MONOSACCHARIDES DR AMINA TARIQ BIOCHEMISTRY

# CarbonsCategory NameRelevant examples 3Triose Glyceraldehyde, Dihydroxyacetone 4TetroseErythrose 5Pentose Ribose, Ribulose, Xylulose 6Hexose Glucose, Galactose, Mannose, Fructose 7HeptoseSedoheptulose 9Nonose Neuraminic acid, also called sialic acid

 Three common sugars share the same molecular formula: C 6 H 12 O 6. 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.

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

 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: Glycogen Starch Cellulose

 Glucose is also called blood sugar as it circulates in the blood at a concentration of 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 isC 12 H 22 O 11 = 2 C 6 H 12 O 6 − H 2 O

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