03. carbohydrates

8/14/2019 03. Carbohydrates

1/53

CARBOHYDRATES


2/53

Introduction

Carbohydrates are carbon compounds thatcontain large quantities of hydroxyl (-OH) groups

The simplest carbohydrates also contain either analdehyde moiety (these are termed

polyhydroxyaldehydes) or a ketone moiety(polyhydroxyketones).

Thus carbohydrates are aldehyde or ketonecompounds with multiple hydroxyl groups

All carbohydrates can be classified as eithermonosaccharides, oligosaccharides orpolysaccharides


3/53

Condensation products oftwo to tenmonosaccharide units, linked by glycosidic bonds,

make up an oligosaccharide. Polysaccharides are much larger, containing

hundreds of monosaccharide units.

The presence of the hydroxyl groups allowscarbohydrates to interact with the aqueousenvironment and to participate in hydrogenbonding, both within and between chains.

Derivatives of the carbohydrates can containnitrogens, phosphates and sulfur compounds.

Carbohydrates also can combine with lipids toform glycolipids or with proteins to formglycoproteins.


4/53

Occurance of carbohdrates

in plants in animals- First product of photo- - blood sugar : D-glucose

synthesis - milk sugar: lactose

- Stored in foods as starch, - stored as glycogen

inulin and hemicelluloses - essential components of

nucleic acids: ribose- Supporting tissue of

- plants: cellulose

- Degradation products:

gums and mucilages

- Miscellaneous : pectins,

glucosides


5/53

CARBOHYDRATES

MONOSACCHARIDES

(simple sugars)DISACCHARIDES POLYSACCHARIDES

Structuralsupport

fructose glucose galactose sucrose maltose lactose starch glycogen cellulose chitin

plants milk sugar beetssugar cane

grains milk storedglucosen plants

forms cellwalls inplants

insectsstoredglucose

n animals

Quickenergy

fruits

Table sugar


6/53

Functions

First, carbohydrates serve as energy stores, fuels,and metabolic intermediates. Second, ribose and deoxyribose sugars form part

of the structural framework of RNA and DNA.

Third, polysaccharides are structural elements inthe cell walls of bacteria and plants.

Fourth, carbohydrates participate in biologicaltransport, cell-cell recognition, activation ofgrowth factors, modulation of the immunesystem

Fifth, carbohydrates are associated with otherentities such as glycosides, vitamins andantibiotics)


7/53

Nomenclature

The predominant carbohydrates encountered in thebody are structurally related to the aldotrioseglyceraldehyde and to the ketotriosedihydroxyacetone.

All carbohydrates contain at least one asymmetrical(chiral) carbon and are, therefore, optically active.

In addition, carbohydrates can exist in either of twoconformations, as determined by the orientation of thehydroxyl group about the asymmetric carbon farthestfrom the carbonyl.

With a few exceptions, those carbohydrates that are of

physiological significance exist in the D-conformation.The mirror-image conformations, called enantiomers, are

in the L-conformation.


8/53


9/53


10/53


11/53

Monosaccharides

The monosaccharides commonly found in humans are classified accordingto the number of carbons they contain in their backbone structures. The

major monosaccharides contain four to six carbon atoms.

# Carbons Category Name Relevant examples3 Triose Glyceraldehyde, Dihydroxyacetone4 Tetrose Erythrose5 Pentose Ribose, Ribulose, Xylulose6 Hexose Glucose, Galactose, Mannose, Fructose7 Heptose Sedoheptulose9 Nonose Neuraminic acid also called sialic acid


12/53

Properties

Differences in structures of sugars areresponsible for variations in properties

PhysicalCrystalline form; solubility; rotatory power

ChemicalReactions (oxidations, reductions, condensations)

Physiological

Nutritive value (human, bacterial); sweetness; absorption


13/53

The aldehyde and ketone moieties of thecarbohydrates with five and six carbons willspontaneously react with alcohol groups presentin neighbouring carbons to produceintramolecular hemiacetals or hemiketals,respectively.This results in the formation of five- or six-membered

rings.

Because the five-membered ring structureresembles the organic molecule furan, derivativeswith this structure are termed furanoses.

Those with six-membered rings resemble theorganic molecule pyran and are termedpyranoses.


