Disaccharides

Disaccharides

These are formed by condensation of 2 molecules   of monosaccharides bound together by glycosidic bond .Its genral formula is Cn (H2O) n-1

A. The most important disaccharides are:

1. Maltose = a - glucose + a - glucose (a 1-4 glycosidic bond)
2. Isomaltose =a- glucose + a - glucose (a 1-6 glycosidic bond)
3. lactose = β -glucose + β –galactose(β1-4 glycosidic bond)
4. Soucrose ==a- glucose + β – fructose(a 1- β2 glycosidic bond)
5. cellobiose = β  - glucose + β  - glucose (β 1-4 glycosidic bond)
6. Trehalose = a - glucose + a - glucose (a 1-1 glycosidic bond)

B. Naming (Nomenclature) of glycosidic bonds :

Glycosidic bonds between sugars are  named according to :

1. The number of the connected carbons
2. The position of the carbon of the sugar .If it is in a position the linkage is an a - bond .If it in the β position , the linkage is β – bond  . Example : Lactose consists of β – glucopyranose and β – glucopyranose .The bond is between carbon 1 of β – glucopyranose and carbon 4 of glucopyranose . The bond is therefore β 1-4 galactoseodic linkage .

Maltose : 

Also called malt sugar :

• Structure :it is formed of 2 molecules of  a-D glucopyranose linked together by a1-4 glycosidic bond
• Sources:

Malt :  

Maltose is produced during digestion of starch by amylase enzyme .

• It is Maltose contains free carbonyl (aldehyde) group ,so having the properties :
• It is a reducing agent(can reduce Benedict's )
• It can be present in a and b forms
• It can form characteristic osazone crystals

Isomaltose : 

• Structure :It is similar to maltose,being formed of 2 molecules of a - D glucopyranose but linked together a 1-6 glycosidic bond.
• Sources : isomaltose is produced during digestion of starch and glycogen by amaltase enzyme .
• proprties :The same as maltose

Lactose :

• Structure :It is formed of 2 molecules of b- D – glucopyranose   galactopyranose linked byb 1-4 glycosidic bond
• Sources : It is sugar present in milk ..It may appear in urine in late pregnancy and during lactation 
• proprties : Lactose contains free carbonyl group . 

 so having the following properties:

1. It is reducing sugar (can reduce Benedict's reagent)
2. It can be present in a  and   b   forms
3. It can form characteristic osazone crystals
4. Lactose is digested by intestinal enzyme to galactose and glucose  .

Sucrose :

• Structure :It is formed of molecules of a- D glucopyranose – b- D – fructofuranose linked together by a 1 ­­b2 glycosidic bond
• Sources: cane and beet sugar .It is also present pineapple and carrot
• proprties :sucrose  contains no free carbonyl group (because both the anomeric carbons:carbon 1 of a-glucose and carbon 2 of b- fructose are involved in glycosidic bond ).

 so fructose has the following proprties :

1. It is not a reducing sugar (cannot reduce Benedict's reagent)
2. It cannot be present in a  and   b   forms
3. It cannot form osazone crystals
4. Sucrose is dextrorotatory .On hydrolysis by invertase (sucrase) enzyme, it give mixture of equal number of glucose and fructose molecules .This mixture is called invert sugar and it is levorotatory

Invert Sugar :

• Structure :It is a sugar that contains equal number of both glucose and fructose molecules (unbound)
• Sources: Bee honey , By hydrolysis of sucrose by sucrase (invertase) enzyme
• proprties : Invert sugar contains free carbonyl so it has the same proprties  as lactose and maltose (it is reducing can be presented in a  and   b   forms, and can form osazone)

Trehalose :

• Structure :It is non- reducing sugar ,formed of 2 units of a- D glucopyranose linked together by a1 ® 1glycosidic bond
• Sources:  It is present in fungi and yeast .It is major sugar of fungi and insect hemolymph
• Importance : Trehalose can be used a sweetener and preservative for foods .It can be also used in organ and tissue preservation solutions that provide improved viability of an organ such as a heart or lung.