Chapter 11:Carbohydrates: Structure and Biological Function

MARIA CRISTINA R. RAMOS, Ph.D. Professor , Department of

Chemistry

Selaginella lepidophylla- a desert plant which can adapt to dehydrating condition by synthesizing trehalose,

and can resume normal metabolism when water is available-hence its nickname is “resurrection plant.

Roles of Carbohydrates

• Storage of metabolic fuel

• Structural framework of nucleic acids

• Structural elements in the cell walls of bacteria and plants

• Linked to proteins and lipids which are involved in cell recognition

Monosaccharides are classified by their number of

carbon atoms

Hexose

Heptose

Octose

TrioseTetrose

Pentose

FormulaName

C3H6 O3C4H8 O4

C5H1 0O5

C6H1 2O6

C7H1 4O7C8H1 6O8

Monosaccharides Stereoisomers

Carbohydrates contain many chiral carbons

OPTICAL ISOMERS

• Stereosiomers can be distinguished using plane of polarized light

Naming stereoisomers

Based on the structure of L- and D-glyceraldehyde

O H O H C C H – C – OH HO – C – H

HO – C – H H – C – OH

H – C – OH HO – C – H

H – C – OH HO – C – H

CH2OH CH2OH

D-glucose L-glucose

D- sugars are the most common carbohydrates. D-

refers to the right hand orientation of the OH on

the chiral carbon farthest from the carbonyl carbon

L-sugars: L refers to the left hand orientation of the

OH on the chiral carbon farthest from the carbonyl

carbon

EXERCISE

• Draw the possible structure(s) of

1. aldotetrose

2. ketotetrose

Glucose

• The most common sugar

• Known as dextrose

• Also known as blood sugar

C

C OHH

C HHO

C OHH

C OHH

CH2OH

D-glucose

OH

Galactose

• One of the monosaccharides in the disaccharide lactose

• Found in plant gums and pectins polysaccharides

• An epimer of glucose at C4• Converted to glucose during metabolism

Fructose

• One monosaccharide in the disaccharide sucrose (table sugar).

• Called levulose or fruit sugar • Found in honey and fruits • Sweeter than sucrose and glucose • Commercially prepared as high-

fructose sugars from corn starch for sweetness

• An epimer of glucose at C2• Readily converted to glucose in metabolism

by isomerization

C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

D-fructose

CARBOHYDRATES IN CYCLIC STRUCTURES

Because of the tetrahedral nature of carbon bonds, pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar.

The representation above reflects the chair configuration of the glucopyranose ring more accurately than the Haworth projection.

Fructose forms either

a 6-member pyranose ring, by reaction of the C2 keto group with the OH on C6, or

a 5-member furanose ring, by reaction of the C2 keto group with the OH on C5.

CH2OH

C O

C HHO

C OHH

C OHH

CH2OH

HOH2C

OH

CH2OH

HOH H

H HO

O

1

6

5

4

3

2

6

5

4 3

2

1

D-fructose (linear) -D-fructofuranose

The formation of hemiketals and hemiacetals results in an asymmetric carbon atom.

Isomers that differ only in their configuration about the new asymmetric carbon are called anomers, the new assymetric carbon is called anomeric carbon.

a-anomer has the hydroxyl group at the right side of the anomeric carbon

-anomer has the hydroxyl group at the left side of the anomeric carbon

Anomers in aqueous solution freely interconvert between the and forms, unless the hydroxyl group attached to the

anomeric carbon is linked to another molecule. This process of interconvertion is

called MUTAROTATION

EXERCISE

D-Mannose exists in aqueous solution as a mixture of and forms. Draw the Haworth projection formula of the two anomers.

Reactions of Monosaccharides

Oxidation-Reduction

• Oxidation

Oxidation using a mild oxidizing agent

Oxidation using a strong oxidizing agent

Oxidation to Uronic Acids

CHO

CH2OH

OHHHHOOHHOHH

CHO

COOH

OHHHHOOHHOHH OH

OH

COOHO

HOHO

D-Glucose

enzyme-catalyzedoxidation

D-Glucuronic acid(a uronic acid)

Reduction to alditol

Reduction of ribose to 2’-deoxyribose by ribonucleotide reductase

Reaction with dilute base

Reaction with a concentrated base-

fragmentation

Esterification

Amino Derivatives

Formation of Glycosides

• Treatment of a monosaccharide, all of which exist almost exclusively in a cyclic hemiacetal form, with an alcohol gives an acetal

