Carbohydrates PDF

Module 5A: Carbohydrates BBIO109 - Biochemistry Prepared by: John Dale Mateo, MAEd  Learning Objectives Classify carbohydrates based on the number of carbons in the structure and as an aldose or ketose Identify the chirality of carbohydrates and use Fischer projection to deter...

Module 5A: Carbohydrates BBIO109 - Biochemistry Prepared by: John Dale Mateo, MAEd  Learning Objectives Classify carbohydrates based on the number of carbons in the structure and as an aldose or ketose Identify the chirality of carbohydrates and use Fischer projection to determine the D and L enantiomers Draw the Haworth structure for carbohydrates Describe and explain the structural features of disaccharides and polysaccharides Develop a carbohydrate diet meal Develop an artwork showing the different structures of sugar Ice Cream Yummy, Ice Cream Good! Carbohydrates Carbohydrates such as table sugar, lactose in milk, and cellulose are all made of carbon, hydrogen, and oxygen Simple sugars, which have formulas of 𝑪𝒏 (𝑯𝟐 𝑶)𝒏 were once thought to be hydrates of carbon, thus the name carbohydrate. Carbohydrates In photosynthesis, energy from the Sun is used to combine the carbon atoms from carbon dioxide and the hydrogen and oxygen atoms of water into carbohydrate glucose. Carbohydrates In the body, glucose is oxidized in a series of metabolic reactions known as respiration, which releases chemical energy to do work in the cells. 01 Types of Carbohydrates  Types of Carbohydrates Simplest carbohydrate Monosaccharides Cannot be split or hydrolyzed into smaller carbohydrate Consist of two monosaccharide units joined together Disaccharides Can be split by hydrolysis with a presence of an acid or enzyme to give monosaccharides Are carbohydrates that are naturally occurring polymers containing many monosaccharide units. Polysaccharides Can be completely hydrolyzed to yield many molecules of monosaccharides. One Sugar to Rule Them All: Meet the Monosaccharides  MONOSACCHARIDES Monosaccharides are sugars that have a chain of three to eight carbon atoms, one in a carbonyl group and the rest attached to hydroxyl groups. In an aldose, the carbonyl group is on the 1st carbon (-CHO), an aldehyde. In a ketose, the carbonyl group is on the 2nd carbon atom as a ketone (C=O)  MONOSACCHARIDES Triose – 3 Carbons Tetrose – 4 Carbons Pentose – 5 Carbons Hexose – 6 Carbons  Let Us Check Your Understanding Classify each of the following monosaccharides as an aldopentose, aldohexose, ketopentose, or ketohexose:  FISCHER PROJECTIONS OF MONOSACCHARIDES A Fischer projection is drawn with vertical and horizontal lines with aldehyde group at the top and –H and –OH groups on the intersecting line.  FISCHER PROJECTIONS OF MONOSACCHARIDES Most monosaccharides to be studied have 5-6 C w/ more than one chiral C. Then the –OH group on the chiral C farthest from the carbonyl group is used to determine the D or L isomer.  FISCHER PROJECTIONS OF MONOSACCHARIDES In each of the mirror images, it is important to understand that the –OH groups on all the chiral carbon atoms are reversed from one side to the other.  Let Us Check Your Understanding Identify the following Fischer projection as D- or L xylose: STRUCTURES OF SOME IMPORTANT MONOSACCHARIDES The hexoses glucose, galactose, and fructose are the most important monosaccharides.  The most common hexose, D-glucose, also known as dextrose and blood sugar, is found in fruits, vegetables, corn syrup, and honey. D-glucose is a building block of the disaccharides sucrose, lactose, and maltose, and polysaccharides such as amylose, cellulose, and glycogen.  Galactose is an aldohexose that is obtained from the disaccharide lactose, which is found in milk and milk products. The only difference in the Fischer projections of D-glucose and D-galactose is the arrangement of the –OH group of carbon 4  In a condition called galactosemia, an enzyme needed to convert galactose to glucose is missing. The accumulation of galactose in the blood and tissues can lead to cataracts, mental retardation, and cirrhosis.  Fructose, also called levulose and fruit sugar, is a ketohexose. The structure of fructose differs at carbons 1 and 2 by the location of the carbonyl group. Fructose is the sweetest of the carbohydrates, twice as sweet as sucrose  Let Us Check Your Understanding Ribulose has the following Fischer projection. Identify the compound as D- or L-ribulose.  HAWORTH STRUCTURES OF MONOSACCHARIDES Molecules of monosaccharides normally exist in a cyclic structure formed when a carbonyl group and a hydroxyl group in the same molecule react to give ring structures known as Haworth structures. The most stable for of pentoses and hexoses are five- or six- atom rings.  DRAWING HAWORTH STRUCTURES Turn the open-chain condensed structural formula clockwise 90˚. This places the –OH groups on the right of C2 and C4 (blue) and C5 Step 1 (red) of the vertical open chain below the carbon atoms. The –OH group on the left (green) of the open chain is drawn above C3.  