Chapter 20 Carbohydrates

Chapter 20 Carbohydrates

Carbohydrates • Carbohydrates (or saccharides) consist of only carbon, hydrogen and oxygen. • Carbohydrates come primarily from plants, however animals can also biosynthesize them • The “Carbon Cycle” describes the processes by which carbon is recycled on our planet - Energy from the sun is stored in plants, which use photosynthesis to convert carbon dioxide and water to glucose and oxygen - In the reverse process, energy is produced when animals oxidize glucose during respiration

Simplified Carbon Cycle

Carbohydrates Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis. Monosaccharide:A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate. Simple sugars.Can’t be split into smaller carbohydrate units - Examples: glucose, fructose, galactose, ribose Monosaccharides have the general formula CnH2nOn, where n varies from 3 to 8. • Aldose: A monosaccharide containing an aldehyde group. • Ketose: A monosaccharide containing a ketone group.

Carbs • Disaccharides are two monosaccharides bonded together - Can be split into two monosaccharides using an acid or enzyme catalyst - Examples: sucrose (table sugar), lactose (milk sugar) • Polysaccharidesare polymers of monosaccharides - Used for storage of carbohydrates - Can be split into many monosaccharides with acid or enzymes - Examples: starch, cellulose, glycogen

Types of Carbs

Monosaccharides • Are chiral! The suffix -ose indicates that a molecule is a carbohydrate. • The prefixes tri-, tetra, penta, and so forth indicate the number of carbon atoms in the chain. • Those containing an aldehyde group are classified as aldoses. • Those containing a ketone group are classified as ketoses. • There are only two trioses: • Often aldo- and keto- are omitted and these compounds are referred to simply as trioses. • Although “triose” does not tell the nature of the carbonyl group, it at least tells the number of carbons.

Monosacharides Glyceraldehyde, the simplest aldose, contains one stereocenter and exists as a pair of enantiomers.

Monosaccharides Fischer projection:A two-dimensional representation for showing the configuration of tetrahedral stereocenters. • Horizontal lines represent bonds projecting forward from the stereocenter. • Vertical lines represent bonds projecting to the rear. • Only the stereocenter is in the plane. *Terminal carbonyl groups (aldehydes –CHO and carboxylic acids –COOH) are written vertically on Fisher diagram

Monosaccharides In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde. • D-monosaccharide: the -OH on its penultimate carbon is on the right in a Fischer projection. • L-monosaccharide: the -OH on its penultimate carbon is on the left in a Fischer projection. *Fischer got lucky. For sugars, D=R and rotates light clockwise (+); L=S and counter (-). BUT, does the Fischer projection for D appear to be R or S? EXPLAIN!

Get a Model Kit • Before building the molecule, draw the D- Glyceraldehyde enantiomer (last slide) on your paper as both a wedge diagram and a Fischer diagram. Answer the following: • 1- Based solely on the Fischer diag, assign priority to the groups (Ch 15 p.427). Reading the order of the groups, what direction would you assign,clockwise or counterclockwise?__ • 2- Build the molecule, based on the Fischer. Is the carbonyl C group pointing toward you or away from you?____ What about the H and –OH? _______ • 3- Now, rotate the molecule, putting the lowest ranked group away from you. Approximately, how many degrees did you turn the molecule?____Where is the –OH after the rotation? • 4- Is the direction now clockwise, or counter?_____________ • 5- In terms of ONLY seeing a (similar) Fischer diagram, what will you have to do (in your head) before determining D or L?

D,L-Monosaccharides • The most common D-tetroses and D-pentoses are: • The three most common D-hexoses are: You do NOT have To memorize these! *See table 20.1 and 20.2

Sugars (Monosaccharides) • All other sugars are classified based on the position of the hydroxyl group farthest away from the carbonyl, but not the one on the “end”. • Penultimate carbon- Point of reference that refers to the next to last C atom in the chain. • See Table 20.1 and 20.2 (penultimate C is in red)

