Lecture 3 Chemistry of Biomolecules Biochemistry I PDF

Biochemistry I Chemistry of Biomolecules Lecture 3 Dr. Mohamed Hemdan 1 Disaccharides A disaccharide is composed of 2 monosaccharide units held by a glycosidic bond. They are subdivide...

Biochemistry I Chemistry of Biomolecules Lecture 3 Dr. Mohamed Hemdan 1 Disaccharides A disaccharide is composed of 2 monosaccharide units held by a glycosidic bond. They are subdivided based on the presence or absence of a free reducing group (anomeric C) into 2 types: a)Reducing disaccharides (with free aldehyde or keto group); e.g. lactose and maltose b) Non-reducing disaccharides (without free aldehyde or keto group); e.g. Sucrose 2 1 3 Disaccharides Non-reducing Reducing Disaccharides Disaccharides Maltose Lactose (Gal + G) Sucrose (G + F) (G + G) α- (1 2) α- (1 4) β- (1 4) 4 2  Reducing Disaccharides; e.g. Maltose & Lactose Maltose Lactose (G + G) (Gal + G) α- (1 4) β- (1 4) α- (1 4) β- (1 4) Intermediate product of starch amylases Found only in milk 5 Acid hydrolysis or maltase enzyme 6 3  Acid hydrolysis or lactase enzyme 7 Non-reducing Disaccharides, e.g Sucrose Sucrose (Glucose + Fructose) Table sugar 8 4  Sucrose 9 Acid hydrolysis or sucrase (invertase) enzyme 10 5  Oligosaccharides Contain from 2-10 monosaccharide units joinedin glycosidic bonds: - Disaccharides (2 units) e.g. maltose andsucrose. - Trisaccharides (3 units); e.g. Raffinose, Isomaltotriose, etc.. Oligosaccharides can have many functionsincluding cell recognition and cellbinding. Glycolipids have an important role in the immune response. In biology, glycosylation is the process by which a carbohydrate is covalently attached to an organic molecule – forming glycoproteins and glycolipids. 11 N-linked glycosylation helps determinethe polypeptide folding. Cell surface proteins and extracellular proteins are O-glycosylated. Glycosylated Biomolecules: Both glycoproteins and glycolipids have a covalently attached carbohydrate to their respective molecule. They are very abundant on the surface of the cell, and their interactions contribute to theoverall stability of the cell. 12 6  Glycoproteins 1. Glycoproteins have distinct Oligosaccharide structures which contribute greatly tovarious properties of the glycoproteins. 2. It is these properties that become important for critical functions such as - Antigenecity, - solubility, and - resistance to proteases. 3. Glycoproteins are relevant as cell-surface receptors, cell-adhesion molecules, immunoglobulins, and tumor antigens. 13 Glycolipids Glycolipids are important for: -cell recognition, -modulating the function of membrane receptors. Glycolipids are generally present in the lipidbilayer. They can serve as receptors for cellular recognition and cell signaling. Glycolipids has chaperone activity for its relevanceto HIV infection and portal fortoxins. 14 7  Glycoproteins & Glycolipids 1. Cell Recognition: - All cells are coated with glycoproteins or glycolipids, which help determine cell types. - Lectins, (proteins that bind carbohydrates), can recognize specific oligosaccharides in cell recognition. - Glycolipids determine blood types (A,B, AB & O). 2. Cell adhesion: - Lectins as well mediate cell-adhesion with oligosaccharides. - Selectins (a family of lectins) mediate certain cell-cell adhesion processes, including those of leukocytes to endothelial cells. 15 3. In immune response, endothelial cells can express certain selectins in response to damage or injury to the cells, which helps the white blood cell to eliminate the infection. 4. Role in mother-to-child transmission of HIV-1: Human milk oligosaccharides possess an inhibitory effects of on HIV-1 mother- to- child transmission. 16 8  Sources of Oligosaccharides Oligosaccharides are found in fibres,in fruits, vegetables. Gal Oligo Saccharide (GOS) isnaturally found in soybeans. Fructose Oligo Saccharide (FOS), GOS,and inulin are available asnutritional supplements in capsules, tablets, …. 