Sugars and Carbohydrates Biochem 4511 Figures Essentials of Biochemistry 3E Pratr Cornely Principles of Biochemistry 5th Ed Moran et al Lehninger Principles of Biochemistry 5th Ed Nelson Cox Biochemistry 3rd Ed Voet Voet Fundamentals of Biochemistry 2nd Ed Ed Voet Voet Pratt Voet Carbohydrates Carbohydrates Carbon hydrates with a rough chemical composition of C H2O n Saccharides from the Greek sakcharon meaning sugar Monosaccharide Monomer unit of a sugar molecule Oligosaccharide Small polymer of monosaccharides Polysaccharide Larger polymer of monosaccharide groups linked together Glycoprotein Oligosaccharides linked to protein Glycolipid Oligosaccharide linked to lipids Sugars and Carbohydrates are Abundant Up to 50 of all biological carbon Simple Triose Monosaccharides C H2O n n 3 3 Aldose Aldose Ketose Ketose Ketone as the carbonyl group Aldose Aldehyde as the carbonyl group Sugars are optically active and chiral Linear Sugars are Often Drawn as j Fischer Projections Remember Horizontal groups project above the page Vertical groups project behind the page Stereochemistry and Numbering D or L D Defined by comparing the asymmetric center t furthest f th t from f the th carbonyl b l group to the C2 of glyceraldehyde For glucose glucose determined by C5 1 2 3 Stereochemistry D L Sugars enantiomers D Sugars are more common in nature Epimers Epimers E i M l Molecules l sugars iin thi this case id identical ti l exceptt ffor rotation around one asymmetric carbon Aldehyde Sugars Aldoses You need to learn the name and structure of ALL boxed sugars Note the nomenclature Aldo Prefix for aldehyde Triose 3 carbon sugar T t Tetrose 4 carbons b Pentose 5 carbons Hexose 6 carbons Ketone Sugars Ketoses You need to learn the name and structure of ALL boxed b d sugars Note the nomenclature Keto Prefix for ketone Number from the terminal carbon closest to the carbonyl Therefore the carbonyl group is l located t d on C2 Furanose and Pyranose Structures Under physiological conditions linear pentoses and hexoses are rarely present 3 linear and 97 cyclic glucose Rather the linear sugars undergo an intramolecular reaction to form 5 or 6 membered rings g Furanose Sugars with a 5 membered ring Pyranose Sugars with a 6 membered ring 6 membered Formation of Hemiacetals and Hemiketals Sugar molecules convert from their linear form to a ring structure by nucleophilic attack of the carbonyl by an alcohol to form either a hemiacetal or hemiketal depending on the identity of the carbonyl process the carbonyl y becomes a new stereocenter In this p and is referred to as the anomeric carbon C Conversion of Glucose to its Pyranose Form and Anomers Up Down The same sugar can be illustrated in different ways ways Fischer Fi h projection Haworth H th projection 2014 John Wiley Sons Inc All rights reserved Chair conformation Mutarotation Conversion between and anomers In physiological conditions D glucose is a mixture of 64 anomer 36 anomer and 1 of the linear form Anomers have differing physical properties including optical rotation t ti and d will ill h have diff differing i reactivities ti iti with ith many enzymes Definitions So Far Haworth Projections Simplified method of drawing ring structures with the plane of the ring perpendicular to the page NOT geometrically precise Anomers A pair of stereoisomers that differ only in conformation around the anomeric carbon the carbonyl carbon in the linear form anomer The OH substituent of the anomeric carbon is on the opposite face of the ring from the CH2OH group which helps to define D L stereochemistry anomer The Th OH substituent b tit t off the th anomeric i carbon b iis on the same face of the ring as the CH2OH group that helps to define D L stereochemistryy Isomerization of carbohydrates H H H H H H H Cyclization of Glucose to Glucofuranose Glucose can also cyclize to form glucofuranose and are defined in furanose sugars as described for pyranoses D glucofuranose D glucofuranose Fructose Cyclization Fructose forms furanose and pyranose rings D Fructopyranose Fructose Cyclization Fructose forms two furanose and two pyranose anomers Reducing Sugars A An aldehyde ld h d group may act as a reducing d i agent and d may b be oxidized to carboxylic acid A sugar g containing g an aldehyde y g group p is called reducing g sugar Fructose and other ketoses can spontaneously isomerize to aldoses and therefore also can get oxidized However when this happens fructose is no longer fructose it becomes glucose Monosaccharide Derivatives Oxidation Aldonic acids Aldehyde oxidized to carboxylic acid Uronic acids Primary alcohol oxidized to carboxylic acid Monosaccharide Derivatives Reduction Aldehyde and ketone groups can be reduced under mild conditions to yield alditols Derivative of Ribose Derivative of Xylose Lipid components Xylitol is a popular sugar alcohol that is used as a sweetener in chewing gum Monosaccharide Derivatives Dehydroxy Sugars Base sugar scaffold for DNA Monosaccharide Derivatives Phosphorylated Sugars Each of these sugar derivatives is an important metabolite which we will discuss and learn in the coming weeks Modified Sugars in Biology Glycosidic Bonds A Any bonding b di off the th sugar anomeric i group or form f generates a glycosidic bond After the anomeric alcohol reacts reacts the glycosidic bond is stable and the anomers can no longer interconvert by mutarotation Glycosidases Enzymes which selectively cleave glycosidic bonds are required to break the formed bond Disaccharides Linking sugars through a glycosidic bond Non Reducing Non Reducing End Reducing g End Common Glucose Disaccharides There are multiple types of glucose glucose disaccharides Maltose is the disaccharide unit of starch linkage Cellobiose is the disaccharide unit of cellulose linkage Lactose Milk sugar The only onl disaccharide with ith a linkage linkage that can be digested b by humans Some individuals develop p lactose intolerance after infancyy due to an inability to metabolize lactose resulting from a loss of the lactose processing enzyme D galactosidase lactase Lactose is digested for us by bacteria in cultured milk cheese and sour cream Sucrose Table Sugar Most abundant disaccharide in nature Sucrose is a major intermediate product of photosynthesis transported from leaves to other plant tissues tissues Sucrose has no reducing ends since the linkage is through the anomeric carbons Could also be named Fru 2 1 Glc Sucrose Table Sugar Sucrose is the most common dietary sugar