11/15/17 CHAPTER 12: Carbohydrates: Structure and Function OUTLINE • 12.1 Role of Carbohydrates • 12.2 Monosaccharides • 12.3 Complex Carbohydrates • 12.4 Carbohydrate Catabolism • 12.5 Oligosaccharides as Cell Markers CHAPTER 12: Carbohydrates: Structure and Function WHAT ARE CARBOHYDRATES? • Glucose and its derivatives are carbohydrates: Ø Carbohydrates are simple organic molecules that have a shared basic chemical Formula: Cn(H2O)n Ø The name “carbo + hydrate” represents that Fact that they are made from CO2 and H2O by photosynthesis • About halF oF all earth’s solid carbon is Found in two polymers of glucose found in plants: Ø Starch = major energy storage molecule Ø Cellulose = major structural component oF the plant cell wall (aka. “fiber”) CHAPTER 12: Carbohydrates: Structure and Function THE SIMPLEST CARBOHYDRATES • Monosaccharides are carbohydrates that cannot be hydrolyZed into simpler carbohydrates: Ø These are the Fundamental building blocks For all other carbohydrates (oFten called “simple sugars”) Ø All have Formulas of based on the basic pattern: Cn(H2O)n • Monosaccharides have speciFic Functional groups: 1. An aldehyde OR a ketone (not both!) 2. Several (two or more) alcohol (-OH) groups 1 11/15/17 CHAPTER 12: Carbohydrates: Structure and Function STRUCTURE & NOMENCLATURE OF MONOSACCHARIDES • Monosaccharides are classiFied by two features: 1. Length of their main carbon chain (utilize standard IUPAC naming For # oF carbons) 2. Whether they contain an aldehyde or ketone group • Names always end with –ose • Two common hexoses: Ø Both C6H12O6 Ø Glucose = aldose Ø Fructose = ketose CHAPTER 12: Carbohydrates: Structure and Function CHIRALITY AND D-SUGARS • Note that monosaccarides have multiple chiral carbons in their structures: Hexoses have 4 1 chiral carbons 2 3 4 The position oF the OH at the carbon furthest from the carbonyl determines whether the sugar is in the D-or L-form: Naturally occurring sugars are the D-isomer form CHAPTER 12: Carbohydrates: Structure and Function DIASTEREOMERS • Many simple sugars diFFer only in the arrangement of –OH groups around chiral carbons: Ø If multiple chiral centers are present, it is possible to get non-superposable isomers that are not mirror images Ø We call these Diastereomers • Common hexoses are a good example: Ø D-glucose and D-galactose are diastereomers Ø DiFFer only in the sterochemistry around the 4th carbon in their main chain 2 11/15/17 CHAPTER 12: Carbohydrates: Structure and Function MONOSACCARIDES ARE TYPICALLY RINGS • In aqueous solution, common monosaccharides usually Form 5- or 6-membered ring structures: Ø Caused by an internal chemical rearrangement between the carbonyl carbon & and alcohol Ø The cyclic structure is more stable in water • For hexoses, two rings are possible: Ø For aldoses (glucose) – 6-member ring called a pyranose forms Ø For ketoses (fructose) – 5-member ring called a furanose forms CHAPTER 12: Carbohydrates: Structure and Function HAWORTH PROJECTIONS OF SUGARS • Haworth projections are used to indicate the 3- dimensional orientation of atoms in a ring: Ø In the ring forms of a monosaccharide, a D-sugar always has the C6 carbon CH2OH group positioned above the ring Ø There is a simple way to remember glucose….. D-Glucose D-Fructose CHAPTER 12: Carbohydrates: Structure and Function a- AND b- FORMS OF SUGARS • The carbon atom bonded to both the ring oxygen atom and a hydroxyl group is known as the anomeric carbon. Ø The –OH group on the anomeric carbon may be either in a down position or an up position. Down = a Up = b a-D-Glucose b-D-Glucose • The numbering scheme For carbons Found in a monosaccharide starts at the anomeric carbon 3 11/15/17 CHAPTER 12: Carbohydrates: Structure and Function STRUCTURAL EQUILIBRIUM OF MONOSACCHARIDES IN SOLUTION • The cyclic Forms oF sugars continuously open and close in aqueous solution: Ø This results in an equilibrium that favors the ring forms, but also including the open chain Form Ø Note that both a- and b- forms are present in solution a-D-Glucose b-D-Glucose 36% D-Glucose 64% 0.2% CHAPTER 12: Carbohydrates: Structure and Function MODIFIED MONOSACCHARIDES • The –OH groups on a simple sugar can be easily modiFied to add diFFerent chemical groups: Ø Recall the type of reactions that work well with alcohols! • These modiFications include the Following: 1. Amino sugars, with an amine replacing the –OH. 2. Phosphosugars, with a phosphate ester at an –OH 3. Deoxy sugars, which are missing a hydroxyl group, and instead have an extra –H at that position. 