond: (1 4) 103

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109 Example. Arrangement starts with non-reducing end left anomeric & enantiomeric with prefixes Cyclic configuration with suffix Atoms between glycosidic bonds the number inside bracket between residues Not all oligosaccharides are dimeric; also possible to have trimers, tetramer and bigger 109

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111 Sucrose - 2 Monosaccharides = -D-Glucose & -D- fructose Glucose = aldohexose = pyranose ; Fructosee = ketohexose = furanose -C-1glucose attached to fructose Not reducing sugar - why? because both anomeric groups are involved in glycosidic bond But free glucose and fructose are reducing sugars. When sucrose is digested, hydrolysed glucose & frctose energy 111

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113 Lactose A disaccharide formed from -D- galactose & - D-glucose Galctose = epimer of glucose - ie. Reverse position at C-4 Glycosidic bond = (1 4) between anomeric C- 1 ( form) of galactose and C-4 carbon of glucose 113

114 Because anomeric carbon of glucose is NOT involved in the bond formation, can be in either & Lactose = reducing sugar because the groups at the anomeric carbon atom (glucose) is not involved in glycosidic bond formation; hence can react with oxidising agents 114

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116 Lactose= milk sugar LACTOSE INTOLERANCE Human can be allergic to milk or milk products why??? lack of lactase (breaks lactose to galactose and glucose) lactose will accumulate Lactose will be acted on by lactase bacteria produce Hydrogen gas, CO 2 & organic acids problems with digestion bloating and diarrhea Although lactose can be degraded to galactose, galactose has to be isomerised to glucose before being absorbed can accumulate GALACTOSEMIA mental retardation 116

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118 Maltose a disaccharide hydrolytic product of starch 2 molecules of D-glucose - -D-glucose & -D-glucose joined by (1 4) bond Different from celobiose o o o hydrolysis of cellulose different glycosidic bond - D-glucose attached through (1 4)bond maltose can be digested by humans, cellobiose cannot 118

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120 Epimers are diastereomers that differ in configuration of only one stereogenic center Diastereomers are a class of stereoisomers that are nonsuperposable, non-mirror images of one another 120

121 Epimers 121

122 POLYSACCAHARIDE various functions sequence of monmer determines the primary structure normally simple monomers 1 type of monomer = homopolysaccharide more = heteropolysaccharide normally not complex not more than 2 residues Cf. Protein & nucleic acids - well defined length polysaccharide chain - random length - 122

123 Storage Polysaccharide 1. Important examples - amylose & amylopectin - starch in plants and & glycogen in mammals and bacterial cells 2. amylose, amylopectin & glycogen = homopolysaccharide (glucan) - deposited in the liver, otot = polymer -D-glucopyranose- Difference bond between residues 123

124 amylose - linear, α(1 4) Amylopectin & glycogen α(1 4) + α(1 6), branched polymer Glycogen - more branched; if not they are very similar 124

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126 Regular and simple structure regular secondary structure α(1 4) bond, each residue leans slightly compared to the previous resisue helical conformation However, the helix is not stable- e.g. amylose form random coils Iodine can stabilise helix because it can fit the core of the helix Complex blue in colour 126

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128 Glycogen & amylopectin cannot get blue colour??? Branches inhibit formation of helix To form a helix need 12 residue for ever turn amylopectin residues, glycogen - 8 residue not enough 128

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131 STRUCTURAL POLYSACCHARIDE Plants do not use/synthesise structural protein use specialised polysaccharide animal use both Cellulose main polymer in plants - woody/fibrous Polymer linear - D-glucose ( glucan) Joined through (1 4)bond 131

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133 Animals- can digest starch can cleave bond Cellulose - cannot requires symbiotic bacteria - produce enzyme cellulase Ruminant - OK- cellulase is present White ants protozoa can digest cellulse Fungi - eg. mushroom live on rottting wood, etc Other polysaccharides also found. e.g. xylans = polymer (1 4) - linked D- xylopyranose Glucomanan = hemicellulose 133

