Sugars and Carbohydrates Biochem 4511 Figures 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 Voet Voet Pratt 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 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 Fischer Projections Remember Horizontal groups project above the page Vertical groups project behind the page Stereochemistry and Numbering How do you define the stereochemistry for a sugar with multiple chiral centers Defined by comparing the asymmetric center furthest from the carbonyl group with the C2 of glyceraldehyde For glucose determined by C5 1 2 3 Stereochemistry D L Sugars enantiomers D Sugars are more common in nature except L arabinose Epimers Epimers Molecules sugars in this case identical except for 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 Tetrose 4 carbons Pentose 5 carbons Hexose 6 carbons Ketone Sugars Ketoses You need to learn the name and structure of ALL boxed sugars Note the nomenclature Keto Prefix for ketone Number from the terminal carbon closest to the carbonyl Therefore the carbonyl group is located on C2 Furanose and Pyranose Structures Under physiological conditions linear pentoses and hexoses are rarely present Rather the linear sugars undergo an intramolecular reaction to form 5 or 6 membered rings The linear structure is a convenient but inaccurate way to portray monosaccharides Similar to writing H2N CH2 COOH for glycine which is really a zwitterion under physiological conditions Furanose Sugars with a 5 membered ring Pyranose Sugars with a 6 membered ring 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 In this process the carbonyl becomes a new stereocenter and is referred to as the anomeric carbon C Conversion of Glucose to its Pyranose Form a and b Anomers 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 OH substituent of the anomeric carbon is on the same face of the ring as the CH2OH group that helps to define D L stereochemistry Mutarotation Conversion between and anomers Mutarotation is the process of interconverting between and anomers through the linear intermediate to reach equilibrium 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 and will have differing reactivities with many enzymes Cyclization of Glucose to Glucofuranose Glucose is not limited to linear and pyranose conformations it 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 An aldehyde group may act as a reducing agent and may be oxidized to carboxylic acid A sugar containing an aldehyde group is called reducing sugar 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 Monosaccharide Derivatives Dehydroxy Sugars Base sugar scaffold for DNA Monosaccharide Derivatives Phosphorylated Sugars Each of these sugar derivatives is an important metabolite which you will discuss and learn in the coming weeks Modified Sugars in Biology Glycosidic Bonds Any bonding of the sugar anomeric group or form generates a glycosidic bond After the anomeric alcohol 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 N Glycosidic Bonds Glycosidic bonds are not limited to reactions between alcohols and carbonyls When linked through a nitrogen referred to as an N glycosidic bond Relatively important in biology DNA and RNA Disaccharides Linking sugars through a glycosidic bond Non Reducing End Reducing End Common Glucose Disaccharides The linkages in disaccharides are very important 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 disaccharide with a linkage that can be digested by humans Some individuals develop lactose intolerance after infancy 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 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 consumed about 50 kg per year for the average american Cleaved by sucrase to glucose and fructose