Biochemistry

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Carbohydrates


General molecular formula Cn(H2O)n
Appeared to be hydrates of carbon. not all carbohydrates have this empirical formula: deoxysugars, aminosugars 





Carbohydrate - polyhydroxy aldehyde, ketones.

General characteristics


Most carbohydrates are found naturally in bound form rather than as simple sugars  

  

Polysaccharides (starch, cellulose, inulin, gums) Glycoproteins and proteoglycans (hormones, blood group substances, antibodies) Glycolipids (cerebrosides, gangliosides) Glycosides Nucleic acids

Classification of carbohydrates


Monosaccharides
Trioses, tetroses, pentoses, hexoses



Disaccharides
Maltose, sucrose, lactose



Oligosaccharides


3 to 9



Polysaccharides or glycans
 

Homopolysaccharides Heteropolysaccharides

D-Glucose in Nature
 

The most abundant carbohydrate is D-glucose. Cells of organisms oxidize glucose for energy:



In animals excess glucose is converted to a polymer called glycogen.



   

Disaccharides On hydrolysis give two molecules of monosaccharides E.g Sucrose (Cane sugar) Lactose (milk sugar) Maltose (malt sugar)

Polysaccharides



Starch, cellulose, glycogen On the hydrolysis of each of them, they yields large number of monosaccharides.

Monosaccharides



 



also known as simple sugars classified by 1. the number of carbons and 2. whether aldoses or ketoses most (99%) are straight chain compounds D-glyceraldehyde is the simplest of the aldoses (aldotriose) all other sugars have the ending ose (glucose, galactose, ribose, lactose, etc…)

Monosaccharides

• General formula (CH2O)n

• • • • •

Triose: n = 3 (e.g., glyceraldehyde) Tetrose: n = 4 Pentose: n = 5 (e.g., ribose) Hexose: n = 6 (e.g., glucose) Heptose: n = 7

CONCEPTS OF ISOMERS

Two or more different compounds which contain the same number and types of atoms and the same molecular weights.



Stereoisomers: Enantiomers and Diastereomers Stereoisomers Are not constitutional isomers since they have the constituent atoms connected in the same sequence! They only differ in the arrangement of their atoms in space! Stereoisomers can be subdivided into two categories: Enantiomers: Are stereoisomers whose molecules are mirror images of each other. (These are like our hands). The molecules of enantiomers are not superimposeable







Diastereomers: Are stereoisomers that are not mirror images of each other as indicated in (Fig.).

Monosaccharides

Represented by Fischer projections
CHO
H H C C O OH

H

C
CH2OH

OH

CH2OH

D-Glyceraldehyde

Emil Fischer Nobel Prize 1902

D- and L- Notation


Prior to determination of absolute configurations, the 19th century chemists assigned arbitrary designations to structures: HC O HC O

H

OH CH2OH

HO

H CH2OH

(R)-(+)-glyceraldehyde
D-glyceraldehyde

(S)-(–)-glyceraldehyde
L-glyceraldehyde

D- and L- Notation


If the OH group attached to the bottom-most chirality center is on the right, it is a D-sugar:



The D- or L- together with the common name of the monosaccharide completely describes the structure, since the relative configurations at all chirality centers is implicit in the common name.

Aldotetroses


Aldotetroses have two chirality centers hence 22 = 4 stereoisomers:

Enantiomers and epimers
H H C H H C C O OH OH OH OH H C C C O H H HO HO H H these two aldotetroses are enantiomers. They are stereoisomers that are mirror images of each other C C C C C O H H OH OH HO HO HO H H C C C C C O H H H OH

CH2OH

CH2OH CH2OH CH2OH

these two aldohexoses are C-4 epimers. they differ only in the position of the hydroxyl group on one asymmetric carbon (carbon 4)

Epimers


A pair of diastereomers that differ only in the configuration about a single carbon atom are said to be epimers. Epimers H H HO HO H O OH H H OH CH2OH H H HO H H O OH H OH OH CH2OH...
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