Carbohydrates of Physiologic Significance (Lecture I & II) PDF

Carbohydrates of Physiologic Significance (Lecture I & II) Dr. Wasanthi Subasinghe Department of Biochemistry and Clinical Chemistry  ILOs At the end of this s...

Carbohydrates of Physiologic Significance (Lecture I & II) Dr. Wasanthi Subasinghe Department of Biochemistry and Clinical Chemistry  ILOs At the end of this session students should be able to § Identify the major monosaccharides, disaccharides and polysaccharides in human body § Explain following properties/features of monosaccharides § nomenclature of monosaccharides (number of “C”, functional group) § Isomerism (structural isomers, epimers, optical isomers, D/L) § Recognize anomeric carbon and reducing and nonreducing sugars § Outline glycosidic bond formation and name the glycosidic bond  ILOs cont. At the end of this session students should be able to § Describe the structure and functions of following carbohydrates § disaccharides (maltose, lactose, sucrose) § Storage polysaccharides (glycogen and starch) § Complex carbohydrates (glycosaminoglycans, glycoproteins, proteoglycans) Lecture/tutorials à ILO ß Additional reading  Carbohydrates § CH the most abundant compounds found in nature (cellulose: 100 billion tons annually) § Saccharides — sugars § Polyhydroxy aldehydes/ Polyhydroxy ketones § Synthesized in plants and animals Definition: polyhydroxylated compounds having at least 3carbon atoms § Molecular formula Cx(H2O)y § Not all CH have this empirical formula: deoxysugars, aminosugars  Functions § Sources of energy § Components of DNA and RNA § Intermediates in the biosynthesis of FA and proteins § Participate in biological transport, cell-cell recognition, activation of growth factors, modulation of the immune system § Lubricantà mucous membranes, Skeletal joints § Structural component - Cellulose - Glycosaminoglycans § Clinical applications  CARBOHYDRATES Carbohydrates Classification of CH Monosaccharides Oligosaccharides Derived Disaccharides Carbohydrates Aldoses Ketoses Oxidation Polysaccharides Reduction Amino sugars Homopolysaccharides Deoxy sugars Heteropolysaccharides A blood test for glucose levels Oxidation of monosaccharides Dye Colored Dye H O2 H2O2 OH C O C O H OH H OH HO H HO H Glucose Oxidase H OH H OH H OH H OH CH2OH CH2OH D-Glucose D-Gluconic Acid (An Aldonic Acid) § Anomeric C ® COOH § glucose ® gluconic acid and hydrogen peroxide § Oxidation by glucose oxidase enzyme : highly specific test for glucose  Clinical Applications: Glucometer and Dipstick Glucose oxidase enzyme gluconic acid +ferricyanide-àferrocyanide à current Glucose oxidase enzyme hydrogen peroxide à colored derivative  Reducing and Non-Reducing Sugars ► Reducing sugars: Free anomeric C that can be oxidized Benedict’s test for reducing sugars - O H | | C=O C=O Brick red | + 2 Cu 2+ + 5 OH- | + 2 Cu2O + 3H2O H-C-OH H-C-OH | Blue | CH2OH CH2OH reducing sugars are readily oxidized, reducing Cu2+ in Benedict’s reagent. Not specific à False positive (Penicillin, streptomycin, salicylates)  Reduction § catalytically (hydrogen and a catalyst) or enzymatically § the resultant product is a polyol or sugar alcohol GLUT1 abundance in the barriers of the brain and retina Opacification of the lens most common cause of glyceraldehyde reversible blindness (51% forms glycerol glucose form sorbitol worldwide)  Biochemical basis of cataract formation in diabetics Insulin independent glucose uptake Hexokinase saturated –activation of aldose reductase pathway Sorbitol accumulation (faster production/slow conversion) Polar character of the sorbitol prevents its intracellular removal through diffusion hyperosmotic effect that results in an infusion of fluid to countervail the osmotic gradient. accumulation of polyols leads to a collapse and liquefaction