CHEM 121: chapter 16 - Louisiana Tech Universityupali/chem121/slides/chapter-1… · · 2011-01-24include glucose (dextrose), fructose (levulose), galactose, xylose and ... 4 H

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18-1 Chemistry 121, Winter 2011, LA Tech

Introduction to Organic Chemistry and Biochemistry

Instructor Dr. Upali Siriwardane (Ph.D. Ohio State)

E-mail: [email protected]

Office: 311 Carson Taylor Hall ; Phone: 318-257-4941;

Office Hours: MWF 8:00 am - 10:00 am;

TT 9:00 – 10:00 am & 1:00-2:00 pm.

December 17, 2010 Test 1 (Chapters 12-13)

January 19, 2011 Test 2 (Chapters 14,15 & 16)

February 7, 2011 Test 3(Chapters 17, 18 & 19)

February 23, 2011 Test 4 (Chapters 20, 21 & 22)

February 24, 2011 Comprehensive Make Up Exam:

Chemistry 121(01) Winter 2010-11


Chapter 18:Carbohydrates


Chapter 18: Carbohydrates 18.1 Biochemistry--An Overview

18.2 Occurrence and Functions of Carbohydrates

18.3 Classification of Carbohydrates

18.4 Chirality: Handedness in Molecules

18.5 Stereoisomerism: Enantiomers and Diastereomers

18.6 Designating Handedness Using Fischer Projections

18.7 Properties of Enantiomers

18.8 Classification of Monosaccharides

18.9 Biochemically Important Monosaccharides

18.10 Cyclic Forms of Monosaccharides

18.11 Haworth Projection Formulas

18.12 Reactions of Monosaccharides

18.13 Disaccharides

18.14 General Characteristics of Polysaccharides

18.15 Storage Polysaccharides

18.16 Structural Polysaccharides

18.17 Acidic Polysaccharides

18.18 Glycolipids and Glycoproteins

18.19 Dietary Considerations and Carbohydrates


Biochemistry

Biochemistry is the study of the chemical processes in

living organisms. It deals with the structure and function

of cellular components, such as proteins, carbohydrates,

lipids, nucleic acids, and other biomolecules.

• Carbohydrates

• Lipids

• Proteins

• Nucleic Acids

• Use of carbohydrates as an energy source

2


The Study of Living Things Biochemistry is the study of the chemical

substances found in living organisms and the

chemical interactions of these substances with

each other.

A biochemical substance is a chemical substance

found within a living organism.

Two types of biochemical substances:

bioinorganic substances and bioorganic

substances.

• Bioinorganic substances : water and inorganic salts.

• Bioorganic substances : carbohydrates, lipids,

proteins, and nucleic acids


Occurrence and Functions of Carbohydrates

―hydrates of carbon‖: Cn (H2O)2

Occurrence

Different objects such as sheets of paper, insect skeletons,

fruits, cotton fabrics and ropes have one common feature:

they all contain carbohydrates.

Functions

The chemical structure of carbohydrates, with their many

hydroxyl groups and the ability to assume

various spatial configurations, makes it possible for them to

form nearly unlimited combinations

with other carbohydrate molecules, as well as with proteins

and lipids. The resulting structures

perform important biological functions.


Classification of Carbohydrates Monosaccharides They consist of one sugar containing 3,4,5,6 and 7 carbon atoms and are

usually colorless, water-soluble, crystalline solids. Some

monosaccharides have a sweet taste. Examples of monosaccharides

include glucose (dextrose), fructose (levulose), galactose, xylose and

ribose.

Disaccharides a sugar (a carbohydrate) composed of two monosaccharides.

Oligosaccharide An oligosaccharide is a saccharide polymer containing a small number

(typically 3-10 monosaccharides

Polysacharides Are relatively complex carbohydrates. They are polymers made up of

many monosaccharides joined together by glycosidic bonds. They

are insoluble in water, and have no sweet taste.


Classification of Sugars

Number of carbon atoms

Triose sugar: three carbon atoms

Tetroses sugar: four carbon atoms

Pentoses sugar five carbon atoms

Hexoses sugar: six carbon atoms

Number fo units (Saccharide-sugar units)

Monosaccharide (one sugar unit);

Disaccharide (two sugar units);

Oligosaccharide (2 to 10 sugar units);

Polysaccharide (over 10 sugar units).

3


Chirality: Handedness in Molecules

A "chiral" molecule is one that is not superimposable with its mirror image. Like left and right

hands that have a thumb, fingers in the same order, but are mirror images and not the same,

chiral molecules have the same things attached in the same order, but are mirror images and not

the same.


