Carbohydrates

Chapter 12

Educational Goals 1.  Given a Fischer projection of a monosaccharide, classify it as either

aldoses or ketoses. 2.  Given a Fischer projection of a monosaccharide, classify it by the

number of carbons it contains. 3.  Given a Fischer projection of a monosaccharide, identify it as a D-

sugar or L-sugar. 4.  Given a Fischer projection of a monosaccharide, identify chiral

carbons and determine the number of stereoisomers that are possible.

5.  Identify four common types of monosaccharide derivatives. 6.  Predict the products when a monosaccharide reacts with a reducing

agent or with Benedict’s reagent. 7.  Define the term anomer and explain the difference between α and β

anomers. 8.  Understand and describe mutarotation.

Educational Goals 9.  Given its Haworth projection, identify a monosaccharide either a

pyranose or a furanose. 10. Identify the anomeric carbon in Haworth structures. 11. Compare and contrast monosaccharides, disaccharides,

oligosaccharides, and polysaccharides. 12. Given the structure of an oligosaccharide or polysaccharide,

identify the glycosidic bond(s) and characterize the glycosidic linkage by the bonding pattern [for example: β(1⟶4)].

13. Given the Haworth structures of two monosaccharides, be able to draw the disaccharide that is formed when they are connected by a glycosidic bond.

14. Understand the difference between homopolysaccharides and heteropolysaccharides.

15. Compare and contrast the two components of starch. 16. Compare and contrast amylopectin and glycogen. 17. Identify acetal and hemiacetal bonding patterns in carbohydrates.

•  Carbohydrates are also known as sugars.

Introduction to Carbohydrates

•  Carbohydrates are an abundant biomolecule. – More than 50% of the carbon in organic

compounds is found in carbohydrates

– Plants use photosynthesis to store energy in glucose, a simple sugar 6 CO2 + 6 H2O + Energy à C6H12O6 + 6 O2

Introduction to Carbohydrates •  Carbohydrates are a large class of naturally occurring

polyhydroxy aldehydes and ketones. •  Monosaccharides (also known as simple sugars) are the

simplest carbohydrates containing 3-7 carbon atoms. A sugar containing: –  an aldehyde is known as an aldose –  a ketone is known as a ketose

Classification of Carbohydrates •  Carbohydrates are grouped into 3 classes: – Monosaccharides are the simplest sugars and serve as the building blocks of larger molecules

Example: Glucose

– Oligosaccharides contain 2-10 monosaccharides bonded together (building block = residue) Example: Sucrose

– Polysaccharides contain more than 10 residues Example: Complex Carbohydrates

Monosaccharides •  Monosaccharides are polyhydroxy ketones

or aldehydes with 3 or more carbons.

Naming Monosaccharides • Carbohydrate nomenclature is unique to

“sugar chemistry” — we do not name monosaccharides using the IUPAC rules.

•  Monosaccharide names end in “ose” • Monosaccharides can be classified by:

• Carbonyl group: aldose or ketose

• Number of carbons: triose, tetrose, etc.

• Both: aldotriose, ketotriose, and so on…

Naming Monosaccharides

Naming Monosaccharides Examples:

Name each of the following monosaccharides as an aldose or ketose & according to its number of C atoms.

You try it:

Stereoisomers in Carbohydrates •  Carbohydrates are chiral molecules since they have

carbon atoms carrying four different groups. •  The simplest three carbon sugar is glyceraldehyde.

This sugar exists as a pair of enantiomers.

•  Enantiomers have the same physical properties except they behave differently in the way they rotate polarized light and the way they are affected by catalysts.

Stereoisomers in Carbohydrates

•  Remember: Compounds with n chiral carbon atoms has a maximum of 2n possible stereoisomers and half that many pairs of enantiomers (mirror images).

Stereoisomers in Carbohydrates

• This aldotetrosose, has 2 chiral carbon atoms and a total of 22 = 4 possible stereoisomers (2 pairs of enantiomers).

The D and L Families of Sugars: Drawing Sugar Molecules

•  Fischer Projections represent three-dimensional structures of stereoisomers on a flat page.

•  A chiral carbon atom is represented in the Fisher projection as the intersection of two crossed lines.

The D and L Families of Sugars: Drawing Sugar Molecules

•  Bonds that point above the page are shown as horizontal lines.

The D and L Families of Sugars: Drawing Sugar Molecules

Bonds that curve behind and below the page are shown as vertical lines.

In a Fischer projection, the aldehyde or ketone carbonyl group of a monosaccharide is always placed toward the top of the page.

Fischer Projections of Sugar Molecules

Fischer Projections of Sugar Molecules Example: Glyceraldehyde

Conventional representation

Fischer Projections of Sugar Molecules Example: Glucose

Monosaccharides are divided into two families: D form and L form sugars.

