Carbohydrates

Dr Imran Siddiqui PhD, MPhil, MBBS

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

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”

Definition

Carbohydrates are polyhydric alcohols (-OH) with aldehyde or ketone.

Figure 7.1. Examples of monosaccharides found in humans, classified according to the number of carbons they contain.

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

Figure 7.4C-2 and C-4 epimers and an isomer of glucose.

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.

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

Two Cyclic Forms of Glucose

Figure 7.6. The interconversion of the α and β anomeric forms of glucose (mutarotation).

• 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

Types of Carbohydrates

Monosaccharides are the simplest carbohydrates Empirical formula = (CH2O)n

Disaccharides consist of two monosaccharides Polysaccharides contain many monosaccharides

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

Structural representation of sugars

• Fischer projection: straight chain representation

• Haworth projection: simple ring in perspective• Conformational representation: chair and boat

configurations

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

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

Chair and boat conformations of a pyranose sugar

2 possible chair conformationsof b-D-glucose

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

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

Special monosaccharides: amino sugarsConstituents of mucopolysaccharides

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

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

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

Memorize!

Memorize!

•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

•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

Less common glucose disaccharides

Isomaltose (alpha 1,6)

Gentiobiose (beta 1,6)

Laminaribiose (beta 1,3)

Cellobiose (beta 1,4)

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

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

Sucralose (Splenda)

About 600 times more sweet than sucrose

Figure 7.11Abnormal lactose metabolism.

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)

Honey also contains glucose and fructose along withsome volatile oils

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

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

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

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

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

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;

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

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

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)

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

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)

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

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