Monosaccharides

Glyceraldehyde contains a stereocenter and exists

as a pair of enantiomers

CHO

CH OH

CH2OH

CHO

C

CH2OH

HHO

(S)-Glycer-

aldehyde

(R)-Glycer-

aldehyde


Fischer Projection Formulas

Fischer projection: a two dimensional

representation for showing the configuration of a

tetrahedral stereocenter

• horizontal lines represent bonds projecting forward

• vertical lines represent bonds projecting to the rear

• the first and last carbons in the chain are written in full;

others are indicated by the crossing of bonds

CHO

CH OH

CH2OH

CHO

H OH

CH2OH

(R)-Glyceraldehyde

convert to a Fischerprojection

(R)-Glyceraldehyde


Stereoisomerism: Enantiomers and Diastereomers

A Fischer projection is the most useful projection for

discovering enantiomers. Compare the Glyceraldehyde

enantiomer structures in this diagram.

D- and L-Monosaccharides CHO

H OH

CH2OH

CHO

CH2OH

HHO

D D

L-GlyceraldehydeD-Glyceraldehyde

[]25

= +13.5° []25

= -13.5°

D-monosaccharide: a monosaccharide that, when written as a

Fischer projection, has the -OH on its penultimate carbon on the

right

L-monosaccharide: a monosaccharide that, when written as a

Fischer projection, has the -OH on its penultimate carbon on the

left

4


Properties of Enantiomers

Enantiomers have, when present in a symmetric

environment, identical chemical and physical properties

except for their ability to rotate plane-polarized light by

equal amounts but in opposite directions.

A mixture of equal parts of an optically active isomer and

its enantiomer is termed racemic and has a net rotation of

plane-polarized light of zero.

Enantiomers of each other often do have different

chemical properties related to other substances that are

also enantiomers. Since many molecules in the bodies of

living beings are enantiomers themselves, there is often a

marked difference in the effects of two symmetrical

enantiomers on living beings, including human beings.


Diastereomers

Have more than one chiral centers

Diastereomers (or diastereoisomers) are

stereoisomers that are not enantiomers (non-

superposable mirror images of each other).

Diastereomers can have different physical

properties and different reactivity. In another

definition diastereomers are pairs of isomers that

have opposite configurations at one or more of

the chiral centers but are not mirror images of

each other.


Diastereomers of Tartaric acid Tartaric acid contains two asymmetric centers, but two of the "isomers"

are equivalent and together are called a meso compound. This

configuration is not optically active, while the remaining two isomers

are D- and L- mirror images, i.e., enantiomers. The meso form is a

diastereomer of the other forms.


What is Plane Polarized Light?

http://en.wikipedia.org/wiki/Plane_(mathematics)

http://en.wikipedia.org/wiki/Polarized_light

http://en.wikipedia.org/wiki/Racemic

http://en.wikipedia.org/wiki/Polarized_light

http://en.wikipedia.org/wiki/Stereoisomer

http://en.wikipedia.org/wiki/Enantiomer

http://en.wikipedia.org/wiki/Superpose



http://en.wikipedia.org/wiki/Tartaric_acid

http://en.wikipedia.org/wiki/Meso_compound



http://en.wikipedia.org/wiki/Optical_activity

5


Optically active of enantiomers

CHO

H OH

CH2OH

CHO

CH2OH

HHO

D D


[]25

= +13.5° []25

= -13.5°


D- and L-Monosaccharides

In 1891, Emil Fischer made the arbitrary

assignments of D- and L- to the enantiomers of

glyceraldehyde

CHO

H OH

CH2OH

CHO

CH2OH

HHO

D D


[]25

= +13.5° []25

= -13.5°


Aldoses:

Monosaccarides with aldehyde functional

group. E.g. D-glucose

Ketoses:

Monosaccarides with keto functional

group. E.g. D-fructose

Functional Groups


Aldoses

6


Aldoses

Hexoses


Cyclization of D-glucose

-D - glucose

- D - glucose

H

OH

O H

OH

H

OH H

OH

CH 2

OH

H

C

C

C

C

C

CH 2

OH

OH

OH

H

OH H

HO

H

H

O H

O H

OH

OH

H

H

H

OH

CH 2

OH

H

OH


Fischer & Haworth Projection

In solutions less than 1% of a sugar will be in the

linear form shown as Fischer projection

The normal form of most sugars is in a cyclic

hemiacetal form shown as Haworth projection


Converting Fischer to Haworth

Projection

7


-Cyclic form of Gulose


Aldopentoses

18-27 Chemistry 121, Winter 2011, LA Tech 18-28 Chemistry 121, Winter 2011, LA Tech

Two monsaccharides connected by a bridging O atom called a

glycosidic bond as in sucrose.