•  D: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the right.

•  L: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the left.

Fischer Projections of Sugar Molecules

Fischer Projections of Sugar Molecules

D: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the right. L: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the left.

Monosaccharides •  We will briefly survey some important pentoses and

hexoses, and their derivatives.

D-glucose, also called dextrose or blood sugar, is the most important monosaccharide in human metabolism.

D-fructose, or fruit sugar, is most common natural ketose • Honey is 40% fructose

Monosaccharides

•  In deoxy sugars a hydrogen atom replaces one or more of the -OH groups in a monosaccharide.

Monosaccharide Derivatives

– D-ribose and its derivative D-2-deoxyribose (deoxy = minus one oxygen atom) are found in various coenzymes and in DNA.

•  In amino sugars an -OH group of a monosaccharide has been replaced by an amino (-NH2) group. D-glucosamine is an example.

Monosaccharide Derivatives

• D-glucosamine is an amino derivative in which an amino group replaces one hydroxyl group

•  In alcohol sugars the carbonyl group of a monosaccharide has been reduced to an alcohol group. Sorbitol is an example.

Monosaccharide Derivatives

Monosaccharide Derivatives

• Sorbitol and Xylitol are used as sweeteners • Ribitol is found in the coenzyme FAD

•  In carboxylic acid sugars, an aldehyde or alcohol group of a monosaccharide has been oxidized to form a carboxyl group.

Monosaccharide Derivatives

[O]

Reactions of Monosaccharides •  Reactions of monosaccharides are rxns of

carbonyl and hydroxyl groups (chapter 11). – Aldehyde and ketone groups can be reduced – Aldehyde and alcohol groups can be oxidized

The reduction of the C=O group in an aldehyde or ketone produces alcohol sugars.

Example:

Reduction Monosaccharides

Oxidation of Monosaccharides – The oxidation of the aldehyde C=O group

produces carboxylic acid sugars.

[O]

Oxidation of Monosaccharides – Benedict’s reagent is a copper compound that

will oxidize only aldehyde groups (aldoses) and not alcohols.

A sugar that reacts with Benedict’s solution is called a reducing sugar since it reduces the ion Cu2+ à Cu+

Benedict’s Reagent

Oxidation of Monosaccharides NOTE: Some ketoses give positive results for

Benedict’s test because they rearrange to aldehydes in the strongly basic Benedict solution.

Oxidizable Aldehydes

Monosaccharides: Their Cyclic Form •  A hydroxyl group in a monosaccharide can react

with the carbonyl to form a cyclic hemiacetal. – Hemiacetals are made by the reaction of an aldehyde

with an alcohol.

A hemiacetal contains a C atom bonded to an -OH and an -OR group.

Monosaccharides: Their Cyclic Form – A monosaccharide contains both an alcohol and an

aldehyde group. –  It can react with itself to form a cyclic hemiacetal.

Open Chain to Cyclic Form Mechanism

Mechanism will not be

on the Exam

Monosaccharides: Their Cyclic Form – Cyclic forms of monosaccharides are usually drawn

with the Haworth Projection in which the ring is viewed from the side at an angle.

The edge of the ring closest to the viewer is drawn with a bold line for perspective. Substituents on the ring in a Haworth projection are either “up” or “down”

Monosaccharides: Their Cyclic Form – The pair of cyclic hemiacetals with the OH on the

hemiacetal carbon in different positions are called anomers.

– For D-sugars: • The α-anomer has the OH pointing down. • The β-anomer has the OH pointing up.

Monosaccharides: Their Cyclic Form Example: The open-chain form of D-galactose

with its cyclic anomers.

Monosaccharides: Their Cyclic Form – In solution, the open chain and cyclic forms of a

monosaccharide are in equilibrium:

• If we start with a pure open chain or cyclic form in solution, the optical rotation of the solution will change until equilibrium is achieved and the concentrations of the different forms remain constant.

The change in optical rotation observed as the system approaches equilibrium is called mutarotation.

Monosaccharides: Their Cyclic Form – The cyclic forms of monosaccharides can be

named as derivatives of the heterocyclic ethers furan (5 members) and pyran (6 members).

Monosaccharides: Their Cyclic Form – Example: The aldopentose D-ribose forms a cyclic

furanose (the deoxy form is also shown below)

Drawing Haworth Projection in Online Homework

– Note that Haworth Projection can be drawn with or without some of the hydrogens bound to ring-carbons:

Oligosaccharides •  Oligosaccharides are short polymers containing

2-10 monosaccharide residues.

The residues are bonded to each other by glycosidic bonds. – A glycosidic bond is the ether linkage formed

when an acetal is made by reacting a hemiacetal of a monosaccharide with a hydroxyl on another sugar.