14/53


15/53

Cyclic Fischer Projection of -D-Glucose Haworth Projection of -D-Glucose

Such structures can be depicted by either Fischer or Haworth style diagrams.


16/53

The rings can open and re-close, allowing rotation tooccur about the carbon bearing the reactive carbonyl

yielding two distinct configurations ( and ) of thehemiacetals and hemiketals. The carbon about which this rotation occurs is the

anomeric carbon and the two forms are termedanomers.

Carbohydrates can change spontaneously between the and configurations-- a process known asmutarotation.

When drawn in the Fischer projection, theconfiguration places the hydroxyl attached to theanomeric carbon to the right, towards the ring.

When drawn in the Haworth projection, theconfiguration places the hydroxyl downward.


17/53


18/53

Simple sugars also differ in the positionsof hydroxyl groups and hydrogen around

the ring

Causes variations in:

1. Solubility

2. Sweetness

3. Rates of fermentation


19/53

Disaccharides

Covalent bonds between the anomerichydroxyl of a cyclic sugar and the hydroxyl of asecond sugar (or another alcohol containingcompound) are termed glycosidic bonds, andthe resultant molecules are glycosides.

The linkage of two monosaccharides to formdisaccharides involves a glycosidic bond.

Several physiogically important disaccharidesare sucrose, lactose and maltose


20/53

Reducing sugars

Reactive aldehyde and ketone groups are

not involved in linking, they are free

to react and reduce Fehlings solution.

Maltose: reducing sugar

Sucrose: nonreducing sugar


21/53

Sucrose

Sucrose

Prevalent in sugar cane and sugar beets, is composedof glucose and fructose through an -(1,2) glycosidicbond.


22/53

Lactose

Lactose

is found exclusively in the milk of mammals andconsists of galactose and glucose in a -(1,4) glycosidicbond


23/53

Maltose

the major degradation product of starch, is composed of2 glucose monomers in an -(1,4) glycosidic bond.


24/53

Oligosaccharides

Trisaccharide: raffinose (glucose, galactoseand fructose)

Tetrasaccharide: stachyose (2 galactoses,

glucose and fructose) Pentasaccharide: verbascose (3 galactoses,glucose and fructose)

Hexasaccharide: ajugose (4 galactoses, glucose

and fructose)


25/53

Oligosaccharides occur widely as components of

antibiotics derived from various sources


26/53

Polysaccharides

Most of the carbohydrates found in nature occurin the form of high molecular weight polymerscalled polysaccharides.

The monomeric building blocks used to generatepolysaccharides can be varied; in all cases,

however, the predominant monosaccharidefound in polysaccharides is D-glucose.

When polysaccharides are composed of a singlemonosaccharide building block, they are termedhomopolysaccharides.

Polysaccharides composed of more than one typeof monosaccharide are termedheteropolysaccharides.


27/53


28/53

Glycogen

Glycogen is the major form of stored carbohydrate inanimals. This crucial molecule is a homopolymer of glucose in-(1,4) linkage; it is also highly branched, with-(1,6)branch linkages occurring every 8-10 residues.

Glycogen is a very compact structure that results fromthe coiling of the polymer chains. This compactness allows large amounts of carbon

energy to be stored in a small volume, with little effecton cellular osmolarity

Complete hydrolysis yields glucose Glycogen and iodine gives a red-violet color


29/53

Starch

Starch is the major form of storedcarbohydrate in plant cells.

Its structure is identical to glycogen, except

for a much lower degree of branching (aboutevery 20-30 residues).

Unbranched starch is called amylose;branched starch is called amylopectin


30/53

Amylose and amylopectin are the 2 forms of starch. Amylopectin

is a highly branched structure, with branches occurring every 12

to 30 residues


31/53

Suspensions of amylose

in water adopt a helicalconformation

Iodine (I2) can insert inthe middle of the amylose

helix to give a blue colorthat is characteristic anddiagnostic for starch


32/53

Cellulose

Cellulose, the other major polysaccharide of glucose foundin plants, serves a structural rather than a nutritional role. Cellulose is one of the most abundant organic compounds

in the biosphere.

It is an unbranched polymer of glucose residues joined by-1,4 linkages.

The configuration allows cellulose to form very long,straight chains.

Fibrils are formed by parallel chains that interact with oneanother through hydrogen bonds.

