Monosaccharides (Simple sugars)• They may exist in a linear molecule or in ring

forms.• They are classified according to the number of

carbon atoms in their molecule. 5 carbons are called pentoses ex. Ribose 6 carbons are called hexoses ex. Glucose• Many forms exists as isomers. Isomers are

molecules which have the same empirical formula (recipe) but have different structures (shapes) due to arrangement of the atoms in the molecule. This also gives them different properties. Glucose and fructose both have the empirical formula C6H12O6, but they have different structural formulas or shapes.

• MONOSACCHARIDES ARE THE BUILDING BLOCKS FOR ALL OTHER CARBOHYDRATES!

Monosaccharides

Aldoses (e.g., glucose) have an aldehyde group at one end.

Ketoses (e.g., fructose) have a keto group, usually at C2.

C

C OHH

C HHO

C OHH

C OHH

CH2OH

D-glucose

OH

C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

D-fructose

Trioses a three-carbon simple sugar. formed during the metabolic breakdown of hexoses in muscle metabolism.

Tetroses are four-carbon sugars. One tetrose, erythrose, is a intermediate in the hexose monophosphate shunt for the oxidation of glucose.

Pentoses are five-carbon sugar molecules.

Ribose and deoxyriboseRibose forms part of ribonucleic acid (RNA), and deoxyribose forms part of deoxyribonucleic acid (DNA).

D-ribulosean intermediary in the pentose phosphate shunt

D-lyxosewhich is found in heart muscle

D-xylose and D-arabinose which are components of glycoproteins.

The Fisher projection representations for the pentoses, are called open-chain structures. However, the predominant form for pentoses is a ring structures. Recall that aldehydes react with alcohols to form hemiacetals. In the case of ribose, the aldehyde can react with the alcohol at carbon number 4 to form two different compounds..

The Fischer and Haworth projection are related as follows:

1) The groups on the right side of the Fisher projection are written below the plane in the Haworth projection. Those on the left side are written above the plane.

2) One exception to rule 1 occurs at carbon 4 in pentoses and at carbon 5 in hexoses because of the nature of the creation occurring there. At these carbons, rule 1 is reversed.

3.) At carbon 1 form is indicated by the –OH being written below the plane; the form has the –OH above the plane.

4.) In both projections the CH2OH group, which has no chiral carbon, is written as a unit.

Haworth Projections

◦aldopentoses also form cyclic hemiacetals

◦the most prevalent forms of D-ribose and other pentoses in the biological world are furanoses

OH ()

H

HOH OH

H HOHOCH2

H

OH ()

HOH H

H HOHOCH2

-D-Ribofuranose(-D-Ribose)

-2-Deoxy-D-ribofuranose(-2-Deoxy-D-ribose)

Hemiacetal & hemiketal formation

An aldehyde can react with an alcohol to form a hemiacetal.

A ketone can react with an alcohol to form a hemiketal.

O C

H

R

OH

O C

R

R'

OHC

R

R'

O

aldehyde alcohol hemiacetal

ketone alcohol hemiketal

C

H

R

O R'R' OH

"R OH "R

+

+

HEXOSES

The six-carbon sugars.

The most common of all the carbohyrates.

The most important as far as the human body is concerned are glucose, galactose and fructose.

The have the same molecular formula but have diff. structural formulas; they are isomers.

GlucoseIs an aldohexose and can be represented structurally as

H O

OH

H

OHH

OH

CH2OH

H

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose -D-glucose

23

4

5

6

1 1

6

5

4

3 2

H

CHO

C OH

C HHO

C OHH

C OHH

CH2OH

1

5

2

3

4

6

D-glucose (linear form)

is known commonly as dextrose, or grape sugar.It is a white crystalline solid that is

soluble in water and insoluble in most organic liquids.

It is found, along with fructose, in many fruit juices.

It can be prepared by the hydrolysis of sucrose, a disaccharide, or by the hydrolysis of starch, polysaccharides.

It is normally found in the bloodstream and in the tissue fluids.

