MONOSACCHARIDES

DR AMINA TARIQBIOCHEMISTRY

# Carbons Category Name Relevant examples

3 Triose Glyceraldehyde, Dihydroxyacetone

4 Tetrose Erythrose

5 Pentose Ribose, Ribulose, Xylulose

6 Hexose Glucose, Galactose, Mannose, Fructose

7 Heptose Sedoheptulose

9 Nonose Neuraminic acid, also called sialic acid

Three common sugars share the same molecular formula: C6H12O6. Because of their six carbon atoms, each is a hexose.

They are: glucose, "blood sugar", the immediate

source of energy for cellular respiration galactose, a sugar in milk (and yogurt),

and fructose, a sugar found in honey.

The simplest monosaccharides of biological interest have n=3 (trioses):

Glyceraldehyde and Dihydroxyacetone

GLYCERALDEHYDE It is the smallest sugar It has got three carbons Its carbon 2 is asymmetric carbon All monosaccharides having more than 3

carbons will have two or more asymmetric carbon atoms.

For D and L designation all monosaccharides are compared with glyceraldehyde.

Thus it is called a reference sugar.

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

L-glyceraldehydeD-glyceraldehyde

L-glyceraldehydeD-glyceraldehyde

For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group.

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

DIHYDROXYACETONE It is the only exception of a

monosaccharide that does not possess any asymmetric carbon.

MONOSACCHARIDES Monosaccharides are carbohydrates that

cannot be hydrolyzed to obtain smaller molecules of carbohydrate.

White crystalline solids , very soluble in water, have sweet taste.

Glucose Glucose is by far the most common

carbohydrate and classified as a monosaccharide,

an aldose, a hexose, and is a reducing sugar. Principal MS utilized for energy

purposes.

It is also known as dextrose, because it is dextrorotatory.

Glucose units combine and give rise to:GlycogenStarchCellulose

Glucose is also called blood sugar as it circulates in the blood at a concentration of 65-110 mg/mL of blood.

Normally trace amounts in urine

Increased amounts in diabetes mellitus.

Glucose is initially synthesized by chlorophyll in plants using carbon dioxide from the air and sunlight as an energy source. Glucose is further converted to starch for storage.

an aqueous sugar solution contains only 0.02% of the glucose in the chain form, the majority of the structure is in the cyclic chair form.

Fructose Fructose is classified as a: monosaccharide, the most important ketose sugar, a hexose, and is a reducing sugar. Fructose, along with glucose are the

monosaccharides found in disaccharide, sucrose.

Sweetest of all sugars.

Fruit juices. Honey. Hydrolysis of cane sugar and of inulin. Can be changed to glucose in the liver and

so used in the body. Main source of energy for the

spermatozoa Levorotatory (in sucrose

dextrorotatory) Hereditary fructose intolerance

Galactose Galactose is classified as a: monosaccharide, an aldose, a hexose, and is a reducing sugar.

Hydrolysis of lactose Galactose is more commonly found in the

disaccharide, lactose or milk sugar. It is found as the monosaccharide in peas. Constituent of glycolipids and

glycoproteins. C-4 epimer of glucose Galactose is part of nerve and brain

biochemicals, so milk is essential to infants.

Pentoses Xylulose- uronic acid pathway Arabinose and xylose- glycoproteins Ribose - RNA Deoxyribose - DNA

` All are classified as: monosaccharides, aldoses, Pentoses, and are reducing sugars.

Ribose Ribose and its related compound,

deoxyribose, are the building blocks of the backbone chains in nucleic acids, better known as DNA and RNA.

Ribose is used in RNA and deoxyribose is used in DNA.

The presence or absence of the -OH group on carbon (#2) is an important distinction between ribose and deoxyribose.

Ribose has an alcohol at carbon # 2, while deoxyribose does not have the alcohol group.

Deoxy hexoses also present in milk and blood group substances.

Disaccharides Three common disaccharides: Sucrose — common table sugar = glucose

+ fructose Lactose — major sugar in milk = glucose

+ galactose Maltose — product of starch digestion =

glucose + glucose

The resulting linkage between the sugars is called a glycosidic bond. The molecular formula of each of these disaccharides isC12H22O11 = 2 C6H12O6 − H2O

Sucrose

Sucrose is made from glucose and fructose units

Sucrose or table sugar Cane, beet sugar, Pineapple Has no free anomeric carbon Is a non-reducing sugar

Dextrorotatory Invert sugar No osazone crystals In sucrase deficiency, malabsorption leads

to diarrhea and flatulence

Maltose Glucose+ Glucose Digestion by amylase or hydrolysis of

starch. Reducing sugar Baby and invalid foods

Lactose Glucose + Galactose Milk sugar May occur in urine during pregnancy In lactase deficiency, malabsorption leads

to diarrhea and flatulence Certain bacteria can ferment lactose to

lactic acid - souring of milk. ( lactobacillus)

LACTULOSE - galactose and fructose Osmolar laxative Relieves Constipation