8



According to the conventions proposed by Fischer

• D-monosaccharide: a monosaccharide that, when

written as a Fischer projection, has the -OH on its

penultimate carbon on the right

• L-monosaccharide: a monosaccharide that, when

written as a Fischer projection, has the -OH on its

penultimate carbon on the left



Following are

• the two most common D-aldotetroses and

• the two most common D-aldopentoses

D-Erythrose D-Threose D-Ribose 2-Deoxy-D-

ribose

CH2 OH

CHO

OH

OHH

H

CH2 OH

CHO

OH

HHO

H

CH2 OH

CHO

OH

OHH

H

CH2 OH

CHO

OH

HH

H

OHH OHH



• and the three common D-aldohexoses

CHO

H

OHH

HO

OHH

D-GlucosamineD-Glucose D-Galactose

CH2 OH

OHH

CHO

H

OHH

HO

HHO

CH2 OH

OHH

CHO

H

NH2H

HO

OHH

CH2 OH

OHH



Amino sugars

• N-acetyl-D-glucosamine is a component of many

polysaccharides, including connective tissue such as

cartilage; it is also a component of chitin, the hard

shell-like exoskeleton of lobsters, crabs, and shrimp

CHO

OH

OH

H

NH2

H

H

HO

H

CH2 OH

CHO

OH

OH

H

H

H

H

HO

H2 N

CH2 OH

CHO

OH

OH

H

NHCCH3

H

H

HO

H

CH2 OH

OCHO

OH

H

H

NH2

H

HO

HO

H

CH2 OH

4

2

D -M an nosamin e

(C-2 stereoisomer

of D-g lucosamine)

D-Glucosamine D-G alactosamine

(C-4 stereoisomer

of D-g lucosamine)

N-Acetyl -D-

g lucosamine

9


Classification of Monosaccharides

Monosaccharides have the general formula CnH2nOn

the most common have from 3 to 9 carbons

Triose (3) , tetrose(4), pentose(5), hexose(6)

• aldose: a monosaccharide containing an aldehyde

group: E.g. D-glucose

• ketose: a monosaccharide containing a ketone group:

E.g. D-Fructose


Carbohydrates: Monosaccharides

Carbohydrate: a polyhydroxy aldehyde, a polyhydroxy

ketone, or a polymeric substance that gives these

compounds on hydrolysis

Monosaccharide: a carbohydrate that cannot be hydrolyzed

to a simpler carbohydrate

• monosaccharides have the general formula CnH2nOn

• the most common have from 3 to 9 carbons

• aldose: a monosaccharide containing an aldehyde

group: E.g. D-glucose

• ketose: a monosaccharide containing a ketone group:

E.g. D-Fructose


Monosaccharides

• monosaccharides are classified by their number of

carbon atoms

hexose

heptose

octose

triose

tetrose

pentose

FormulaName

C3 H6 O3

C4 H8 O4

C5 H10 O5

C6 H12 O6

C7 H14 O7

C8 H16 O8


Aldoses: Trioses, Tetroses and Pentoses

10


Aldoses: Hexoses


Important ketoses:

pentoses hexose heptoses


Monosaccharides

• there are only two trioses

• often aldo- and keto- are omitted and these compounds

are referred to simply as trioses

• although this designation does not tell the nature of the

carbonyl group, it at least tells the number of carbons

CHO

CHOH

CH2OH

CH2OH

C=O

CH2OH

Dihydroxyacetone (a ketotriose)

Glyceraldehyde (an aldotriose)


Biochemically Important Monosaccharides

• Glucose is the most common monosaccharide consumed

and is the circulating sugar of the bloodstream.

-Insulin and glucagon regulate blood levels of glucose.

• Galactose, a component of lactose (milk sugar) is also

found in some plant gums and pectins.

-Galactosemia results from inability to metabolize galactose.

-If treated, galactosemia can be managed medically.

Untreated galactosemia may result in mental retardation,

liver damage, or death.

• Fructose is slightly sweeter than glucose. It is an

intermediary in metabolism and is found in many fruits.

• Ribose and deoxyribose are aldopentose components of

DNA and RNA.

11


Cyclic Forms of Monosaccharides Intramolecular cyclization of simple sugars tend to exist

primarily in cyclic form through hemiacetal or hemiketal

formation. It is the most stable arrangement.

C

C

C

CH2OH

C

C

OH

O

H C

C

C

CH2OH

C

C

O

OH

aldehyde hemiacetal


- and - anomers

The -OH group that forms can be above or

below the ring resulting in two forms -

anomers

and are used to identify the two forms.

- OH group is down compared to CH2OH

(trans).

- OH group is up compared to CH2OH (cis).