“Metabolism of Carbohydrates,” glucose requires no digestion and can be given intravenously to patients who are unable to take food by mouth.

is found in the urine of patients suffering from diabetes mellitus and is an indication of this disease

GALACTOSE An isomer of glucose, is also an aldohexose.Glucose and galactose differ from each other only in the configuration of the H and OH about a single carbon atom.Two sugars that differ only in the configuration about a single carbon atom are called epimers.D-Galactose is converted to D-glucose in the liver by a specific enzyme called an epimerse.is present in some glycoproteins and glycolipids.

Galactosemiasevere inherited disease, results in the inability of infants to metabolize galactose because of a difiency of either the enzyme galactose 1-phosphate uridyl transferase or the enzyme galactokinase. The galactose concentration increases in the blood the urine (galactosuria).

D-galactose

a-D-galactose

FRUCTOSEcan be represented as a straight-chain or as a ring compound.The ring structure is predominant. Note that the ring structure represents a hemiketal.often called levulose, or fruit sugar and it occurs naturally in fruit juices and honey. It can be prepared by the hydrolysis of sucrose, a disaccharide, and also by the hydrolysis of insulin, polysaccharides found in Jerusalem artichokes.the most soluble and also the sweetest of all sugars, being 75 percent sweeter than glucose.   

 Fructosemia,fructose intolerance, is an inherited disease due to a deficiency of a deficiency of the enzyme fructose 1-phosphate aldolase. An infant suffering from this disease experiences hypoglycemia, vomiting, and severe malnutrition. Such a condition is treated by placing the infant on a low-fructose diet.  

a-D-fructose

Reactions of the Hexoses

Hexoses, which are either aldoses or ketoses, show reducing properties. This reducing property is the basis of the test sugar in the urine and in the blood.

When a reducing agent is treated with an oxidizing agent such as Cu2+ complex, a red-orange precipitate of copper(I) oxide(Cu2O) is formed

aldehyde + Cu2+ acid + Cu2O + water

deep blue red-orange solution precipitate

heat

NaOH

In this reaction the aldehyde is oxidized to the corresponding acid.

When glucose is treated with Cu2+ complex ion and the mixture is heated, the reaction is as follows:

heat +

Cu2O + NaOH

Cu2+

D-glucose

Copper(II) Copper(I)Complex ion oxide(deep-blue color) (red-orange precipitate) D-gluconic

acid

Benedict's Reagent

• Benedict's reagent is a solution of the citrate complex of CuSO4 in water. It is used as a test for "reducing sugars." Cu2+ is a weak oxidizing agent.

• A reducing sugar is one which has an aldehyde function, or is in equilibrium with one that does.

• A positive test is the formation of a red precipitate of Cu2O.

+ 2Cu2+RCH

O

5HO–+ + Cu2ORCO–

O

3H2O+

Examples of Reducing Sugars

• Aldoses: because they have an aldehyde function in their open-chain form.

• Ketoses: because enolization establishes an equilibrium with an aldose.

CH2OH

C O

R

CHOH

C OH

R

CH

CHOH

R

O

oxidized by Cu2+

Oxidation

If the aldehyde end of the molecule is oxidized, the product is named and –onic acid. When the aldehyde end of glucose is oxidized, the product is called gluconic acid.If the alcohol at the end opposite the aldehyde is oxidized, the product is called a –uronic acid. The ooxidation of the alcohol end of glucose yields glucuronic acid.If both ends of the glucose molecule are oxidized at the same time, the product is called saccaharic acid.

Gluconic acid Glucuronic acid Saccharic acid

REDUCTION

When glucose is reduced, sorbitol is formed. Sorbitol accumulation in the eyes is a major factor in the formation of cataracts due to diabetes.Reduction of galactose yields dulcitol.Reduction of fructose yields a mixture of mannitol and sorbitol.

MannitolUsed in the treatment of malignant brain tumors.a

FERMENTATION

Glucose ferments in the presence of yeast, forming ethyl alcohol and carbon dioxide. This reaction will not readily occur in the absence of yeast. Yeast contains certain enzymes that catalyze this particular reaction.

+

Glucose ethyl alcohol

enzymes

Fructose will also ferment; galactose will not readily ferment. Pentoses do not ferement in the presence of yeast

Fermentation of Phosphate Esters

Phosphate esters such as D-glyceraldehyde 3-phosphate and dihydroxyacetone phosphate esters involved in glycolysis.

Amino sugars

D-glucosamine

An amino group in place of an –OH group. These amino sugars have been found in nature.