134 Cellulose Not confined to plants only Marine Invetebrata - eg. Tunicates - cellulose in the mantle in connective tissue - human 134

135 Tunicates 135

136 Tunicates 136

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138 CHITIN Similar to cellulose smalll difference - homopolymer N-asetil-Dglucosamine - minor constituent in fungi and algae- replace cellulose Role in invertebrate - exoskeleton of arthropod & mollusks 138

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142 GLYCOSAMINOGLYCAN In vertebrata - previously known as mucopolysacharide chondroitin sulphate keratan sulphate connective tissue dermatan sulphate skin Hyaluronic acid 1. All are polymers = repeating units of disaccharides 2. Sugar (CHO) = N- asetylglucosemine 142

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145 Main functions of glycosaminoglycan formationn of matrix that bind protein components with connective tissue eg. Proteoglycan inside cartilage - filemental structure synthesised from hyaluronic acid with protein core Protein - keratin sulphate chain and chondroitin sulphate chains are attached Structure - collagen fibers becomes compact and strong Bonds electrostatic between sulphate and basic collagen side chains 145

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147 Non Structural function of glycosaminoglycan Hyaluronic acid very soluble in water- found in-synovial fluid- lubricating agent for joints vitreous humor eyes agent for increasing fluidity Heparin - anticoagulant in body tissues bound strongly to blood proteins (prothrombin III) prevets enziymes in the coagulation of blood 147

148 Polysaccharides in bacterial cell walls Gram + & Gram based on cell wall Gram + = peptidoglycan = polysaccharide peptide complex which are multilayered and crosslinked Gram - = single layer of peptidoglycan covered with membrane layer 148

149 Gram + bacteria 149

150 Gram - bacteria 150

151 Importance of other peptidoglycas A few antibiotics inhibit bacterial growth by preventing peptidoglycan layers Lysozymes can dissolve cell walls cell lysis bacterial death Also found in bacteriophage, white of egg, eyedrops of humans 151

152 GLYCOPROTEIN Many proteins are bound to saccharide = glycoprotein Different functions Saccharide chain (= glycan) bound to protein -2 ways Bound to N of the amino group of asparagine - (N- Iinked Glycans) Through N-acetylgalactosamine Differet structures complex branched structures Different functions protein indicators old proteins that need to be destroyed 152

153 ABO blood group system, the classification of human blood based on the inherited properties of red blood cells (erythrocytes) as determined by the presence or absence of the antigens A and B, which are carried on the surface of the red cells. These antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group system Persons may thus have type A, type B, type O, or type AB blood 153

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155 eg. immunoglobin sialic acid residues will be cleaved slowly, then receptor will recognise and bind to the protein, engulf the protein O-linked Glycans Different functions e.g.. Antartic fish have glycoprotein which acts as an anti freez fluids do not freeze although temperature below freezing point Mucins - glycoprotein in salive increase viscosity of fluid 155

156 Humans can produce antibodies against the A & B, but NOT O; i.e. O is antigenic antigenic Normally, antibodies react against other antigens. eg. Type A carries antibodies against B - therefore when receiving blood type B - will clot & precipitate Blood type O carries antibodies against A & B cannot receive blood type A and B; but CAN donate to both Bllod Type AB carries A & B antigens; therefore no antibodies against B ; only donatieto AB only 156

157 Oligosaccharides as CELL MARKERS Blood group antigens a cell recognition phenomenon Cells need to be marked (on the surface) so that they can interact with other cells can recognize own cells from other external cells In animals there a layer of saccharides bound to a protein or lupid in the membranes e.g. glycocalyx - can interact with bacteria in the intestine; collagen To act as signal, need to have a specific protein bound specifically - imunoglobulin Other examples lectin interact between cells and proteins in the intercellulr matrix to maintain the structure of tissues and organs 157