HH OH

HHO

HOH

OH

H

CH2OHO

CH3OHH

+

-H2O

OCH2OH

H

OH

OCH3H

HOH

OHH

H

OCH2OH

H

OH

HH

HOH

OHH

OCH3

(-D-Glucose)-D-Glucopyranose

Methyl -D-glucopyranoside(Methyl -D-glucoside)

anomeric carbon

+

+

Methyl -D-glucopyranoside(Methyl -D-glucoside)

glycosidicbond

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 -1,4-glycosidic bond

– maltose is a reducing sugar

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 -1,4-glycosidic bond to carbon 4 of D-glucopyranose

– lactose is a reducing sugar

Sucrose

– is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets

– sucrose is a nonreducing sugar

Sweeter than sugar, and no calories

N-Glycosides

French Fries a la Plastic

EXERCISE

A. Draw the chair conformations for -D mannopyranose and -D-mannopyranose

B. Draw the chair conformation of 1. methyl - and -D-mannopyranoside2. -D-mannopyranosyl-(1-4)- -D-glucopyranoside

Polysaccharides

Storage Polysaccharides

Glycogen

• is the energy-reserve carbohydrate for animals

• glycogen is a branched polysaccharide of approximately 106 glucose units joined by -1,4- and -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

Starch

• starch can be separated into amylose and amylopectin

• amylose is composed of unbranched chains of up to 4000 D-glucose units joined by -1,4-glycosidic bonds

• amylopectin contains chains up to 10,000 D-glucose units also joined by -1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by -1,6-glycosidic bonds

Structural Polysaccharides

CelluloseCellulose

• is a linear polysaccharide of D-glucose units joined by -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

Plant Polysaccharides

Hemicellulose chains are shorter than cellulose and may be branched

Pectin

• Is a heteropolymer containing galacturonate and rhamnose residues

• Used a gelling agent in the preparation of jams and jellies

Chitin

• Present in exoskeletons of arthropods

• Composed of N-acetylglucosamine, linked in (1-4)

Bacterial Cell Walls

• The bacterial cell wall is a unique structure which surrounds the cell membrane.

• Maintaining the cell's characteristic shape• Countering the effects of osmotic pressure• Consist of many layers of peptidoglycan

connected by amino acid bridges

Peptidoglycan

• is composed of an alternating sequence of N-acetyl-muraminic acid (NAM) and N-acetylglucosamine (NAG) joined by -1,4-glycosidic bonds.

O

NHO

OCH2OH

OO

NHO

CH2OH

O

O=C O=CCH3 CH3

CHH3C

C=O

NH

Ala

Gln

Lys

Ala

L

D

L

D

C=O

NH-(Gly)5C----

(CH2)4NH-C-(Gly)5-NH-----

To tetrapeptide side chainsO

O

• in Staphylococcus aureus, the cross link is a tetrapeptide

• this tetrapeptide is unusual in that it contains two amino acids of the D-series, namely D-Ala and D-Gln

• each tetrapeptide is cross linked to an adjacent tetrapeptide by a pentapeptide of five glycine units

The NAM-NAG polysaccharide is in turn cross-linked by

small peptides

Bacterial polysaccharides form a biofilm

A biofilm is attached to a surface and

harbors a community of embedded bacteria

that contributes to biofilm production and

maintenance

A Pseudomonas aeruginosa biofilm

Glycoproteins

Components of the cell membrane, where they play variety of roles in cell adhesion

Glycoprotein Functions

• Glycoproptein and Cancer

• Protein Turnover

• Viral Growth

• Antifreeze Glycoprotein

Protein turnover is initiated by removal of carbohydrate residues by hydrolysis

Specific enzymes are responsible for

oligosaccharide assembly

• Glycosyltransferase-catalyzes the synthesis of oligosaccharides through the formation of glycosidc bond

Protein glycosylation takes part in the lumen of the endoplasmic

reticulum and in the golgi complex

Processing of an N-linked oligosaccharides

PROTEOGLYCANS contain long

glycosaminoglycans

Glycosaminoglycans (GAGs) are anionic polyanionic polysaccharides chains

made of repeating disaccharide units

• play important roles in the structure and function of connective tissues

• polysaccharides based on a repeating disaccharide where one of the monomers is an amino sugar and the other has a negative charge due to a sulfate or carboxylate group

• Glycosaminoglycans tend to be negatively charged, because of the prevalence of acidic groups.

• GAGs are usually attached to proteins to form proteoglycans

Common GAGs

• heparin: natural anticoagulant

• hyaluronic acid: a component of the vitreous humor of the eye and the lubricating fluid of joints

• chondroitin sulfate and keratan sulfate:: components of connective tissue

Chondroitin 6-sulfate

• The repeating disaccharide units of N-acetylgalactosamine ang glucoronic acid

• can be sulfated in either 4 or 6 position of GalNAc

• They are prominent components of cartilage tendons, ligaments and aorta

• Palate is made from chondroitin sulfate

LECTINS

• Proteins that bind specific carbohydrate structures

• Are ubiquitous, being found in plants, animals and microorganisms