17 Examples of Trisaccharides Trisaccharide Unit 1 Bond Unit 2 Bond Unit 3 Isomaltotriose glucose α(1→6) glucose α(1→6) glucose Nigerotriose glucose α(1→3) glucose α(1→3) glucose Maltotriose glucose α(1→4) glucose α(1→4) glucose Maltitriulose glucose α(1→4) glucose α(1→4) fructose Raffinose galactose α(1→6) glucose β(1→2) fructose 18 9 19 Polysaccharides Polysaccharides [Greek poly = many; sacchar= sugar] are complex carbohydrates, composed of > 10 to up to several thousand monosaccharides arranged in chains The most common monosaccharidesthat occur in polysaccharides are glucose, fructose, galactose and mannose. 20 10  Polysaccharides Homo- Hetero- polysaccharides polysaccharides Are composed of a Are composed of more than single mono-saccharide one type ofmonosaccharides building block. (e.g. GAGs) Glycosaminoglycans (GAGs) - Starch - Hyaluronic acid, Heparin (Amylose,Amylopectin,Dextrins) - Chondroitin sulfate - Glycogen - Dermatan sulfate, - Cellulose - Keratan sulfate - Chitin. Pectins, Agar 21 A) Homo-polysaccharides 1. Glucosans (Such as, starch, glycogen, cellulose) 2. Fructosans (such as, inulin) 3. Galactosans 4. Mannosans 22 11  1. Starch A polymer composed of D- glucose units held by Glycosidic bonds. It is composed of 2 components: - Amylose (water insoluble, unbranched), 15-20% - Amylopectin (soluble in water, branched), 80-85% 23 Amylose Amylose is a predominantly long unbranched helical chains of polysaccharide in which glucose units are linked by α-(1,4) glycosidic bonds. Amylose molecules may comprise between 200–6000 glucose units, varying between different starch types. Stains deep blue with iodine. Amylose hydrolysis yields maltose. 24 12 25 26 13  Amylopectin water-soluble, branched polymer of α-D-glucose units. Glucose units are linked in a linear way with α(1→4) glycosidic bonds, while branching takes place with α(1→6) bonds every 24 to 30 glucose residues. Stains red to violet with iodine. 27 28 14 29 Amylose Amylopectin Inner part with 15-20% of starch outer part with 80-85% of starch Straigh, unbranched chain of α-D- branched chain of α-D-glucose glucose units (300-6000) units (24-30) Glucose residues are linked by Glucose residues are linked α(1→4) glycosidic bonds by α(1→4) and α(1→6) glycosidic bonds Gives blue color with iodine Gives reddish violet color with iodine Lower molecular weight Higher molecular weight 30 15  2. Dextrins Are partial hydrolytic products of starch by acids or enzymes. They are mixture of polymers of D-glucose units joined by α-(1,4) or α-(1,6) glycosidic bonds. Starch is sequentially hydrolyzed through different dextrins and finally to maltose and glucose. The intermediates include: soluble starch, amylodextrins, erythrodextrins and achro-dextrins. 31 32 16  3. Glycogen 1. It is the major stored carbohydrate in animals so it is referred to as animal starch. 2. It is present at high concentration in the liver, muscles and brain. 3. The structure of glycogen is similar to amylopectin with more number of branches. Where, each branch contains 8-12 glucose units. 33 Glycogen Glycogen Structure. The blue balls represent glucose linked by α1,4 glycosidic bonds. The red balls represent glucose at branch points where there are both α1,4 and α1,6 glycosidic bonds. The orange balls represent the reducing ends of the polymeric chains of α1,4-linked glucoses. The area in the box is expanded to show the actual structure of the glucose monomers in both α-1,4- and α-1,6 glycosidic linkages. 34 17  Glycogen Amylopectin Found in animals Found in plants More branched , more Less branched, less compacted compacted and more molecular and less molecular weight weight Each branch comprises 8-12 D- Each branch comprises 24-30 D- glucose residues glucose residues Gives reddish brown color with Gives reddish violet color with iodine iodine 35 THANKYOU 36 18