- and forms of D-glucose

-D - glucose

- D - glucose

H

OH

O H

OH

H

OH H

OH

CH 2

OH

H

C

C

C

C

C

CH 2 OH

OH

OH

H

OH H

HO

H

H

O H

O H

OH

OH

H

H

H

OH

CH 2 OH

H

OH 18-44 Chemistry 121, Winter 2011, LA Tech

Haworth Projection Formulas

• the anomers of D-glucopyranose

CHO

OH

H

OH

H

HO

H

H OH

CH2OH

H

H OH

HHO

HOH

OH

H

CH2OH

O

C

OH

HHO

HOH

H

CH2OH

OHO

H

OH

H OH

HHO

HH

OH

H

CH2OH

O

D-Glucose

-D-Glucopyranose (-D-Glucose)

()

()

-D-Glucopyranose

( -D-Glucose)

anomeric carbon

+

anomericcarbon

5

5

1

1

redraw to show the -OH on carbon-5 close to the

aldehyde on carbon-1

H

12


Converting Fischer to Haworth

Projection


Haworth Projections

• 5- and 6-membered hemiacetals are represented as

planar pentagons or hexagons viewed through the

edge

• most commonly written with the anomeric carbon on

the right and the hemiacetal oxygen to the back right

• - means that -OH on the anomeric carbon is cis to the

terminal -CH2OH; - means it is trans

• a 6-membered hemiacetal is shown by the infix -pyran-

• a 5-membered hemiacetal is shown by the infix -furan-

OOPyranFuran


Cyclic Structures in Haworth Projections

Aldopentoses also form cyclic hemiacetals

• the most prevalent forms of D-ribose and other

pentoses in the biological world are furanoses

• the prefix deoxy- means ―without oxygen‖

OH ( )

H

HOH OH

H HO

HOCH2

H

OH ( )

HOH H

H HO

HOCH2

-D-Ribofuranose

(-D-Ribose)

-2-Deoxy-D-ribofuranose

(-2-Deoxy-D-ribose)


Cyclic Structures

D-Fructose, a 2-ketohexose, also forms a cyclic

hemiacetal

O

HO

HOCH2

H

H

HO

CH2OH

OHH

OH

HO

HOCH2

H

H

HO

H

H

HO

HOHOHOCH2

CH2OH

O

HCH2OH

OH

5

5

1

2

2

()

-D-Fructofuranose(-D-Fructose)

-D-Fructofuranose(-D-Fructose)

()

