Biochemistry

Carbohydrates
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General molecular formula Cn(H2O)n
Appeared to be hydrates of carbon. not all carbohydrates have this empirical formula: deoxysugars, aminosugars
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Carbohydrate - polyhydroxy aldehyde, ketones.

General characteristics
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Most carbohydrates are found naturally in bound form rather than as simple sugars
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Polysaccharides (starch, cellulose, inulin, gums) Glycoproteins and proteoglycans (hormones, blood group substances, antibodies) Glycolipids (cerebrosides, gangliosides) Glycosides Nucleic acids

Classification of carbohydrates
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Monosaccharides
Trioses, tetroses, pentoses, hexoses

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Disaccharides
Maltose, sucrose, lactose

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Oligosaccharides
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3 to 9

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Polysaccharides or glycans
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Homopolysaccharides Heteropolysaccharides

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

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

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

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

Monosaccharides
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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.

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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

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

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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
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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
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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 Epimers H HO HO H H O H H OH OH CH2OH

D(+)-Galactose

D(+)-Glucose Diastereomers

D(+)-Mannose

POLARIMETER

Dextrorotatory -plane polarized light rotated to clockwise (or to the right)
Levoratatory - plane polarized light rotated to counterclockwise.

POLARIMETRY
Measurement of optical activity in chiral or asymmetric molecules using plane polarized light Molecules may be chiral because of certain atoms or because of chiral axes or chiral planes

Measurement uses an instrument called a polarimeter

polarimetry
Magnitude of rotation depends upon:
1. the nature of the compound 2. the length of the tube (cell or sample container) usually

expressed in decimeters (dm)
3. the wavelength of the light source employed; usually either sodium D line at 589.3 nm or mercury vapor lamp

at 546.1 nm
4. temperature of sample 5. concentration of analyte in grams per 100 ml

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D

T

 observed x 100 = lxc

D = Na D line T = temperature oC  obs : observed rotation in degree (specify solvent) l = length of tube in decimeter c = concentration in grams/100ml [] = specific rotation

Specific rotation of various carbohydrates at 20oC
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D-glucose D-fructose D-galactose L-arabinose D-mannose D-arabinose D-xylose Lactose Sucrose Maltose+ Invert sugar Dextrin

+52.7 -92.4 +80.2 +104.5 +14.2 -105.0 +18.8 +55.4 +66.5 +130.4 -19.8 +195

Glucose (dextrose)
Most common monosaccharide. Commercially from starch. Mutarotation ---The optical changes of glucose in water solution to constant value

20D = +520
 - D - glucose -> 20D = 113 D - glucose 20D = 52 4) -DGlucopyranose
CH2 OH O OH HO OH O OH OH OH CH2 OH O

Cellobiose 4-0-b-D-Glucopyranosyl (1->4)-b-D-Glucopyranose
CH2 OH O OH HO OH OH O OH CH2 OH O OH

Sucralfate (Carafate)

Polysaccharides
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homoglycans (starch, cellulose, glycogen, inulin) heteroglycans (gums, mucopolysaccharides) characteristics:
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polymers (MW from 200,000) White and amorphous products not sweet not reducing; do not give the typical aldose or ketose reactions) form colloidal solutions or suspensions

Starch
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most common storage polysaccharide in plants composed of 10 – 30% -amylose and 70-90% amylopectin depending on the source the chains are of varying length, having molecular weights from several thousands to half a million

STARCH

The reserve carbohydrate of plants. Occurs as granules in the cell. Made of amylose and amylopectin. Amylose --- ploymer of -D- Glucose (1->4) linkagestraight-chain.
CH2 OH O OH OH OH O OH OH O OH CH2 OH O OH CH2 OH O OH CH2 OH O

O
OH

Amylose and amylopectin are the 2 forms of starch. Amylopectin is a highly branched structure, with branches occurring every 12 to 30 residues

suspensions of amylose in water adopt a helical conformation

iodine (I2) can insert in the middle of the amylose helix to give a blue color that is characteristic and diagnostic for starch

(in starch)

(in cellulose)

Cellulose
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Polymer of b-D-glucose attached by b(1,4) linkages Yields glucose upon complete hydrolysis Partial hydrolysis yields cellobiose Most abundant of all carbohydrates
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Cotton flax: 97-99% cellulose Wood: ~ 50% cellulose

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Gives no color with iodine Held together with lignin in woody plant tissues

POLYSACCHARIDE

Cellulose --- polymer of b-D-Glucose (1, 4) linkage. Repeating cellobiose moiety.

CH2 OH O OH OH OH O

CH2 OH O OH O

CH2 OH O OH

CH2 OH O

O

OH

OH

OH

OH

n

Structure of cellulose

Glycogen
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also known as animal starch stored in muscle and liver present in cells as granules (high MW) contains both (1,4) links and (1,6) branches at every 8 to 12 glucose unit complete hydrolysis yields glucose glycogen and iodine gives a red-violet color hydrolyzed by both  and b-amylases and by glycogen phosphorylase

GLYCOGEN
Animal starch.  - (1 -> 4) linkage and  - (1 -> 6) linkage 12 : 1

RELATIVE SWEETNESS OF DIFFERENT SUGARS

Sucrose Glucose Fructose Lactose Invert Sugar Maltose Galactose

100 74 174 16 126 32 32

CARBOHYDRATE DETERMINATION

1.

Monosaccharides and Oligosaccharides A. Enzymatic Method

1.
2. B.

Glucose oxidase
Hexokinase

Chromatography Method

1.
2. 3. 2.

Paper or thin layer chromatography
Gas chromatography Liquid column chromatography

Polysaccharides

Glucose Oxidase System
Glucose Oxidase

D-Glucose + O2
Peroxidase

Gluconic Acid + H2O2

H2O2+ 0 - Dianisidine
(Colorless)

2 H2O + Oxidized 0-Dianisidine
(Brown)

H3 CO H2 N

OCH3 NH2

H3 CO HN

OCH3 NH

Synthetic Sweeteners