Module 16: Carbohydrates

What are the properties of stereoisomers?

Molecules that differ only on their spatial arrangement. Their properties are chiral and optical

Aldopentoses have ___ chiral centers

3

Carbohydrates general formula

Cn(H2O)n

Blood samples for research or medical tests sometimes have heparin added. Why is this done?

Heparin is an anticoagulant. Its presence prevents blood clotting.

How can we detect the reducing sugar in a lab?

Use Tollen's reagent

what is pectin a polymer of?

a galactose derivative

hemiacetal

aldehyde + alcohol

two main ways to classify carbohydrates by carbonyl functional group

aldose or ketose

How does the presence of alpha-bonds versus beta-bonds influence the digestibility of glucose polymers by humans? Hint: There are two effects.

Humans can digest glucose polymers containing alpha-bonds but not beta. For this reason, glycogen and starch are digestible, whereas cellulose is not. In addition, cellulose is fibrous rather than branched, which makes the polymer insoluble in water and inaccessible to digestion by humans.

What is the chemical difference between a sugar phosphate and a sugar involved in a glycosidic bond?

In a sugar phosphate, an ester bond is formed between one of the sugar hydroxyls and phosphoric acid. A glycosidic bond is an acetal, which can be hydrolyzed to regenerate the two original sugar hydroxyls.

What is the metabolic basis for the observation that many adults cannot ingest large quantities of milk without developing gastric difficulties?

In some cases, the enzyme that degrades lactose (milk sugar) to its components—glucose and galactose—is missing. In other cases, the enzyme isomerizes galactose to glucose for further metabolic breakdown.

You are a teaching assistant in a general chemistry lab. The next experiment is to be an oxidation-reduction titration involving iodine. You get a starch indicator from the stockroom. Why do you need it?

Iodine is the reagent that will be added to the reaction mixture in the titration. When the end point is reached, the next drop of iodine will produce a characteristic blue color in the presence of the indicator.

Main function of carbohydrates: Oligosaccharides - Polysaccharides -

Major energy source, structural components, oligosaccharides - key role in cell-cell interactions and immune recognition polysaccharides - essential structural components of several classes of organisms

Where can we find chitin?

Major structural component of exoskeletons of invertebrates, such as insects and crustaceans. Also occurs in cell walls of algae, fungi, and yeasts

What is chitin a polymer of?

N-acetyl-β-D-glucosamine

What makes up peptidoglycan?

N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

How are amylose and amylopectin similar and how are they different?

Similar because they both have alpha-1,4-chains and are glucose homopolymers but different because amylopectin has branches and amylose does not.

Why are (the branched structures of glycogen) important?

The branched structure of glycogen is important because the debranching enzymes can only break down the glucose chains from the ends, not the middle of the chain. So, branching allows the chains to have more ends gives the debranching enzymes and glycogen phosphorylase more ends to react with. This allows for a faster release of glucose, which is very important when there's a need for quick energy.

What are some of the main differences between the cell walls of plants and those of bacteria?

The cell walls of plants consist mainly of cellulose, whereas those of bacteria consist mainly of polysaccharides with peptide crosslinks.

What are the consequences of accumulating proteoglycans?

The consequences of accumulating proteoglycans are skeletal deformities and severe mental retardation

No animal can digest cellulose. Reconcile this statement with the fact that many animals are herbivores that depend heavily on cellulose as a food source.

The digestive tract of these animals contains bacteria that have the enzyme to hydrolyze cellulose.

How many chiral centers are there in the open-chain form of glucose? In the cyclic form?

There are four chiral centers in the open-chain form of glucose (carbons two through five). Cyclization introduces another chiral center at the carbon involved in hemiacetal formation, giving a total of five chiral centers in the cyclic form.

How does the cyclization of sugars introduce a new chiral center?

Two different orientations with respect to the sugar ring are possible for the hydroxyl group at the anomeric carbon. The two possibilities give rise to the new chiral center.

How can we test for starches in a lab?