Three Important Monosaccharides • D-Glucose (aldose) is the most common monosaccharide - Primary fuel for our cells, required for many tissues - Main sources are fruits, vegetables, corn syrup and honey - Blood glucose is maintained within a fairly small range - Some glucose is stored as glycogen, excess is stored as fat • D-Galactose (aldose) comes from hydrolysis of the disaccharide lactose - Used in cell membranes of central nervous system - Converted by an enzyme into glucose for respiration (lack of this enzyme causes galactosemia, which can cause retardation in infants if not treated by complete removal from diet) • D-Fructose (ketose) is the sweetest carbohydrate - Converted by an enzyme into glucose for respiration - Main sources are fruits and honey - Also obtained from hydrolysis of the disaccharide sucrose

Structures of Glucose, D-Galactose and D-Fructose • Note that in nature, only the D enantiomers of sugars are used • What is the relationship between D-glucose and L-glucose? • What is the relationship between D-glucose and D-galactose? • What is the relationship between D-glucose and D-fructose? (*use these choices for each question: enantiomers, diastereomers or constitutional isomers?)

Amino Sugars- omit in 2017?? Amino sugars contain an -NH2 group in place of an -OH group. • Only three amino sugars are common in nature: D-glucosamine, D-mannosamine, and D-galactosamine. HW- Why do many ‘older’ people take glucosamine supplements?

Cyclic Structure • Aldehydes and ketones react with alcohols to form hemiacetals (Chapter 17). • Cyclic hemiacetals form readily when the hydroxyl and carbonyl groups are part of the same molecule and their interaction can form a five- or six-membered ring. Alcohol+Aldehyde (or alcohol+ketone) Hemiacetal

Cyclic Structures of Monosaccharides • Recall that an alcohol can react with an aldehyde or ketone to form a hemiacetal • If the alcohol and aldehyde or ketone are in the same molecule, a cyclic hemiacetal is formed • Monosaccharides in solution are in equilibrium between the open-chain and ring forms, and exist primarily in the ring form

Haworth Projection • A Haworth projection is a common way of representing the cyclic structure of monosaccharides with a simple three-dimensional perspective

Drawing Haworth Structures for Cyclic Forms Don’t have time…know last 3 things • Step 1: Number the carbons in the chain and turn the Fischer projection of the open-chain form clockwise 90 degrees - Hydroxyl groups that were on the right are now on the bottom, and hydroxyl groups that were on the left are now on the top (they will stay on bottom or top in the Haworth structure) • Step 2: Rotate around so that C-6 sticks up from C-5, and the hydroxyl group on C-5 points towards C-1 • Step 3: Form the cyclic hemiacetal by bonding the hydroxyl O to the carbonyl C and moving the hydroxyl H to the carbonyl O • Note: For C-6 aldose sugars, the cyclic hemiacetal has a new chiral carbon at C-1 - The two possible stereoisomers are called anomers - The alpha anomer has the hydroxyl group down - The beta anomer has the hydroxyl group up

Haworth Projections • D-Glucose forms these two cyclic hemiacetals.

Haworth Projections • Groups bonded to the carbons of the ring then lie either above or below the plane of the ring. (*Remember your stereoisomer lab!) • Stereoisomers that differ in configuration only at the anomeric carbon are called anomers. • In a ring, the –OH on the alpha anomer is down and axial; Beta is up and equatorial. *Most Glucose in our bodies is Beta because it’s farther away and more stable. BUILD A MODEL of alpha and beta glucose. • The anomeric carbon of an aldose is C-1; that of the most common ketoses is C-2 (because there is a branch off of the “first” C and the C in the branch is #1). See next slide. *For 2017, focus on aldose only (glucose)

Haworth Projections D-Fructose (a 2-ketohexose) also forms a five-membered cyclic hemiacetal. *For this test, you only have to know that 6 C ketose sugar (fructose) has anomeric C at #2. 6 C Aldose (glucose) has anomeric C on #1

Haworth Projections A six-membered hemiacetal ring is called a pyranose, and a five-membered hemiacetal ring is called a furanosebecause these ring sizes correspond to the heterocyclic compounds furan and pyran. *DO NOT have to know furanose for TEST. Focus on pyranoses (glucose)

Conformations • When going from the Haworth projection to the chair conformation, the anomeric carbon’s substituent that points down in the Haworth projection is going to be axial, and the substituent that points up in the Haworth projection is going to be equatorial. An axial –OH on the anomeric carbon makes the sugar an α sugar, while an equatorial –OH on the anomeric carbon makes the monosaccharide a β sugar.