DRAWING HAWORTH STRUCTURES Fold the carbon chain into a hexagon and bond the O on C5 to the carbonyl group. With C2 and C3 as the base of a hexagon, move the remaining carbons upwards. The remaining –OH group (red) on C5 is Step 2 drawn next to the carbonyl carbon, which moves C6 in -𝐶𝐻2 𝑂𝐻 above C5. To complete the Haworth structure, draw a bond from the O of the reacting –OH group (C5) to the carbonyl carbon.  DRAWING HAWORTH STRUCTURES Draw the new –OH group on C1 down to give the α isomer or up to give β isomer. In a Haworth structure, the corners of the ring Step 3 represent carbon atoms. There are two ways to draw the new –OH, either up or down, which gives two possible stereoisomers of D- glucose.  Mutarotation of α- and β-D-Glucose In an aqueous solution, the Haworth structure of α-D-glucose opens to give the open chain of D-glucose with an aldehyde group. However, when the open chain closes again it can form β-D Glucose. In this process called mutarotation, each isomer converts to the open chain and back again. As the ring opens and closes, the –OH group on C1 can form either the α or β isomer  Haworth Structures of Galactose Galactose is an aldohexose that differs from glucose only in the arrangement of the –OH group on C4. It’s Haworth is similar to glucose except that the –OH on C4 is drawn up. It also exist as α and β isomers  Haworth Structures of Fructose Fructose is a ketohexose. The Haworth structure for fructose is a five-atom ring with C2 at the right corner of the ring. The cyclic structure forms when the hydroxyl group on C5 reacts with C2 in the carbonyl group. The new hydroxyl group on carbon 2 gives the α and β isomers of fructose  Let Us Check Your Understanding D-Mannose, a carbohydrate found in immunoglobulins, has the following open-chain structure. Draw the Haworth structure for β- D-Mannose. Let Us Check Your Understanding Let Us Check Your Understanding  Chemical Properties of Monosaccharides In an aldose, the aldehyde group can be oxidized to a carboxylic acid. The carbonyl group in both an aldose and a ketose can be reduced to give a hydroxyl group. The hydroxyl groups can react with other compounds to form a variety of derivatives that are important in biological structures.  Chemical Properties of Monosaccharides OXIDATION OF MONOSACCHARIDES An aldehyde group with an adjacent hydroxyl can be oxidized to a carboxylic acid by an oxidizing reagent such as Benedict’s reagent. Then, the 𝐶𝑢2+ is reduced to 𝐶𝑢+ , which forms a brick-red precipitate of 𝑪𝒖𝟐 𝑶. A carbohydrate that reduces another substance is called a reducing sugar.  Chemical Properties of Monosaccharides OXIDATION OF MONOSACCHARIDES Fructose is also a reducing sugar. In Benedict’s solution, which is basic, a rearrangement occurs between the ketone group on C2 and the hydroxyl group on C1. As a result, fructose is converted to glucose, which produces an aldehyde group with an adjacent hydroxyl that can be oxidized.  Chemical Properties of Monosaccharides REDUCTION OF MONOSACCHARIDES The reduction of the carbonyl group in monosaccharides produces sugar alcohols, which are also called alditols. D-Glucose is reduced to D-glucitol, better known as D-sorbitol.  Chemical Properties of Monosaccharides REDUCTION OF MONOSACCHARIDES Sugar alcohols such as D-sorbitol, D-xylitol from D-xylose, and D-mannitol from D-mannose are used as sweeteners in many sugar-free products such as diet drinks and sugarless gum as well as products for people with diabetes. Let Us Check Your Understanding Let Us Check Your Understanding Double Trouble: When Sugars Team Up – Disaccharides  DISACCHARIDES A disaccharide is composed of two monosaccharides linked together. The most common disaccharides are maltose, lactose, and sucrose. When they are split by water (hydrolysis) in the presence of an acid or an enzyme, the products are two monosaccharides. DISACCHARIDES  Maltose, or malt sugar, is obtained from starch and is found in germinating grains. When maltose in barley and other grains is hydrolyzed by yeast enzymes, glucose is obtained, which can undergo fermentation to give ethanol. Maltose is used in cereals, candies, and the brewing of beverages.  HAWORTH STRUCTURES OF DISACCHARIDES In the Haworth structure of a disaccharide, a glycosidic bond is an ether bond that connects two monosaccharides.  HAWORTH STRUCTURES OF DISACCHARIDES In maltose, a glycosidic bond forms between the –OH groups of C1 and C4 of two α-D-Glucose molecules with a loss of a water molecule. The glycosidic bond in maltose is designated as an α-1,4 linkage to show that an α-OH on C1 is joined to C4 of the 2nd Glucose molecule.  HAWORTH STRUCTURES OF DISACCHARIDES Because the 2nd glucose molecule still has a free –OH group on C1, it can form an open chain, which allows maltose to form both α and β isomers. The open chain provides an aldehyde group that can be oxidized, making maltose a reducing sugar.  