1

anomericcarbon

5

1

2

13


Fischer & Haworth Projection

In solutions less than 1% of a sugar will be in the

linear form shown as Fischer projection

The normal form of most sugars is in a cyclic

hemiacetal form shown as Haworth projection


-Cyclic form of Gulose


Aldopentoses


Aldoxeoses

14


Reactions of Monosaccharides

Reduction: The carbonyl group of a monosaccharide can be

reduced to an hydroxyl group by a variety of reducing

agents, including NaBH4

Oxidation: The -CHO group can be oxidized to –COOH

Reducing sugar: any carbohydrate that reacts with an

oxidizing agent to form an aldonic acid

Oxidation: OH to –COOH

Enzyme-catalyzed oxidation of the 1° alcohol at carbon-6 of

a hexose gives a uronic acid

Glucose Assay: The analytical procedure most often

performed in the clinical chemistry laboratory is the

determination of glucose in blood, urine, or other

biological fluid


Reduction to Alditols

The carbonyl group of a monosaccharide can be

reduced to an hydroxyl group by a variety of

reducing agents, including NaBH4


Reduction to Alditols

• name alditols by replacing the -ose of the name of the

monosaccharide by -itol

• sorbitol is found in the plant world in many berries and

in cherries, plums, pears, apples, and seaweed; it is

about 60% as sweet as sugar

• other common alditols include

CH2 OH

CH2 OH

OHH

OHH

CH2 OH

CH2 OH

OHH

HHO

OHH

CH2 OH

HHO

HHO

OHH

CH2 OH

OHH

D-Mannitol XylitolErythritol


Oxidation to Aldonic Acids

The -CHO group can be oxidized to -COOH

• reducing sugar: any carbohydrate that reacts with an

oxidizing agent to form an aldonic acid

15


Oxidation to Uronic Acids

Enzyme-catalyzed oxidation of the 1° alcohol at

carbon-6 of a hexose gives a uronic acid


Oxidation to Uronic Acids

• the body uses glucuronic acid to detoxify foreign

alcohols and phenols

• these compounds are converted in the liver to

glycosides of glucuronic acid and then excreted in the

urine

• the intravenous anesthetic propofol is converted to the

following water-soluble glucuronide and excreted

O

OHOHO

OH

COO-

HO

Propofol A urine-soluble glucuronide


Glucose Assay

The analytical procedure most often performed in

the clinical chemistry laboratory is the

determination of glucose in blood, urine, or other

biological fluid

• this need arises because of the high incidence of

diabetes in the population


Glucose Assay

The glucose oxidase method is completely specific

for D-glucose

+

+

glucose

oxidase

D-Gluconic acid

Hydrogen

peroxide

- D-Glucopyranose

OHOH

HOHO

CH2 OHO

H2 O2

O2 + H2 O

CO2 H

CH2 OH

OHH

HHO

OHH

OHH

16


Glucose Assay

• O2 is reduced to hydrogen peroxide, H2O2

• the concentration of H2O2 is proportional to the

concentration of glucose in the sample

• in one procedure, hydrogen peroxide is used to oxidize

o-toluidine to a colored product, whose concentration

is determined spectrophotometrically

NH2

CH3

H2 O2peroxidase

colored product+

2-Methylaniline(o-Toluidine)


Ascorbic Acid (Vitamin C)

L-Ascorbic acid (vitamin C) is synthesized both

biochemically and industrially from D-glucose

CHO

CH2 OH

OHH

HHO

OHH

OHH

CH2 OH

OHH

HHO

O

OH

O

D-Glucose

both biochemialand industrial

syntheses

L-Ascorbic acid (Vitamin C)


Ascorbic Acid (Vitamin C)

• L-ascorbic acid is very easily oxidized to L-

dehydroascorbic acid

• both compounds are physiologically active and are

found in most body fluids

CH2OH

OHH

HHO

O

OH

O

CH2OH

OHH

HO

O

O

O

L-Ascorbic acid

(Vitamin C)L-Dehydroascorbic acid

oxidation

reduction


Disaccharides

A disaccharide forms by reaction of the -OH group on the

anomeric carbon of one monosaccharide with an –OH

group of a second monosaccharide.

• The linkage between monosaccharides in a disaccharide is

referred to as a glycosidic linkage and is named

according to the number of the carbon at which the

linkage begins and the carbon on the second

monosaccharide at which the linkage ends.

-The glycosidic linkage is also designated or β, depending

upon whether the conformation at the anomeric carbon is

up or down.

17


Two monsaccharides connected by a bridging O atom called a glycosidic

bond as in sucrose.

Disaccharides


Sucrose

Table sugar, obtained from the juice of sugar cane

and sugar beet


Lactose

The principle sugar present in milk

• about 5 - 8% in human milk, 4 - 5% in cow’s milk


Maltose

From malt, the juice of sprouted barley and other

cereal grains

18


Uses for polysaccharides

• Storage polysaccharides

Energy storage - starch and glycogen

• Structural polysaccharides

Used to provide protective walls or lubricative

coating to cells - cellulose and

mucopolysaccharides.

• Structural peptidoglycans

• Bacterial cell walls

Polysaccharides


Starch

Starch is used for energy storage in plants

• it can be separated into two fractions; amylose and

amylopectin

• amylose is composed of unbranched chains of up to

4000 D-glucose units joined by -1,4-glycosidic bonds

• amylopectin is a highly branched polymer of D-

glucose; chains consist of 24-30 units of D-glucose

joined by -1,4-glycosidic bonds and branches created

by -1,6-glycosidic bonds


Starch

• at the cellular level, the biochemical basis for this

classification is a group of relatively small membrane-

bound carbohydrates


• Straight chain that forms coils (1 4) linkage.

O

O

H

HO

H

OH

CH2OH

O

OH

OH

H

HO

HOH2C

O O

H

OH

H

HO

HOH2C

O

O

HOH

HHO

HOH2C

O

O

H

HO

H

OH

CH2OH

O

OH

OHH

OH

CH2OH

O

O

HHO

HOH

CH2OH

Amylose starch

19


• Energy storage of animals.

• Stored in liver and muscles as granules.

• Similar to amylopectin but more highly

branched.

O

O

O

O

O

O

O

O

O

O

O

O

O

O c

(1 6) linkage

at crosslink

c

Glycogen


Glycogen

The reserve carbohydrate for animals

• a nonlinear polymer of D-glucose units joined by -1,4-

and -1,6-glycosidic bonds

• the total amount of glycogen in the body of a well-

nourished adult is about 350 g (about 3/4 of a pound)

divided almost equally between liver and muscle


• Most abundant polysaccharide. • (1 4) glycosidic linkages.

• Result in long fibers - for plant structure.