Using iodine, the presence of starch will make iodine a very dark blue

distinction between alpha and beta anomers

alpha - means trans beta - means that -OH on anomeric carbon is cis to terminal -CH2OH

amylases hydrolyze _____ linkages

alpha(1-4)

What is glycogen made up of?

alpha-D-glucose

amino sugar are sugars with a substituted ____ group as part of its structure

amino (-NH2)

What makes up starch?

amylose and amylopectin

a glycosidic bond forms when _____ reacts with ____ to give full acetal

anomeric C reacts with alcohol

the carbonyl carbon becomes a ____ in aldoses and ketoses

anomeric carbon (bound to 2 O atoms)

Why are glycosaminoglycans important?

are part of lubrication of joints and are part of connective tissues.

What type of linkages does cellulose have?

beta-linkages

What are glycosidic linkages

bonds that join a carbohydrate to another group that may or may not be a carbohydrate.

proteoglycans are glycoproteins with HIGH _____ content

carbohydrate

right on fischer projection becomes ____ on haworth. Left on fisher becomes ____ on haworth

down, up

difference between furanose and pyranose

furanose is a cyclic sugar with a 5-membered ring and pyranose is a cyclic form with 6-membered ring

cellulose is a polymer of

glucose

cyclization of sugars results from internal _____ or _____ formation

hemiacetal, hemiketal

structure of aldose

monosaccharide containing aldehyde group (D-glucose)

structure of ketose

monosaccharide containing ketone group (D-fructose)

structure of dihydroxyacetone

no chiral carbon

What is the difference between oligosaccharides vs. polysaccharides?

oligosaccharides are few monosaccharides linked. Polysaccharides are formed when many monosaccharides bonded together

reactions of monsaccharides: oxidation to ____ Reduction to ____ Reduction to _____ Formation of _____

oxidation to lactone reduction to deoxy reduction to alcohols formation of phosphate esters

Where is pectin found?

plant cell walls

structure of glyceraldehyde

simplest carbohydrate that contains 1 chiral carbon. exists in 2 isomeric forms that are mirror images of each other. Picture is of D-glyceraldehyde

Explain why lactose is a reducing sugar, whereas sucrose is not.

sucrose has no free aldehyde or keto group

What is the main purpose of glycoproteins?

to add variety to proteins, basis of ABO blood grouping

What are the structures of amylose and amylopectin?

■ Amylose: linear glucose homopolymer made up of alpha-1,4 linkages ■ Amylopectin: branched glucose homopolymer made up of alpha-1,4-linkages, but the branches are connected with an alpha-1,6-linkage.

Define the following terms: polysaccharide, furanose, pyranose, aldose, ketose, glycosidic bond, oligosaccharide, glycoprotein

-A *polysaccharide* is a polymer of simple sugars, which are compounds that contain a single carbonyl group and several hydroxyl groups. -A *furanose* is a cyclic sugar that contains a five-membered ring similar to that in furan. -A *pyranose* is a cyclic sugar that contains a six-membered ring similar to that in pyran. -An *aldose* is a sugar that contains an aldehyde group; -a *ketose* is a sugar that contains a ketone group. -A *glycosidic bond* is the acetal linkage that joins two sugars. -An *oligosaccharide* is a compound formed by the linking of several simple sugars (monosaccharides) by glycosidic bonds. -A *glycoprotein* is formed by the covalent bonding of sugars to a protein.

calculate the number of possible stereoisomers

2^n, where n is the number of chiral centers

Aldohexoses have ___ chiral centers

4

What is the difference between a polysaccharide and a monosaccharide?

A polysaccharide is a polymer formed by many sugars or carbohydrates bonded together and a monosaccharide is the simplest form of a carbohydrate that contain a single carbonyl group and 2 or more -OH groups.

A reducing sugar is any sugar that can act as a _______ that contains a free ____ group. Usually an aldehyde but ketoses can be reducing sugars if they can isomerize to an aldose.

A reducing sugar is any sugar that can *act as a reducing agent* that *contains a free carbonyl group*. Usually an aldehyde but ketoses can be reducing sugars if they can isomerize to an aldose.

