carbohdrates 2013
TRANSCRIPT
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Carbohydrates
Dr Imran Siddiqui PhD, MPhil, MBBS
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Objectives
• Define Carbohydrates• List the biomedical importance of
carbohydrates• Write down the Empirical formula of
Carbohydrates• Classify with examples various types of
carbohydrates• Explain isomerism in carbohydrates
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Significance of Carbohydrates• Most abundant organic compound in nature• A major source of energy from our diet• Storage form of energy in plants and animals• Cell membrane component• Structural component of many organisms• Composed of the elements C, H and O and
empiric formula is C(H2O)n
• Also called hydrates of carbon OR saccharides, which means “sugars”
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Definition
Carbohydrates are polyhydric alcohols (-OH) with aldehyde or ketone.
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Figure 7.1. Examples of monosaccharides found in humans, classified according to the number of carbons they contain.
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Isomers and epimers• Compounds that have same chemical formula but have
different structures = ISOMERS• e.g., fructose, glucose, mannose, & galactose are all isomers
of each other, having same formula C6H12O6• If 2 monosacch differ in configuration around only one
specific C atom (with exception of carbonyl C), they are defined as epimers of each other (of course they are also isomers) e.g., – glucose & galactose are C-4 epimers, their structures
differ only in the position of –OH group at C 4. – Glucose & mannose are C-2 epimers
Note: carbons in sugars are numbered beginning at end containing the carbonyl C i.e., aldehyde or keto group
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Figure 7.4C-2 and C-4 epimers and an isomer of glucose.
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B. Enantiomers- A special type of isomerism is found in the pairs of structures
that are mirror images of each other. These mirror images = enantiomers, & the 2 members of the pair are designated as D- & L-sugar. Vast majority of sugars in humans are D-sugars
Figure 7.5Enantiomers (mirror images) of glucose.
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C. Cyclization of monosaccharides
- Less than 1% of each of the monosacch with 5 or more C’s exist in the open-chain (acyclic) form. Rather, they are predominantly found in ring form, in which aldehyde (or ketone) group has reacted with an alcohol group on the same sugar
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Two Cyclic Forms of Glucose
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Figure 7.6. The interconversion of the α and β anomeric forms of glucose (mutarotation).
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• Anomers are isomers which differ in placement of hydroxyl on C1
• The –OH is drawn down for the -anomer, and up for the -anomer
-D-Glucose -D-Glucose
OCH2OH
OHOH
OH
OH
OCH2OH
OHOH
OH
OH
and Anomers for D-Glucose
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Types of Carbohydrates
Monosaccharides are the simplest carbohydrates Empirical formula = (CH2O)n
Disaccharides consist of two monosaccharides Polysaccharides contain many monosaccharides
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MonosaccharideUnbranched chain of 3-8 C atomsOne is carbonyl; others attached to -OH
Aldoses contain an aldehyde group (carbon 1)
Ketosescontain a ketone group (carbon 2)
CHO
HO H
CH2OH
CHO
HO H
OHH
CH2OH
CH2OH
O
HHO
OHH
OH H
OH
CH2OH
Classification of Monosaccharides
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Structural representation of sugars
• Fischer projection: straight chain representation
• Haworth projection: simple ring in perspective• Conformational representation: chair and boat
configurations
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Rules for drawing Haworth projections
• draw either a six or 5-membered ring including oxygen as one atom
• most aldohexoses are six-membered• aldotetroses, aldopentoses, ketohexoses are 5-
membered
O O
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Rules for drawing Haworth projections
• for D-sugars the last alcohal group (farthest from the carbonyl) is drawn up.
• For L-sugars, it is drawn down• for D-sugars, the OH group at the anomeric
position is drawn down for a and up for b. • For L-sugars a is up and b is down
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Chair and boat conformations of a pyranose sugar
2 possible chair conformationsof b-D-glucose
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Oxidation reactions• Aldoses may be oxidized to 3 types of acids– Aldonic acids: aldehyde group is converted to a carboxyl
group ( glucose – gluconic acid)
– Uronic acids: aldehyde is left intact and primary alcohol at the other end is oxidized to COOH• Glucose --- glucuronic acid• Galactose --- galacturonic acid
– Saccharic acids (glycaric acids) – oxidation at both ends of monosaccharide)• Glucose ---- saccharic acid• Galactose --- mucic acid• Mannose --- mannaric acid
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Special monosaccharides: deoxy sugars
• These are monosaccharides which lack one or more hydroxyl groups on the molecule
• deoxy sugar is 2’-deoxy ribose which is the sugar found in DNA
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Special monosaccharides: amino sugarsConstituents of mucopolysaccharides
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D-Glucose
• Most common hexose
• Found in fruits, corn syrup, and honey
• An aldohexose with the
formula C6H12O6
• Known as blood sugar in the body
• Building block for many disaccharides and polysaccharides
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D-Fructose
• Ketohexose C6H12O6
• Differ from glucose at C1 and C2 (location of carbonyl)
• The sweetest carbohydrate (2x sucrose)
• Found in fruit juices and honey
• Formed from hydrolysis of sucrose
• Converts to glucose in the body
D-Fructose
CH2OH
C
C
O
C
H
OHH
HO
C
CH2OH
OHH
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D-Galactose
• Aldohexose
• Differ from D-glucose at C4
• Not found in the free form in
nature
• Obtained from lactose, a
disaccharide (milk products)
• Important in cellular membranes
in CNS
CHO
C
C
OH
C
H
HHO
HO
C
CH2OH
OHH
H
D-Galactose
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Memorize!
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Memorize!