Chair Conformations • For pyranoses, the six-membered ring is more accurately represented as a chair conformation. *Which is equatorial and which is axial? Which is more stable? WHY?

Chair Conformations • An axial –OH on the anomeric carbon makes the sugar an α sugar, while an equatorial –OH on the anomeric carbon makes the monosaccharide a β sugar. The chair conformation gives a better representation, as the Haworth makes all groups ‘appear’ axial (up and down), but the –OH on the anomeric (shown ‘up’) is really equatorial

Mutarotation- *2017: will not test specifics. • Mutarotation: The change in specific rotation that accompanies the equilibration of a- and b-anomers in aqueous solution. • Example: When either a-D-glucose or b-D-glucose is dissolved in water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms. *2017: ONLY have to know that most glucose in body is Beta

Disaccharides • Formation occurs by dehydration between 2 –OH’s of 2 monosaccharide monomers. One monomer removes –OH and the other removes –H. The O that was not removed forms a bond between the anomeric C’s. The new bond is called a Glycosidic bond. The process is commonly called dehydration synthesis or condensation. • Maltose is glucose+ glucose • Sucrose is glucose+ fructose • Lactose is glucose+ galactose

Disaccharides Disaccharide: a carbohydrate containing two monosaccharide units joined by a glycosidic bond. Sucrose (table sugar) • Sucrose is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets. A water molecule is removed (not shown)

Disaccharides Maltose • Present in malt, the juice from sprouted barley and other cereal grains. • Maltose consists of two units of D-glucopyranose joined by an a-1,4-glycosidic bond. Watch: https://www.youtube.com/watch?v=jyFsOaHmclA

Disaccharides Lactose • Lactose is the principal sugar present in milk; it makes up about 5 to 8 percent of human milk and 4 to 6 percent of cow's milk. • It consists of D-galactopyranose bonded by a b-1,4-glycosidic bond to carbon 4 of D-glucopyranose.

Physical Properties Monosaccharides are colorless crystalline solids, very soluble in water, but only slightly soluble in ethanol. Sweetness relative to sucrose:

Polysaccharides Polysaccharide: A carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds. Starch: A polymer of D-glucose. • Starch can be separated into amylose and amylopectin. • Amylose is composed of unbranched chains of up to 4000 D-glucose units joined by a-1,4-glycosidic bonds. • Amylopectin contains chains up to 10,000 D-glucose units also joined by a-1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by a-1,6-glycosidic bonds.

Polysaccharides • Figure 20.3 Amylopectin, a branched polymer of approximately 10,000 units of D-glucose joined by -1,4-glycosidic bonds.

Polysaccharides • Glycogen is the energy-reserve carbohydrate for animals. • Glycogen is a branched polysaccharide of approximately 106 glucose units joined by a-1,4- and a-1,6-glycosidic bonds. • The total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle.

Polysaccharides Cellulose is a linear polysaccharide of D-glucose units joined by b-1,4-glycosidic bonds. • It has an average molecular weight of 400,000 g/mol, corresponding to approximately 2200 glucose units per molecule. • Cellulose molecules act like stiff rods and align themselves side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds. • This arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength. • It is also the reason why cellulose is insoluble in water.

Polysaccharides Figure 20.4 Cellulose is a linear polymer containing as many as 3000 units of D-glucose joined by b-1,4-glycosidic bonds.

Polysaccharides Cellulose (cont’d) • Humans and other animals can not digest cellulose because their digestive systems do not contain b-glycosidases, enzymes that catalyze the hydrolysis of b-glycosidic bonds. • Termites have such bacteria in their intestines and can use wood as their principal food. • Ruminants (cud-chewing animals) and horses can also digest grasses and hay. • Instead, we have only a-glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen. • Many bacteria and microorganisms have b-glucosidases.

Chapter 20 Carbohydrates End Chapter 20

Chapter 20 Carbohydrates