The -1,4 linkages in glycogen and starch produce a very

different molecular architecture from that of cellulose.


33/53

(in starch)

(in cellulose)


34/53

Cellulose

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

Held together with lignin in woody plant tissues


35/53

Chitin

Chitin is the second most abundantcarbohydrate polymer

Present in the cell wall of fungi and in theexoskeletons of crustaceans, insects and

spiders

Chitin is used commercially in coatings(extends the shelf life of fruits and meats)


36/53

Linear structures of cellulose and chitin(2 most abundant polysaccharides)


37/53

Inulin

-(1,2) linked fructofuranoses

Linear only; no branching

Lower molecular weight than starch

Colors yellow with iodine

Hydrolysis yields fructose

Sources include onions, garlic etc

Used as diagnostic agent for the evaluation ofglomerular filtration rate (renal function test)


38/53

Dextrans

Products of the reaction of glucose and the enzymetransglucosidase from Leuconostoc mesenteroides

Contains (1,4), (1,6) and (1,3) linkages

MW: 40,000; 70,000; 75,000

Used as plasma extenders (treatment of shock) Also used as molecular sieves to separate proteins

and other large molecules (gel filtrationchromatography)

Components of dental plaques


39/53

Dextrins

Produced by the partial hydrolysis of starchalong with maltose and glucose

Dextrins are often referred to as eitheramylodextrins, erythrodextrins orachrodextrins

Used as mucilages (glues)

Also used in infant formulas (prevent the

curdling of milk in babys stomach)


40/53

Glycoconjugates

These are carbohydrates with protein or lipidconjugates

They include

PeptidoglycanProteoglycan

Glycolipids

Glycoprotein


41/53

Glycosaminoglycans (GAGS)

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 100monosaccharides)

Glycosaminoglycans are unbranched polysaccharide chains of

repeating disaccharide units (N-acetylgalactosamine (GalNAc)

or N-acetylglucosamine (GlcNAc) and a uronic acid such asglucuronate or iduronate)


42/53

GAGs of Physiological significance

Involved in a variety of extracellular functions;

Hyaluronic acid (lubricant in synovial joint)

Dermatan sulphate (skin, blood vessels, heart

valves)

Heparin & heparin sulphate (in blood clotting)

Keratin sulphate (skin, cornea, bone, cartilage)


43/53

Glycosaminoglycans

A characteristic of glycosaminoglycans is the presence

of acidic functionalities (carboxylate and/or sulfates)


44/53

Bacterial cell wall

Provide strength and rigidity for the organism

Consists of a polypeptide-polysaccharide knownas petidoglycan or murein

Heteropolymer of alternating N-acetylglucosamine

and N-acetylmuramic acid in -(1,4) glycosidic linkage

Determines the Gram staining characteristic ofthe bacteria

Many antibiotics work by inhibiting normal

synthesis of peptidoglycan in bacteria causingthem to burst as a result ofosmotic lysis


45/53

Structure of Peptidoglycan:Escherichia coli(Gram -)

Structure of Peptidoglycan:Staphylococcus aureus(Gram+)


46/53


47/53

Gram stain morphology of bacteria


48/53

Gram Stain of S. aureusGram Stain of E. coli


49/53

Glycoproteins

Proteins with covalently bonded carbohydrate

Found in almost all organisms

The oligosaccharide content is usually

between 1 to 30%

Sugars maybe neutral sugars, amino sugars,uronic acids or neuramic acid


50/53

The carbohydrate portions of glycoproteinsperform important biological functions

Carry determinants of human ABO blood grouping

Stabilize the protein conformationsAre involved in immunoprotection (cytokines,

immunoglobulins)

Are involved in cell-cell or cell-molecule

recognition events (host-parasite interactions)Are involved in blood clotting


51/53

ABO blood group antigens

Blood group classifications depend ondifferences in oligosaccharide structure

bonded to a protein called glycophorin and

lipids (glycosphingolipids) of RBC membranes

It is the terminal sugar of the oligosaccharidethat distinguishes the different blood group

cells

These minor differences in the sugars result inthe differences in compatibility of the blood

types


52/53

ABO Antigens


53/53

Diseases associated with carbohydratemetabolism include

diabetes mellitus,

galactosemia,glycogen storage diseases, and

lactose intolerance.

03. carbohydrates

Documents