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•Most common disaccharides are•Sucrose, lactose, and maltose
•Maltose hydrolyzes to 2 molecules of D-glucose
•Lactose hydrolyzes to a molecule of glucose and a molecule of galactose
•Sucrose hydrolyzes to a molecules of glucose and a molecule of fructose
Disaccharides
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•galactose-b-(1,4)-fructose•a semi-synthetic disaccharide (not naturally
occurring)•not absorbed in the GI tract•used either as a laxative (Chronulac) or in the
management of portal systemic encephalopathy (Cephulac)
•metabolized in distal ileum and colon by bacteria to lactic acid, formic acid and acetic acid (remove
ammonia)
Lactulose
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Less common glucose disaccharides
Isomaltose (alpha 1,6)
Gentiobiose (beta 1,6)
Laminaribiose (beta 1,3)
Cellobiose (beta 1,4)
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Cellobiose
Cellobiose consists of 2 molecules of glucose linked by a beta-1,4 glycosidic bondIt is usually obtained by the partial hydrolysis of cellulose
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Trehalose is a disaccharide that occurs naturally in insects, plants, fungi, and bacteria. The major dietary source is mushrooms. Trehalose is used in bakery goods, beverages,
confectionery, fruit jam, breakfast cereals, rice, and noodles as a texturizer, stabilizer with a low sweetening intensity
O
CH2OH
H
O O
H
H
H
HO
H
OH
OH
H
OH H
H
OH
OH
H
HOH2C
TREHALOSE
Trehalose
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Sucralose (Splenda)
About 600 times more sweet than sucrose
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Figure 7.11Abnormal lactose metabolism.
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Oligosaccharides
• Trisaccharide: raffinose (glucose, galactose and fructose)
• Tetrasaccharide: stachyose (2 galactoses, glucose and fructose)
• Pentasaccharide: verbascose (3 galactoses, glucose and fructose)
• Hexasaccharide: ajugose (4 galactoses, glucose and fructose)
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Honey also contains glucose and fructose along withsome volatile oils
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Polysaccharides or glycansTypes• homoglycans / homopolysaccharides (starch,
cellulose, glycogen, inulin)• heteroglycans / heteropolysaccharides (gums,
mucopolysaccharides)
Characteristics• Polymers (MW from 200,000)• White and amorphous products (glossy)• not sweet• form colloidal solutions or suspensions
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Tertiary structure - sterical/geometrical conformations
• Rule-of-thumb: Overall shape of the chain is determined by geometrical relationship within each monosaccharide unit
(b 14) - zig-zag - ribbon like b(1 3) & a(14) - U-turn - hollow helix (b 1 2) - twisted - crumpled (16) - no ordered conformation
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Starch
• most common storage polysaccharide in plants
• composed of 10 – 30% -a 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
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Starch
• Main sources of starch are rice, corn, wheat, potatoes
• A storage polysaccharide• Starch is used as an excipient, a binder in
medications to aid the formation of tablets.• Industrially it has many applications such as
in adhesives, paper making, biofuel, textiles
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Cellulose• Polymer of b-D-glucose attached by b(1-->4) linkages• Only digested and utilized by ruminants (cows, deers,
giraffes, camels)• A structural polysaccharide• Yields glucose upon complete hydrolysis• Partial hydrolysis yields cellobiose• Most abundant of all carbohydrates
• Cotton flax: 97-99% cellulose• Wood: ~ 50% cellulose
• Gives no color with iodine
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Products obtained from cellulose
• Microcrystalline cellulose : used as binder-disintegrant in tablets
• Methylcellulose: suspending agent and laxative• Oxidized cellulose: hemostat• Sodium carboxymethyl cellulose: laxative• Cellulose acetate: rayon; photographic film; plastics• Cellulose acetate phthalate: enteric coating• Nitrocellulose: explosives;
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Glycogen• also known as animal starch• stored in muscle and liver (mostly)• present in cells as granules (high MW)• contains both a(1,4) links and a(1,6) branches at
every 8 to 12 glucose unit (more frequent than in starch)
• complete hydrolysis yields glucose• hydrolyzed by both a and b-amylases and by
glycogen phosphorylase
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Inulin• b-(1,2) linked fructofuranoses• linear only; no branching• lower molecular weight than starch• colors yellow with iodine• hydrolysis yields fructose• sources include onions,and garlic, dandelions and
jerusalem artichokes• used as diagnostic agent for the renal function test
Jerusalem artichokes
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Chitin
• Chitin is the second most abundant carbohydrate polymer
• Present in the cell wall of fungi and in the exoskeletons of crustaceans, insects and spiders
• Chitin is used commercially in coatings (extends the shelf life of fruits and meats)
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Dextrans
• products of the reaction of glucose and the enzyme transglucosidase
• contains a (1,4), a (1,6) and a (1,3) linkages• MW: 40,000; 70,000; 75,000• used as plasma extenders (treatment of shock)• components of dental plaques
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Dextrins
• produced by the partial hydrolysis of starch along with maltose and glucose
• dextrins are often referred to as either amylodextrins or erythrodextrins
• used as mucilages (glues)• also used in infant formulas (prevent the
curdling of milk in baby’s stomach)
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Glycoproteins and proteoglycans
• Glycoproteins are proteins conjugated to saccharides lacking a serial repeating unit
• In glycoprotein the protein>>>carbohydrate• Example include enzymes, immunoglobulins or antibodies,
certain hormones
• In Proteoglycans proteins are conjugated to polysaccharides with serial repeating units
• Here carbohydrate>>> protein• Proteoglycans modulate cell processes and make cartilage
flexible and resilient
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Glycosaminoglycans
• they are the polysaccharide chains of proteoglycans• they are linked to the protein core via a serine or
threonine (O-linked)• the chains are linear (unbranched)• the glycosaminoglycan chains are long (over 100
monosaccharides)• they are composed of repeating disaccharides