of lens fibers, which ultimately results in the formation of lens opacities osmotic stress in the lens caused by sorbitol accumulation induces apoptosis in lens epithelial cells Increase oxidative stress/reduced antioxidant actions (↓ NADPH )  Special monosaccharides deoxy sugars § These are monosaccharides which lack one or more hydroxyl groups on the molecule § one quite ubiquitous deoxy sugar is 2’-deoxy ribose which is the sugar found in DNA  Glycosidic bonds § Bond formed between the –OH group on anomeric carbon of a monosaccharide and –OH or –NH group of another compound R-OH + HO-R' à R-O-R' + H2O § – OH group : O-glycosidic bond § –NH group : N-glycosidic bond § Glycosidic bonds between monosaccharides yields di, oligo- and polysaccharides  Naming glycosidic bonds According to the # of connected “C” with regards to the position of anomeric -OH 6 CH 2OH 6 CH 2OH6 CH 2OH 6 CH 2OH 5 O 5 5 O 5 H H H H OOH H H O OH H H H H 1 4 1 4 4 H1 1 4 H OH H OH OHH OH OH H2 OH OH H OH OH OH 3 3 2 3 2 3 2 H OH H OH H OH H OH a-D-glucose b-D-glucose a-D-glucose b-D-glucose 6 CH 2OH 6 CH 2OH 5 O 5 O H H H H H H 1 4 1 4 OH H OH H OH O OH 3 2 3 2 H OH H OH maltose Maltose (α-1, 4) ………………………………………………  Disaccharides § Joining of 2 monosaccharides by O - glycosidic bond: Maltose (α-1, 4) = glucose + glucose Sucrose (α-1, 2) = glucose + fructose Lactose (β-1, 4) = galactose + glucose  Disaccharides: structure and function CH2 OH CH2 OH O O OH a-glucose OH H H H OH H O OH H CH2OH H H H H O H H H OH H OH H OH b -D-galactose b -D-glucose OH Lactose : Milk sugar : b (1 ® 4) bond H OH O CH2OH O CH2 OH CH2 OH H OH H O H H O OH H CH2OH H H H OH OH H OH H O OH H b-fructose H OH H OH Sucrose: a (1 ® 2) bond a-D-glucose b -D-glucose Maltose: a (1 ®4) bond  Polysaccharides Hydrolysable polymers of monosaccharides characteristics: -White and amorphous products (glassy) - not sweet - non reducing 1. Homopolysaccharides : Glycogen, Cellulose 2. Heteropolysaccharides : Glycosaminoglycans 1. Storage Polysaccharides : Glycogen, Starch 2. Structural Polysaccharides : Cellulose, Chitin Glycogen Stored in muscle and liver Main chain: a(1®4) links Branch : a(1®6) at every 8 to 12 glucose unit (more frequent than in starch) Hydrolysis ® glucose Hydrolyzed by glycogen phosphorylase from non reducing end CH 2OH CH 2OH H O O glycogen H H H H H OH H OH H 1 O OH O H OH H OH CH 2OH CH 2OH 6 CH 2 CH 2OH CH 2OH H O H H O H H 5 O H H O H H O H H H H H H OH H OH H OH H 1 4 OH H OH H 4 O O O O OH OH 2 3 H OH H OH H OH H OH H OH  Starch Energy storage used by plants Long repeating chain of a-D-glucose Amylose: straight chain, a (1 ®4) linkage Amylopectin: branched structure, similar to glycogen, less branching Iodine (I2) can insert in the middle of the amylose helix to give a blue color that is characteristic and diagnostic for starch Main sources of starch are rice, corn, wheat, potatoes and cassava Industrial applications : adhesives, paper making, bio-fuel, textiles Cellulose Most abundant polysaccharide b-D-glucose attached by b(1®4) linkages Structural polysaccharide in plants Only digested and utilized by ruminants Gives no color with iodine Uses: binder-disintegrant in tablets, suspending agent, laxative CH2OH CH2OH O H CH2OH O H H O CH2OH O H OH H H O CH2OH O H OH H H H O O H OH H H H O OH H H OH H O H OH H H OH H H OH H H OH H OH  Glycosaminoglycans (GAGs) linear polymers of repeating disaccharides. Monosaccharide units are modified with –COOH, -NH2, sulfated hydroxyl and amino groups. Amino sugar is mostly N-acetyl glucosamine or N-acetyl galactosamine. The uronic sugar is either D- glucuronic acid or L-iduronic acid. Glycosaminoglycans (GAGs) GAGs are strongly negatively-charged due to carboxyl groups Synthesized in the ER & Golgi Degraded by lysosomal hydrolases Functions: -Ground substance - bind to large amounts of water producing the gel-like matrix - lubricant in mucous secretions [mucopolysaccharides] -mediating cell-cell signaling & adhesion Members of GAGs Examples of GAGs are: 1. Chondroitin sulfates 2. Keratan sulfates 3. Hyaluronic acid 4. Heparin 5. Heparan Sulphate 6. Dermatan Sulfate  Chondroitin sulfate Found in tendons, ligament, cartilage and wall of aorta , cornea and skin Relatively much shorter chain In cartilage, they bind collagen and hold fibers in a tight, strong network  Hyaluronic acid non sulfated GAGs forms in the plasma membrane can be very large MW> 1M Forms clear highly viscous solution One of the main components of the extracellular matrix cartilages and tendons Provides tensile strength & elasticity  Heparin highly sulfated GAGs, with anticoagulant properties. It is used in blood banks to prevent clotting and in the prevention of blood clots in patients recovering from serious injury or surgery  Glycoconjugates § Glycoconjugates are compounds that covalently link carbohydrates to proteins and lipids. § Proteoglycans and glycoproteins are two kinds of glycoconjugates that contain protein.  Glycoproteins threonine Proteins that carry covalently CH 3 CH 2OH attached oligosaccharides O O C C O H H In glycoprotein the OH H H protein>>>carbohydrate H H NHCOCH 3 O-glycosidic bonds: –OH groups of serine, threonine asparagine CH 2OH O N-glycosidic bonds : side chain – O H H N C C C NH2 of asparagine residue H H2 OH H O H H NHCOCH 3  Glycoproteins Common CH : glucose, mannose, N-acetylgalactosamine, N-acetylneuraminic acid (NANA) Function Glycoprotein Structural molecule Collagens Transport molecules Transferrin Hormones TSH Enzymes Alkaline phosphatase Immunologic molecules Immunoglobulins, antigens interactions Lectins, antibodies Blood Group Based on terminal monosaccharide of the glycoprotein at the non reducing end  Blood group antigens Found on the red cell membrane, attached to both lipids and proteins Responsible for the determination of the blood group of the individual Erythrocyte glycocalyx –electron microscopy image  ABO blood group system CH of the blood group substances are built by sequential assembly of the respective monosaccharide residues through the action of specific enzyme called glycosyltransferases. A antigen : α-1,3-N-acetylgalactosaminytransferase B antigen : α-1,3-galactosyltransferase O antigen : no transferase (also called H antigen) sugar molecule responsible for specificity Genetically determined ABO blood group system A antigen : α-1,3-N-acetylgalactosaminytransferase B antigen : α-1,3-galactosyltransferase O antigen : no transferase (also called H antigen)  Proteoglycans Proteoglycans are proteins conjugated to polysaccharides with serial repeating units basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan chains carbohydrate>>> protein Found in cell surface or extracellular matrix Functions: modulate cell growth processes, provide flexibility and resiliency to cartilage Proteoglycans Retain moisture and skin elasticity Glycolipids a compound in which a carbohydrate is bound to an -OH of the lipid Main component of cell surface CH many glycolipids are derived from ceramides Ceramide In most cases, sugar is either glucose or galactose Glucocerebroside Glycolipids - Homework i. Glycosingolipids Ceramide and one or more sugar – Galactocerebroside – in brain and neuronal membranes – Glucocerebrosides ii. Gangliosides Have a more complex oligosaccharide attached functions: cell-cell recognition; receptors for hormones Glycation of proteins § Also called non-enzymatic glycation § Aldehyde group of sugars non enzymatically form Schiff bases with amino group of proteins § Subsequently undergoes Amadori rearrangement to stable products HbA1c in diabetes –long term glycemic control