Carbohydrates BL1011 PDF

Carbohydrates BL1011 Dr. Imeobong Antia Image licensed under CC BY-ND  Learning Outcomes Understand how carbohydrates are classified Understand carbohydrate isomers and epimers Understand glycosidic bond nomenclature U...

Carbohydrates BL1011 Dr. Imeobong Antia Image licensed under CC BY-ND  Learning Outcomes Understand how carbohydrates are classified Understand carbohydrate isomers and epimers Understand glycosidic bond nomenclature Understand the structure and function of various carbohydrates  Biomolecules Organic molecules that are formed by living organisms Consists majorly of Carbon, Hydrogen, Oxygen and Nitrogen Four major classes Carbohydrates Proteins Lipids Nucleic acids  Carbohydrates “hydrates of carbon” – empiric formula (CH2O)n for many of the simpler carbohydrates Carbohydrates are the most abundant organic molecules in nature. Various functions Dietary requirement Storage form of energy Component of cell membrane Structural components of many organisms  Carbohydrates – Classification & structure Based on number of sugars:  Carbohydrates – Monosaccharide classification Based on number of carbon atoms:  Carbohydrates – Monosaccharide classification Based on functional group carbonyl carbon (C=O) at the end = aldose Elsewhere = ketose Monosaccharide classification  Monosaccharides – Carbon numbering Numbered beginning at the end that contains the carbonyl carbon (aldehyde or ketone group)  Monosaccharides Monosaccharides generally have molecular formulae that are some multiples of CH2O e.g. glucose (C6H12O6) Most names for sugars end in –ose. Triose, Tetrose, Pentose, Hexose etc. The carbons in sugars are numbered beginning at the end that contains the carbonyl carbon (aldehyde or keto group).  Isomers Compounds that have the same chemical formula but have different structures are called isomers e.g. glucose, galactose and fructose – same chemical formula (C6H12O6) Isomers  Epimers Carbohydrate isomers that differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon) are defined as epimers of each other. Note: galactose and mannose are NOT epimers as they differ in the position of –OH on 2 different  Enantiomers Monosaccharides may also exist as enantiomers (mirror images) The two members of the pair are designated as a D- and an L-sugar. The vast majority of the sugars in humans are D-sugars In the D-isomeric form, the –OH group on *C-5 is on the right.  Cyclisation of monosaccharides Monosaccharides can exist in ring formations Less than 1% exists in the open-chain form Cyclization creates an anomeric carbon (the former carbonyl carbon)- anomers Cyclization of monosaccharides  Cyclization of monosaccharides Monosaccharides can exist in ring formations Less than 1% exists in the open-chain form Cyclization creates an anomeric carbon (the former carbonyl carbon)- anomers  Polymerisation Monosaccharides can join to form di-, oligo- or polysaccharides The bonds that link sugars are called glycosidic bonds (catalysed by glycosyltransferases) Examples of important polysaccharide include: Glycogen, Starch, cellulose (all polymers of glucose)  Naming of glycosidic bonds If this anomeric hydroxyl is in the α configuration, the linkage is an α-bond. If it is in the β configuration, the linkage is a β-bond  Common disaccharides Disaccharide Comprising Glycosidic Structure monosaccharides Bond Lactose “lact” – latin for β-galactose + α- milk glucose β(1→4) Major carbohydrate (glucose can be α or β) in milk Maltose “malt” – French α-glucose + α-glucose Product of starch α(1→4) hydrolysis Used in beer production Sucrose “sucre” – French Common table α-glucose + β- α(1→2) sugar fructose  Reducing sugars If the hydroxyl group on the anomeric carbon is not linked to another compound, then the ring can open The sugar can act as a reducing agent, and is termed a reducing sugar Therefore, all monosaccharides are reducing agents Reducing sugars Such sugars can react with chromogenic agents causing the reagent to be reduced and coloured (e.g. detect sugars in urine). With the aldehyde group of the acyclic sugar becoming oxidized  Glucose – biomedical importance The main sugar in human body Important energy source in all cells Cannot be stored in this form (affects osmotic balance) Glucose is stored in large polymers (glycogen) which are osmotically inactive Ribose in nucleotides and nucleic acids Form glycoproteins, glycolipids and lipids Present in plasma membrane  Polymers of glucose – Glycogen Storage form of glucose in animals (mainly muscle and liver cells) α 1,6 glycosidic bonds α 1,4 glycosidic bonds (α 1,6 glycosidic bond branching every ~10 α-glucose subunit) Similar to amylopectin in starch but more branched Image: Häggström, Mikael (2014)  Polymers of glucose – Starch Storage form of glucose in plants Composed of two long polysaccharides of glucose  Polymers of glucose – Starch α 1,4 glycosidic bonds Linear 1 non-reducing end 1 reducing end α 1,6 glycosidic bonds and α 1,4 glycosidic bonds Branched (every ~20- 30 α1,4 linked glucose residues) Many non-reducing ends Image: Adapted from Giri (2018) DOI:10.1021/bk-2018-1304.ch001  Cellulose Structural carbohydrate - Plant cell walls Most abundant organic molecule on earth Commonly referred to as fibre and is largely indigestible (mammals lack cellulase) Chains of β glucose joined by β 1,4 glycosidic bonds Cellulose vs starch Cellulose Parallel fibres can interact via H-bonds forming a very strong structure  Polymers of glucose Image: Cornell (2016) bioninja Polymers of glucose Starch: Amylose α1-4 glycosidic bonds Amylopectin 1 x α1-6 glycosidic bond ~20-30 x α1-4 glycosidic bonds Glycogen: 1 x α1-6 glycosidic bond ~10 x α1-4 glycosidic bond Cellulose: β1-4 glycosidic linkages  Complex carbohydrates Carbohydrates can be attached by glycosidic bonds to non-carbohydrate structures, including purine and pyrimidine bases (found in nucleic acids), proteins (found in glycoproteins and proteoglycans), and lipids (found in glycolipids) If the group on the non-carbohydrate molecule to which the sugar is attached is an –NH2 group, the structure is an N-glycoside and the bond is called an N-glycosidic link. If the group is an –OH, the structure is an O-glycoside, and the bond is an O-glycosidic link Present on cell surfaces (RBCs, bacterial, mammalian) – oligos  Carbohydrates – summary Carbohydrates are the most abundant organic molecules in nature Can be classified based on number of carbon atoms, sugar subunits or functional group Exist as isomers (same chemical formula, different structures) Carbohydrate functions: short-term energy generation intermediate term energy storage structural components of cells Carbohydrates can be attached by glycosidic bonds to non- carbohydrate structures  Further reading Moran, Laurence A (2014) Principles of Biochemistry, Fifth edition. Chapter 7. Carbohydrates ISBN: 9781292034966 (e-book) Rodwell, Victor W ; Bender, David A ; Botham, Kathleen M ; Kennelly, Peter J ; Weil, P. Anthony (2018) Harper’s Illustrated Biochemistry 31st edition. Chapter 15. Carbohydrates of Physiological significance ISBN: 9781259837937 Carbohydrates – Reading list