Name which, if any, of the following groups are not aldose-ketose pairs: D-ribose and D-ribulose, D-glucose and D-fructose, D-glyceraldehyde and dihydroxyacetone.

All groups are aldose-ketose pairs

What are the components of glycosaminoglycans?

Amino sugar and sugar with sulfate or carboxyl

Glycogen is highly branched. What advantage, if any, does this provide an animal?

Because of the branching, the glycogen molecule gives rise to a number of available glucose molecules at a time when it is being hydrolyzed to provide energy. A linear molecule could produce only one available glucose at a time

What is the main structural difference between cellulose and starch?

Both cellulose and starch are polymers of glucose. In cellulose, the monomers are joined by a beta-glycosidic linkage, whereas in starch they are joined by an alpha-glycosidic linkage.

How does chitin differ from cellulose in structure and function?

Chitin is a polymer of N-acetyl-Beta-D-glucosamine, whereas cellulose is a polymer of D-glucose. Both polymers play a structural role, but chitin occurs in the exoskeletons of invertebrates and cellulose primarily in plants.

Why is the polysaccharide chitin a suitable material for the exoskeleton of invertebrates such as lobsters? What other sort of material can play a similar role?

Chitin is a suitable material for the exoskeleton of invertebrates because of its mechanical strength. Individual polymer strands are cross-linked by hydrogen bonding, accounting for the strength. Cellulose is another polysaccharide cross-linked in the same way, and it can play a similar role.

Two sugars are epimers of each other. Is it possible to convert one to the other without breaking covalent bonds?

Converting a sugar to an epimer requires inversion of configuration at a chiral center. This can be done only by breaking and reforming covalent bonds.

Name which, if any, of the following are epimers of D-glucose: D-mannose, D-galactose, D-ribose.

D-Mannose and D-galactose are both epimers of -glucose, with inversion of configuration around carbon atoms 2 and 4, respectively; D-ribose has only five carbons, but the rest of the sugars named in this question have six.

Explain how the minor structural difference between alpha- and beta-glucose is related to the differences in structure and function in the polymers formed from these two monomers.

Differences in structure: cellulose consists of linear fibers, but starch has a coil form. Differences in function: cellulose has a structural role, but starch is used for energy storage.

Difference in structure of dihydroxyacetone and glyceraldehyde

Dihydroxyacetone does not have a chiral carbon or any chiral structures, glyceraldehyde contains one chiral carbon

What is the difference between an enantiomer and a diastereomer?

Enantiomers are nonsuperimposable, mirror-image stereoisomers. Diastereomers are nonsuperimposable, nonmirrorimage stereoisomers.

How many possible epimers of D-glucose exist?

Four epimers of D-glucose exist, with inversion of configuration at a single carbon. The possible carbons at which this is possible are those numbered two through five.

Why are furanoses and pyranoses the most common cyclic forms of sugars?

Furanoses and pyranoses have five-membered and six-membered rings, respectively. It is well known from organic chemistry that rings of this size are the most stable and the most readily formed.

How does glycogen differ from starch in structure and function?

Glycogen and starch differ mainly in the degree of chain branching. Both polymers serve as vehicles for energy storage, glycogen in animals and starch in plants.

What is the main structural difference between glycogen and starch?

Glycogen exists as a highly branched polymer. Starch can have both a linear and a branched form, which is not as highly branched as that of glycogen.

What is the difference between glycoproteins and proteoglycans?

Glycoproteins are carbohydrate residues covalently linked to polypeptide chain. Proteoglycans are a type of glycoprotein with high carbohydrate content.

What are glycoproteins? What are some of their biochemical roles?

Glycoproteins are ones in which carbohydrates are covalently bonded to proteins. They play a role in eukaryotic cell membranes, frequently as recognition sites for external molecules. Antibodies (immunoglobulins) are glycoproteins.

Distinguish between a homopolymer and a heteropolymer of saccharides.

Homopolymer is made up of the same sugar (glycogen for ex) and a heteropolymer is made up of 2 or more sugars