Lactose, milk sugar, is a disaccharide found in milk and milk products. Lactose makes up 6–8% of human milk and about 4–5% of cow’s milk, and it is used in products that attempt to duplicate mother’s milk. Some people do not produce sufficient quantities of the enzyme lactase needed to hydrolyze lactose. Then lactose remains undigested, which can cause abdominal cramps and diarrhea.  HAWORTH STRUCTURES OF DISACCHARIDES The bond in lactose is a β-1,4- glycosidic bond because the –OH group on C1 of β-D-galactose forms a glycosidic bond with the –OH group on C4 of a D-glucose molecule. Because D-glucose still has a free group on C1, it can form an open chain, which allows lactose to form both α and β isomers.  Sucrose, a disaccharide obtained from sugar beets and sugar cane, contains glucose and fructose. The sugar we use to sweeten our cereal, coffee, or tea is sucrose. Most of the sucrose for table sugar comes from sugar cane (20% by mass) or sugar beets (15% by mass). Both the raw and refined forms of sugar are sucrose.  HAWORTH STRUCTURES OF DISACCHARIDES Sucrose consists of an α-D-glucose and a β-D-fructose molecule joined by an α, β-1,2- glycosidic bond. Unlike maltose and lactose, the glycosidic bond in sucrose is between C1 of glucose and C2 of fructose. Thus, sucrose cannot form an open chain and cannot be oxidized and react with Benedict’s reagent. So it is not a reducing sugar.  Let Us Check Your Understanding Melibiose is a disaccharide that is 30 times sweeter than sucrose. Melibiose has the following Haworth structure: (a) What are the monosaccharide units in melibiose? (b) What type of glycosidic bond links the monosaccharides? (c) Identify the structure as α- or β-melibiose.  How Sweet Is Your Sweetener? Although many of the monosaccharides and disaccharides taste sweet, they differ considerably in their degree of sweetness. Dietetic foods contain sweeteners that are noncarbohydrate or carbohydrates that are sweeter than sucrose.  Blood Types and Carbohydrates The blood types A, B, and O are determined by monosaccharides attached to the surface of red blood cells. The blood types, A, B, and O, all contain the monosaccharides N-acetylglucosamine, galactose, and fucose.  Blood Types and Carbohydrates In type A, galactose is bonded to N-acetylgalactosamine. In type B, the galactose is bonded to a second galactose. In type AB, the monosaccharides of both blood types A and B are found.  Carb Party: Polysaccharides and Their Endless Connections  POLYSACCHARIDES A polysaccharide is a polymer of many monosaccharides joined together. Four important polysaccharides— amylose, amylopectin, cellulose, and glycogen – are all polymers of D-glucose that differ only in the type of glycosidic bonds and the amount of branching in the molecule.  Starch, a storage form of glucose in plants, is found as insoluble granules in rice, wheat, potatoes, beans, and cereals. Starch is composed of two kinds of polysaccharides, amylose and amylopectin.  Amylose, which makes up about 20% of starch, consists of 250–4000 α-D-glucose molecules connected by α-1,4- glycosidic bonds in a continuous chain. Sometimes called a straight-chain polymer, polymers of amylose are actually coiled in helical fashion  Amylopectin, which makes up as much as 80% of starch, is a branched-chain polysaccharide. Like amylose, the glucose molecules are connected by α-1,4-glycosidic bonds. However, at about every 25 glucose units, there is a branch of glucose molecules attached by an α-1,6-glycosidic bond between C1 of the branch and C6 in the main chain.  Starches hydrolyze easily in water and acid to give dextrins, which then hydrolyze to maltose and finally glucose. In our bodies, these complex carbohydrates are digested by the enzymes amylase (in saliva) and maltase (in the intestine). The glucose obtained provides about 50% of our nutritional calories.  Glycogen, or animal starch, is a polymer of glucose that is stored in the liver and muscle of animals. In glycogen, the glucose units are joined by α-1,4- glycosidic bonds, and branches occurring about every 10-15 glucose units are attached by α-1,6-glycosidic bonds  Cellulose is the major structural material of wood and plants. Cotton is almost pure cellulose. In cellulose, glucose molecules form a long unbranched chain similar to that of amylose. However, the glucose units in cellulose are linked by β-1,4-glycosidic bonds.  Humans have enzymes called α-amylase in saliva and pancreatic juices that hydrolyze the α-1,4 glycosidic bonds of starches, but not the β-1,4-glycosidic bonds of cellulose. Thus, humans cannot digest cellulose.  Animals such as horses, cows, and goats can obtain glucose from cellulose because their digestive systems contain bacteria that provide enzymes such as cellulase to hydrolyze β -1,4-glycosidic bonds. Dear Vegetarians, If you are trying to save animals, why are you eating our food?  Let Us Check Your Understanding Identify the polysaccharide described by each of the following: a) a polysaccharide that is stored in the liver and muscle tissues b) an unbranched polysaccharide containing β-1,4- glycosidic bonds c) a starch containing α-1,4- and α-1-6-glycosidic bonds Biochem Notes