4. Glycosides, which have an –OR (ether) at the anomeric carbon instead of an –OH. CHAPTER 12: Carbohydrates: Structure and Function EXAMPLES OF MODIFIED SUGARS Amino sugar Deoxy sugar b-D-Glucoseamine b-D-2-Deoxyribose Glycoside sugar Phosphosugar Methyl glycoside b-D-Glucose-6-phosphate 4 11/15/17 CHAPTER 12: Carbohydrates: Structure and Function PRACTICE PROBLEM Two Forms oF galactose are shown below: • Are these D-sugars or L-sugars? How can you tell? • Name each Form oF galactose: ____-____galactose and ____ - ____galactose • Do the Haworth projections show these monosaccharides to be pyranoses or a furanoses? • Are these monosaccharides chiral? • Number the carbon atoms in the ring. Which carbon atom is the anomeric carbon? CHAPTER 12: Carbohydrates: Structure and Function OLIGOSACCHARIDES & POLYSACCHARIDES • Simple sugars (monosaccharides) can be joined together into chains of various lengths: Ø Short chains (3-10) = oligosaccarides (oligo = “Few”) Ø Long chains (>10) = polysaccharides (poly = “many”) • Monosaccharides are joined by a type of ether bond is called a glycosidic bond. Ø The glycosidic bond can be broken by addition of water in a hydrolysis reaction. CHAPTER 12: Carbohydrates: Structure and Function GLYCOSIDIC BONDS IN DISACCHARIDES • Disaccharides are carbohydrates composed of two monosaccharides joined by a glycosidic bond: Ø Monosaccharides are joined by a condensation reaction between two alcohol groups: a-D-Glucose + a-D-Glucose Maltose + H2O • Disaccharides can vary by: 1. The identity oF the two monosaccharides 2. The type oF linkage between the monomers 5 11/15/17 CHAPTER 12: Carbohydrates: Structure and Function IDENTIFYING DISACCHARIDE STRUCTURES • The glycosidic bond in a disaccharide is named to reFlect its speciFic HOW it joins two monosaccharides: 1. The numbering of each carbon atom associated with the glycosidic bond. 2. Whether the anomeric carbon in the glycosidic bond is in an a- or b- orientation. Maltose Cellobiose 2 glucose with a(1®4) 2 glucose with b(1®4) glycosidic linkage glycosidic linkage CHAPTER 12: Carbohydrates: Structure and Function COMMON BIOLOGICAL DISACCHARIDES Disaccharide Monosaccharide Linkage Stereochemistry Biological Common components Type of Anomeric Source Name Carbon in Linkage Lactose galactose and glucose 1® 4 b Milk sugar Table Sucrose glucose and Fructose 1® 2 a, b sugar Cellobiose glucose 1® 4 b - Maltose glucose 1® 4 a Malt sugar • Common biological dissaccharides are readily hydrolyzed by digestive enzymes: CHAPTER 12: Carbohydrates: Structure and Function POLYSACCHARIDES • The most common Form For biological carbohydrates is in the Form oF polysaccharides: Ø For energy storage Ø To Form tough structural (starch & glycogen) fibers (cellulose) Ø All three of these are polymers of glucose units • Most polysaccharides contain thousands of monosaccharides linked by glycosidic bonds 6 11/15/17 CHAPTER 12: Carbohydrates: Structure and Function ENERGY STORAGE POLYSACCHARIDES • The energy storing polysaccharides consist oF glucose units primarily joined by a(1®4) linkages. • There are three major storage polysaccharides: 1. Amylose Ø Unbranched (linear) chains oF glucose 2. Amylopectin Starch Ø Mostly linear chains with occasional branches (plants) Ø Branch points are a(1®6) glycosidic linkages 3. Glycogen (animals) Ø Highly branched chains with a(1®6) glycosidic linkages CHAPTER 12: Carbohydrates: Structure and Function STARCH & GLYCOGEN STRUCTURE CHAPTER 12: Carbohydrates: Structure and Function CELLULOSE • Cellulose is the most abundant carbohydrate: Ø Major component oF plant cell wall Ø Forms sheet-like structure provides structural support Ø The main component oF wood, paper, grass, and cotton • Cellulose consists oF linear, unbranched glucose units joined by b-(1®4) linkages: Ø Animals lack enzymes to digest these linkages 7 11/15/17 CHAPTER 12: Carbohydrates: Structure and Function CELLULAR IDENTITY & OLIGOSACCHARIDES • Besides their role in energy, carbohydrates also play a role in cell recognition: Ø Oligosaccharides can be covalently bonded to proteins & lipids on the cell membrane Ø Attachment is via an alcohol or amine on the protein or lipid to an alcohol from the sugar • Cell surFace sugars serve as “cell markers” allow immune cells to distinguish host cells From Foreign cells CHAPTER 12: Carbohydrates: Structure and Function THE ABO SYSTEM IN BLOOD • Red blood cells (erythrocytes) have speciFic oligosaccharide markers on them that define the blood groups A, B, AB, or O • The diFFerence between these blood type markers is the presence or absence oF a single sugar: Key: CHAPTER 12: Carbohydrates: Structure and Function CARBOHYDRATES AND ENERGY • The role oF dietary carbohydrates is primarily— though not exclusively—catabolic: Ø We extract energy From carbs For our bodily needs Ø Carbohydrates are converted to CO2 + H2O + energy • The overall process oF extracting energy From complex carbohydrates has 3 phases: 1. The hydrolysis of dietary starch into monosaccharides 2.