Pharm D 1st Prof Biochemistry Notes, Lecture notes of Medical Biochemistry

Download Pharm D 1st Prof Biochemistry Notes and more Lecture notes Medical Biochemistry in PDF only on Docsity!

Chemistry of

Carbohydrates

Dr. Bilal Ahmad Ghalloo

Carbohydrates

They are also called “saccharides”. In Greek, saccharon means sugar.

 Carbohydrates may be defined chemically as aldehyde or ketone derivatives

of polyhydroxy (more than one hydroxy group) alcohols or as compounds that yield these derivatives on hydrolysis.  (^) The carbohydrates are widely distributed both in animal and plant tissues.  (^) Chemically, they contain the elements carbon, hydrogen and oxygen.  (^) The empirical formula of many simple carbohydrates is [CH 2 O] n.

Classification of Carbohydrates

Carbohydrates are classified into four groups:

1. Monosaccharides
2. Disaccharides
3. Oligosaccharides
4. Polysaccharides.

Monosaccharides

 (^) Molecules having only one actual or potential sugar group are called monosaccharides

(Greek, mono = one; saccharide = sugar). They cannot be further hydrolysed into smaller

units.

 Monosaccharides are also called simple sugars. The term sugar is applied to

carbohydrates that are soluble in water and sweet to taste. They consist of a single polyhydroxy aldehyde or ketone unit, and thus cannot be hydrolyzed into a simpler form.  (^) Sugars having aldehyde group are called aldoses and sugars with keto group are

ketoses.

 (^) Depending on the number of carbon atoms, the monosaccharides are named as triose (C3), tetrose (C4), pentose (C5), hexose (C6), heptose (C7) and so on.

Oligosaccharides

 (Greek: oligo = few)  Oligosaccharides consist of a short chain of monosaccharide units (2 to 10 units), joined together by a characteristic bond called glycosidic bond which, on hydrolysis, gives two to ten molecules of simple sugar (monosaccharide) units.  Oligosaccharides are subdivided into different groups based on the number of monosaccharide units present

Homopolysaccharides (Homoglycans)  (^) When a polysaccharide is made up of several units of one and the same type of monosaccharide unit, it is called homopolysaccharide.  (^) The most common homoglycans are:  (^) – Starch  (^) – Dextrins  (^) – Glycogen  (^) - Inulin  (^) – Cellulose.  (^) Some homopolysaccharides serve as a storage form of monosaccharides used as fuel, e.g. starch and glycogen, while others serve as structural elements in plants, e.g. cellulose.

Heteropolysaccharides (Heteroglycans)  (^) They contain two or more different types of monosaccharide units or their derivatives.  (^) Heteropolysaccharide present in human beings is glycosaminoglycans (mucopolysaccharides), e.g.

• Heparin
• Chondritin sulfate
• Hyaluronic acid
• Dermatan sulfate
• Keratan sulfate
• Blood group polysaccharides.

 (^) Monosaccharide in solution is mainly present in ring form.  (^) In solution, aldehyde (CHO) or ketone (C=O) group of monosaccharide react with a hydroxy (OH) group of the same molecule forming a bond hemiacetal or hemiketal respectively.  (^) The aldehyde group of glucose at C-1 reacts with alcohol (OH) group of C-5 or C-4 to form either six membered ring called glucopyranose or five membered ring called glucofuranose , respectively.  (^) However, in case of glucose, the six membered glucopyranose is much more stable than the glucofuranose ring. In the case of fructose, the more stable form is fructofuranose.

Ketose-Aldose isomerism

 (^) Glucose and fructose are isomers of each other having the same chemical (molecular) formula C 6 H 12 O 6 , but they differ in structural formula with respect to their functional groups.

 There is a keto group in position two of fructose and an aldehyde group in

position one of glucose. This type of isomerism is known as ketose-aldose

isomerism.