O O

H

OHH

OH

CH2OH

O O

CH2OH

OHH

HO HO O

H

OHH

OH

CH2OH

O O

CH2OH

OHH

HO HO O

H

OHH

OH

CH2OH

O

CH2OH

OHH

HO H

Structural Polysaccharides: Cellulose


Cellulose

Cellulose is a linear polymer of D-glucose units

joined by -1,4-glycosidic bonds

• it has an average molecular weight of 400,000,

corresponding to approximately 2800 D-glucose units

per molecule

• both rayon and acetate rayon are made from

chemically modified cellulose

20


H OO

H

H

OHH

COO-

HO

H O

OH

O

H

H

NH

CH2OH

H

C O

CH3

H OO

H

H

OHH

COO-

HO

H O

OH H

H

NH

CH2OH

H

C O

CH3

H OO

H

H

OHH

COO-

HO

H O

OH

O

H

H

NH

CH2OH

H

C O

CH3O

• These materials provide a thin, viscous, jelly-

like coating to cells.

• The most abundant form is

hyaluronic acid.

•

(1 3)

(1 4)

Mucopolysaccharides: lubricative coatings

Alternating units of N-acetylglucosamine

and D-glucuronic acid.


Glycolipids and Glycoproteins

Glycolipids: are carbohydrate-attached lipids. Their role is

to provide energy and also serve as markers for cellular

recognition.

Glycoproteins: are the proteins covalently attached to

carbohydrates such as glucose, galactose, lactose,

fucose, sialic acid, N-acetylglucosamine, N-

acetylgalactosamine, etc.

The antigens which determine blood types belong to

glycoproteins and glycolipids (more info later).


Cyclic Structure

Monosaccharides have hydroxyl and carbonyl

groups in the same molecule and exist almost

entirely as five- and six-membered cyclic

hemiacetals

• anomeric carbon: the hemiacetal carbon of a cyclic

form of a monosaccharide

• anomers: monosaccharides that differ in configuration

only at their anomeric carbons


Glycosides

Glycoside: a carbohydrate in which the -OH on its

anomeric carbon is replaced by -OR

H

H OH

HHO

HOH

OH

H

CH2OH

O

CH3 OH H+

-H2 O

O

CH2OH

H

OH

OCH3H

HOH

OHH

H

O

CH2OH

H

OH

HH

HOH

OHH

OCH3

(-D-Glucose)

-D-Glucopyranose

Methyl -D-glucopyranos ide

(Methyl -D-glucoside)

anomeric carbon

+

+

Methyl -D-glucopyranoside

(Methyl -D-glucos ide)

glycos idicbond glycos idic

bond

http://www.web-books.com/MoBio/Free/Ch1B5.htm

21


Glycosides

Glycosidic bond: the bond from the anomeric

carbon of the glycoside to an -OR group

To name a glycoside, name the alkyl or aryl group

bonded to oxygen followed by the name of the

carbohydrate; replace the ending -e by -ide

• methyl -D-glucopyranoside

• methyl -D-ribofuranoside


N-Glycosides

• the anomeric carbon of a cyclic hemiacetal also reacts

with an N-H of an amine to form an N-glycoside

• especially important in the biological world are the N-

glycosides of D-Ribose and 2-deoxy-D-ribose with the

following heterocyclic aromatic amines

HN

N

H

O

O

Uracil

N

N

H

O

NH2

Cytosine

HN

N

H

O

O

Thymine

CH3N

N N

N

H

NH2

HN

N N

N

H

O

H2 N

Adenine Guanine

Pyrimidine bases Purine bases


N-Glycosides

• following is the -N-glycoside formed between D-

ribofuranose and cytosine

H

H

H

H

OHOCH2

HO OH

NH2

O

N

N

anomericcarbon

a -N -glycosidicbond


Blood Group Substances

Membranes of animal plasma cells have large

numbers of relatively small carbohydrates

• these membrane-bound carbohydrates are part of the

mechanism by which cell types recognize each other;

they act as antigenic determinants

• among the first discovered of these antigenic

determinants are the blood group substances

In the ABO system, individuals are classified

according to four blood types: A, B, AB, and O

22



• one of these membrane-bound monosaccharides is L-

fucose

HHO

OHH

CH3

CHO

OHH

HHO

An L-monosaccharide;this -OH is on the left in

the Fischer projection

L-Fucose

Carbon 6 is -CH3 ratherrather than -CH

2OH



A, B, AB, and O blood types

N -Acetyl-D-

galactosamineD-Galactose N-Acetyl-D-

glucosamine

D-GalactoseN-Acetyl-D-

glucosamine

Redblood

cell

Type A

Type B

D-GalactoseType O N-Acetyl-D-

glucosamine

Redblood

cell

Redblood

cell

D-galactose

(-1,4)

(-1,4)

(-1,3)

(-1,3)

(-1,3)

L-Fucose

(-1,2)

L-Fucose

(-1,2)

L-Fucose

(-1,2)


Polysaccharides

• These are biopolymers composed of

hundreds to thousands of simple

sugar units (monosaccharides).