– The glycosidic bond in maltose is referred to as an α-(1à4) bond since the monosaccharide on the left reacts it’s α-anomer hemiacetal at C-1 with a hydroxyl at C-4 on the second monosaccharide

Formation of α and β anomers

•  The glycosidic bond can be either α or β.

An α(1→4) Glycosidic bond

A β(1→4) Glycosidic bond

Oligosaccharides – Example: Cellobiose is a disaccharide formed when the

polysaccharide cellulose is broken down. •  Cellobiose is made by connecting two glucose molecule by a β(1à4) glycosidic bond.

•  Cellobiose cannot be used as a source of glucose by humans since we lack the enzyme to hydrolyze the glycosidic bond.

Oligosaccharides Example: Lactose is a disaccharide found in milk

• Lactose consists of a galactose connected to a glucose residue by a β(1à4) glycosidic bond. –  Sounds like cellobiose! But the OH on C-4 is up in

galactose and down in glucose. • Lactose intolerance is the inability to hydrolyze lactose due to an enzyme deficiency.

Oligosaccharides – Sucrose, or table sugar, is a disaccharide with two twists:

different residues and no hemiacetal. • The glycosidic bond in sucrose is formed between the hemiacetal

C of α-glucopyranose and the hemiacetal C of β-fructofuranose

– This is an α,β-(1↔2) glycosidic bond

Note that, as is the case with monosaccharides, the oligosaccharides can be in equilibrium with their anomers.

Only the end, hemiacetal residue can open and close

Oligosaccharides – Most common oligosaccharides are disaccharides – The following are found in peas and beans

Raffinose (trisaccharide) Stachyose (tetrasaccharide) Verbascose (pentasaccharide)

Stachyose Raffinose

Oligosaccharides –  Raffinose (trisaccharide)

Stachyose (tetrasaccharide) Verbascose (pentasaccharide)

– These oligosaccharides are indigestible since they contain galactopyranose residues involved in α-(1à6) glycosidic bonds that humans lack the enzyme to hydrolyze.

Oligosaccharides Glycolipids are sugar-containing lipids that:

-Are present in nerve cell membranes. -Serve as identifying markers on cell surfaces.

The hemiacetal of a sugar residue is connected to an alcohol group of a lipid by a glycosidic bond.

Oligosaccharides •  “How Sweet It Is!”

– Sweetness is rated in comparison to sucrose, which is assigned a sweetness = 100

– Sweeteners used in our foods can be divided into two classes: natural and artificial • Natural Sweeteners are sugars or derivatives

• Artificial Sweeteners may bear no similarity

to sugars!

Relative Sweetness

Artificial Sweeteners

Polysaccharides •  Polysaccharides contain 10 or more residues

–  In a homopolysaccharide, all the residues are the

same monosaccharide

–  In a heteropolysaccharide, the residues are built from more than one type of monosaccharide

– The primary functions of polysaccharides are to: •  Provide structure (e.g. cellulose) •  Store energy (e.g. starch and glycogen)

Polysaccharides – Cellulose is a homopolysaccharide consisting of long,

linear chains of glucose residues joined by β-(1à4) bonds

Polysaccharides •  Cellulose is so strong because the linear chains can form many

hydrogen bonds with adjacent chains forming sheets of the polymer

•  Wood is about 50% cellulose •  Bacteria in horses, cows, and termites have enzyme cellulase to

hydrolyze β-(1à4) bonds

Polysaccharides – Starch is a homopolysaccharide used by

some plants to store energy; there are 2 components of starch: • 1) Amylose • 2) Amylopectin

Starch: Amylose – Amylose contains chains of glucose residue

connected by α-(1à4) glycosidic bonds.

Starch: Amylose – Unlike cellulose, amylose chains are not

linear but coil into a helix.

• Amylopectin is the other component of starch. •  Amylopectin is similar to amylose in that it contains glucose

residues linked by α-(1à4) glycosidic bonds, BUT in amylopectin this chain branches through additional α-(1à6) glycosidic bonds to residues in other chains.

Starch: Amylopectin

• This branched polysaccharide can be “pruned” simultaneously at numerous points allowing the glucose residues (and their energy) to be released more quickly!

Starch: Amylopectin

Polysaccharides • Glycogen, or animal starch, is very similar to amylopectin, except that the chains in glycogen branch more frequently.

• In amylopectin (left), branches occur every 25 to 30 residues • In glycogen (right), branches occur every 8 to 12 residues

Polysaccharides • Chitin is a homopolysaccharide of the glucose derivative N-acetyl-D-glucosamine .

-Chitin makes up the hard exoskeleton of crustaceans and insects. -The polymer chains hydrogen bond to each other leading to chitin’s rigidity.

Polysaccharides – An example of a heteropolysaccharide is

hyaluronic acid.

Hyaluronic acid is found in the lubricating fluid that surrounds joints, and also in the vitreous humor inside the eye.