Carbohydrates Lecture 2 2024 Final PDF

RMIT Classification: Trusted ONPS2423 INTRODUCTION TO MEDICAL BIOCHEMISTRY Carbohydrates II Danielle Kibell 1  RMIT Classification: Trusted Learning Objectives Describe the structure of α-D(+) glucose and its relation to...

RMIT Classification: Trusted ONPS2423 INTRODUCTION TO MEDICAL BIOCHEMISTRY Carbohydrates II Danielle Kibell 1  RMIT Classification: Trusted Learning Objectives Describe the structure of α-D(+) glucose and its relation to α-D- glucopyranose Differentiate between the α and β isomers of the pyranose and furanose forms of sugars Describe the glycoside or acetal linkage between the two monosaccharides Explain the difference between a reducing sugar and a non-reducing sugar Write the formula for sucrose and explain why it is a non-reducing sugar Illustrate the structure of a polysaccharide using several molecules of glucose Understand the fundamental roles of structural and nutritional polysaccharides Explain the difference between the terms homopolysaccharide and heteropolysaccharide 2 3 RMIT Classification: Trusted Polysaccharides Most abundant of the carbohydrates found in nature – serve as reserve of food substances (nutritional polysaccharides) – structural components of plant cells (structural polysaccharides) Natural carbohydrates are usually found as polymers These polysaccharides can be – homopolysaccharides – consists of one monomer – heteropolysaccharides – consists of more than one monomer – linear – branched Polysaccharides do not have a defined molecular weight. – This is in contrast to proteins because unlike proteins, no template is used to make polysaccharides 3 4 RMIT Classification: Trusted 4 5 RMIT Classification: Trusted Cellulose Cellulose is a linear homopolysaccharide of glucose – Glucose monomers form (1 → 4) linked chains – Hydrogen bonds form between adjacent monomers – Additional H-bonds between chains – Structure is now tough and water-insoluble – Most abundant polysaccharide in nature – Cotton is nearly pure fibrous cellulose Occurs in the framework or supporting structure of plants 5 6 RMIT Classification: Trusted Hydrogen Bonding in Cellulose 6 7 RMIT Classification: Trusted Cellulose Metabolism The fibrous structure and water-insolubility make cellulose a difficult substrate to act on Fungi, bacteria, and protozoa secrete cellulase, which allows them to use wood as source of glucose Most animals cannot use cellulose as a fuel source because they lack the enzyme to hydrolyze (1 →4) linkages Ruminants and termites live symbiotically with microorganisms that produces cellulase Cellulases hold promise in the fermentation of biomass into biofuels 7 8 RMIT Classification: Trusted Chitin Chitin is a linear homopolysaccharide of N-acetyl-D-glucosamine – N-acetyl-D-glucosamine monomers form (1 → 4)-linked chains – Forms extended fibers that are similar to those of cellulose – Hard, insoluble, cannot be digested by vertebrates – Structure is tough but flexible, and water-insoluble – Found in cell walls of algae, fungi and yeasts, and in exoskeletons of insects, spiders, crabs, and other arthropods 8 9 RMIT Classification: Trusted Chitin 9 10 RMIT Classification: Trusted 10 11 RMIT Classification: Trusted Glycosaminoglycans Linear polymers of repeating disaccharide units One monomer is either – N-acetyl-glucosamine or – N-acetyl-galactosamine Forms meshwork with fibrous proteins to form extracellular matrix – Connective tissue – Lubrication of joints Examples: Hyaluronic acid and Chondroitin 11 12 RMIT Classification: Trusted Hyaluronic Acid Hyaluronic acid is a linear heteropolysaccharide of D- glucoronic acid and N-acetyl-D-glucosamine – The repeating unit of hyaluronic acid form (1 → 3)-linked chains – It is a glycosaminoglycan (GAG) providing the cell coat in animal cells – It has a gelatinous nature that lubricates and serves as sticky cement – It is highly water soluble – Found in the vitreous humor of the eye and in the umbilical cord 12 13 RMIT Classification: Trusted Chondroitin Chondroitin is a linear heteropolysaccharide of D-glucoronic acid and N-acetyl-D-galactosamine – The repeating unit of chondroitin form (1 → 3)-linked chains – It has a sulfate ester on either the 4-hydroxyl or the 6- hydroxyl group of the N-acetyl-D-galactosamine – It is water soluble – Found in connective tissues, cartilage, tendons and bones 13 14 RMIT Classification: Trusted 14 15 RMIT Classification: Trusted Heparin and Heparan Sulfate Heparin is linear polymer, 3–40 kDa Heparan sulfate is heparin-like polysaccharide but attached to proteins Heparin is a linear heteropolysaccharide of D-glucoronic acid- 2-sulfate and D-glucosamine sulfate – The repeating unit of heparin form (α1 → 4)-linked chains – Sulfate groups are on some hydroxyl and amino groups Prevent blood clotting by activating protease inhibitor antithrombin 15 16 RMIT Classification: Trusted 16 17 RMIT Classification: Trusted Glycogen Glycogen is a branched homopolysaccharide of glucose – Glucose monomers form (1 → 4) linked chains – Branch-points with (1 → 6) linkers every 8–12 residues – Molecular weight reaches several millions – Functions as the main storage polysaccharide in animals – Found in the liver and muscle tissue – Soluble in water – Storages are mobilized in periods of starvation or fasting – Can be broken down into D-glucose subunits by hydrolysis or enzymatic reaction 17 18 RMIT Classification: Trusted Starch Starch is a mixture of two homopolysaccharides of glucose Amylose is an unbranched polymer of (1 → 4) linked residues Amylopectin is branched like glycogen but the branch-points with (1 → 6) linkers occur every 24–30 residues Insoluble form of starch Molecular weight of amylopectin is up to 200 million Starch is the main storage polysaccharide in plants Natural starch consists of 10-20% amylose and 80-90% amylopectin 18 19 RMIT Classification: Trusted Glycosidic Linkages in Glycogen and Starch 19 20 RMIT Classification: Trusted Glycoproteins A protein with small oligosaccharides attached – Carbohydrate attached via its anomeric carbon – About half of mammalian proteins are glycoproteins – Carbohydrates play role in protein-protein recognition For example: antibodies – Only some bacteria glycosylate few of their proteins – Viral proteins heavily glycosylated; helps evade the immune system 20  RMIT Classification: Trusted Glycoproteins Carbohydrates also play an important part as antigenic determinants – Including in blood groups (A, B, AB and O) The distinction between blood groups depend on the oligosaccharide portions of the glycoproteins on the surface of blood cells In all blood types, the oligosaccharide portion of the molecule contains the sugar L-fucose(6-deoxy-β-L-galactose) – Type A Blood: N-acetylgalactosamine is found at the non-reducing end – Type B Blood: α-D-galactose – Type O Blood: neither of these are present – Type AB Blood: both kinds of oligosaccharide are present In blood transfusions careful consideration of the oligosaccharides present needs to be considered 21  RMIT Classification: Trusted Glycoproteins 22 23 RMIT Classification: Trusted Summary The carbohydrates are found primarily in polymeric form in the structural parts of plants The basic monosaccharide unit contains an aldehyde or ketone group The way in which monosaccharides are linked determines not only the shape of the polymer, but also influences the possibility of hydrolyses that are catalysed by enzymes The monosaccharides formed from the hydrolysis of polysaccharides are the major source of energy for the life of all animals 23 24 RMIT Classification: Trusted Summary The disaccharides sucrose and lactose are two of the most abundant oligosaccharides –Sucrose is composed of D-glucose and fructose –Lactose is composed of β-galactose and D-glucose –Maltose is a disaccharide of starch has D-glucosyl residues The most common polysaccharides are cellulose, plant starches and animal glycogens –Cellulose is a linear polymer of D-glucosyl residues –Starches are composed of two molecular components, amylose and amylopectin 24 25 RMIT Classification: Trusted Summary Glycogens are similar in their chemical structures to amylopectin except they are more highly branched and are larger polymers. Starches and glycogen serve as storage forms of energy Cellulose, starches and glycogens are homoglycans, since they contain only one type of monosaccharide unit The glycosaminoglycans (GAGs) are heteroglycans, as they contain more than one type of monosaccharide unit –Hyaluronic acid, Chondroitin and Heparin 25