• The most common

monosaccharide used

in polysaccharides is glucose.

C

C

C

C

C

CH 2

OH

OH

OH

H

OH H

HO

H

H

O H


Polysaccharides

• Uses for polysaccharides

• Storage polysaccharides • Energy storage - starch and glycogen

• Structural polysaccharides • Used to provide protective walls or

lubricative coating to cells - cellulose and

mucopolysaccharides. • Structural peptidoglycans

• Bacterial cell walls

23


Starch

• Energy storage used by plants • Long repeating chain of -D-glucose

• Chains up to 4000 units • Amylose straight chain

• Amylopectin branched structure

• Starch is a mixture of about 75% amylopectin and 25%

amylose.


Amylose starch

• Straight chain that forms coils (1 4)

linkage.

O

O

H

HO

H

OH

CH2OH

O

OH

OH

H

HO

HOH2C

O O

H

OH

H

HO

HOH2C

O

O

HOH

HHO

HOH2C

O

O

H

HO

H

OH

CH2OH

O

OH

OHH

OH

CH2OH

O

O

HHO

HOH

CH2OH


Amylose starch

Example showing coiled structure

- 12 glucose units

- hydrogens and side chains are omitted.


Amylopectin starch

• Amylopectin differs from amylose only in that it has

side chains. These are formed from

(1 6) links

• Side chains occur every 24-30 units.

• Starch is stored as starch grains. They cannot

diffuse from the cell and have little effect on the

osmotic pressure of the cell.

24


Glycogen • Energy storage of animals.

• Stored in liver and muscles as granules.

• Similar to amylopectin but more highly

branched.

O

O

O

O

O

O

O

O

O

O

O

O

O

O c

(1 6) linkage

at crosslink c


Cellulose • Most abundant polysaccharide.

• (1 4) glycosidic linkages.

• Result in long fibers - for plant

structure.

O O

H

OHH

OH

CH2OH

O O

CH2OH

OHH

HO HO O

H

OHH

OH

CH2OH

O O

CH2OH

OHH

HO HO O

H

OHH

OH

CH2OH

O

CH2OH

OHH

HO H


H OO

H

H

OHH

COO-

HO

H O

OH

O

H

H

NH

CH2OH

H

C O

CH3

H OO

H

H

OHH

COO-

HO

H O

OH H

H

NH

CH2OH

H

C O

CH3

H OO

H

H

OHH

COO-

HO

H O

OH

O

H

H

NH

CH2OH

H

C O

CH3O

Mucopolysaccharides • These materials provide a thin, viscous, jelly-like

coating to cells.

• The most abundant form is hyaluronic acid.

•

• Alternating units of

• N-acetylglucosamine

• and D-glucuronic acid.

(1 4)

(1 3)


Reducing and Nonreducing sugar.

Monosaccharides both aldoses and ketoses

Disaccharides with free hemiacetal or hemiketal

ends:maltose and lactose

Aldoses

Aldehyde sugars should show positive test for the

Benedict's test

Ketoses

give a positive test for Benedict's test because of

the ability of ketoses to get converted to aldoses

(aldehydes) via enediol.

25


Enediol Intermediate


Galactosemia and Lactose

Intolerance.

Galactosemia

Lack of enzymes necessary for the

conversion of galactose to phosphorylated

glucose which is used in the cellular

metabolism or glycolysis

Lactose Intolerance.

Lack of diagestive enzyme, lactase to

break down lactose to glucose and

galactose


Practice Exercise

Answers: a. Not a chiral center b. Not a chiral center

c. Chiral center d. Not a chiral center


Copyright © Cengage Learning. All rights reserved 100

Exercise

26



Interactions Between Chiral

Compounds

Our body responds differently to different

enantiomers:

One may give higher rate or one may be

inactive

• Example: Body response to D form of hormone

epinephrine is 20 times greater than its L isomer



Exercise


Dominant form of monosaccharides with 5 or more C atoms is

cyclic - cyclic forms are in equilibrium with open chain form

Cyclic forms are formed by the reaction of carbonyl group

(C=O) with hydroxyl (-OH) group on carbon 5

Cyclic Hemiacetal Forms of D-Glucose


Practice Exercise

Which of the monosaccharides glucose, fructose, galactose, and

ribose has each of the following structural characteristics?

(There may be more than one correct answer for a given

characteristic)

a. It is a pentose.

b. It is a ketose.

c. Its cyclic form has a 6-membered ring.

d. Its cyclic form has two carbon atoms outside the ring.

Answers:

a. Ribose

b. Fructose

c. Glucose, galactose

d. Fructose

27



Five important reactions of monosaccharides:

• Oxidation to acidic sugars

• Reduction to sugar alcohols

• Glycoside formation

• Phosphate ester formation

• Amino sugar formation

These reactions will be considered with

respect to glucose.

Other aldoses, as well as ketoses, undergo

similar reactions.


Oxidation Oxidation to acidic functional groups present in

sugars: The redox chemistry of

monosaccharides is closely linked to the

alcohol and aldehyde

Oxidation can yield three different types of acidic

sugars depending on the type of oxidizing

agent used:

• Weak oxidizing agents such as Tollens and

Benedict’s solutions oxidize the aldehyde

end to give an aldonic acid.

• A reducing sugar is a carbohydrate that

gives a positive test with Tollens and

Benedict’s solutions.



Oxidizing Agents

Strong oxidizing agents can oxidize both ends

of a monosaccharide at the same time (the

carbonyl group and the terminal primary

alcohol group) to produce a dicarboxylic

acid:

• Such polyhydroxy dicarboxylic acids are known

as aldaric acids.


Oxidization

In biochemical systems enzymes can oxidize

the primary alcohol end of an aldose such as

glucose, without oxidation of the aldehyde

group, to produce an alduronic acid.

28


Amino Sugar Formation

Amino sugar formation: An amino sugar - one

of the hydroxyl groups of a monosaccharide

is replaced with an amino group

In naturally occurring amino sugars the carbon

2 hydroxyl group is replaced by an amino

group

Amino sugars and their N-acetyl derivatives

are important building blocks of

polysaccharides such as chitin


Cellobiose

Cellobiose is produced as an intermediate in

the hydrolysis of the polysaccharide

cellulose:

• Cellobiose contains two b - D-glucose

monosaccharide units linked through a b (1—4)

glycosidic linkage.

O OH

OH

OH

CH2OH

HO

OHOH

OH

CH2OH

O

Cellobiose

(1-4)


Chitin • Similar to cellulose in both function and structure

• Linear polymer with all b (14) glycosidic linkages - it has a

N-acetyl amino derivative of glucose

• Function is to give rigidity to the exoskeleton s of crabs,

lobsters, shrimp, insects, and other arthropods

N-Acetyl

-D-Glucoseamine

O

HN

OH

O

OH

O

OHO

HO O

OH

HO O

OH

O

HO

O

O

O

HN

O

HN

O

HN

O


Acidic polysaccharides - polysaccharides with a repeating

disaccharide unit containing an amino sugar and a sugar with a

negative charge due to a sulfate or a carboxyl group.

Structural polysaccharide present in connective tissue associated

with joints, cartilage, synovial fluids in animals and humans

• Primary function is lubrication necessary for joint

movement

• These are heteropolysaccharides - have more than one type

of monosaccharide monomers is present.

Examples:

• Hyaluronic acid

• Heparin

29


Hyaluronic Acid and Heparin

Hyaluronic acid:

• Alternating residues of N-

acetyl-b-D-glucosamine

and D-glucuronic acid.

• Highly viscous - serve as

lubricants in the fluid of

joints and part vitreous

humor of the eye.

Heparin:

• An anticoagulant-prevents

blood clots.

• Polysaccharide with 15–

90 disaccharide residues

per chain.


A glycolipid is a lipid molecule that has one or

more carbohydrate (or carbohydrate

derivative) units covalently bonded to it.

A glycoprotein is a protein molecule that has

one or more carbohydrate (or carbohydrate

derivative) units covalently bonded to it.


Nutrition

Foods high in carbs content constitute over 50% of the diet of

most people of the world -- a balanced dietary food should

contain about 60% of carbohydrate:

• Corn in South America

• Rice in Asia

• Starchy root vegetables in parts of Africa

• Potato and wheat in North America

Nutritionist divide dietary carbs into two classes:

• Simple carb: dietary monosaccharides or disaccharides -

sweet to taste commonly referred to as sugars - 20 % of the

energy in the US die

• Complex carbs: Dietary polysaccharides -- starch and

cellulose - normally not sweet to taste


Glycemic Foods

A developing concern about intake of

carbohydrates involves how fast the given

dietary carbs are broken down to glucose

within the human body

Glycemic effect refers to:

• how quickly carbs are digested

• how high blood glucose rise

• how quickly blood glucose levels return to

normal

Glycemic index (GI) has been developed for

rating foods

Documents

CHEM 121: chapter 16 - Louisiana Tech Universityupali/chem121/slides/chapter-1… · · 2011-01-24include glucose (dextrose), fructose (levulose), galactose, xylose and ... 4 H