Carbohydrate structure and function

PTT 103: BiochemistryPTT 103: BiochemistryDepartment of Chemical Engineering Technology, UniMAPDepartment of Chemical Engineering Technology, UniMAPSemester 1, 2013/2014Semester 1, 2013/2014khadijahhanim@unimap.edu.mykhadijahhanim@unimap.edu.my

Intro to Polysaccharides

Classification of Polisacharides

HOMOPOLYSACCHARIDES

•Found in abundance in nature•Important examples: starch, glycogen,

cellulose, and chitin•Starch, glycogen, and cellulose all yield

D-glucose when they are hydrolyzed•Cellulose - primary component of plant

cells•Chitin – principal structural component of

exoskeletons of arthropods and cell walls of many fungi; yield glucose derivative N-acetyl glucosamine when it is hydrolyzed.

STARCH

•Amylose

•amylopectin

Starch digestion

GLYCOGEN (Homopolysaccharide)• Glycogen is the storage form of glucose in animals and

humans which is analogous to the starch in plants. • Glycogen is synthesized and stored mainly in the liver and

the muscles. • Structurally, glycogen is very similar to amylopectin with

alpha acetal linkages, however, it has even more branching (every fourth glucose residue in the core of molecules) and more glucose units are present than in amylopectin.

• In glycogen, the branches occur at intervals of 8-10 glucose units, while in amylopectin the branches are separated by 12-20 glucose units

• Various samples of glycogen have been measured at 1,700-600,000 units of glucose..

• The structure of glycogen consists of long polymer chains of glucose units connected by an alpha acetal linkage.

• The branches are formed by linking C # 1 to a C # 6 through an acetal linkages.

STRUCTURE OF GLYCOGEN

Cellulose• Composed of D-glucopyranose residues linked by β(1,4)

glycosidic bonds.• Most important structural polysaccharides of plants• About 1 third of plants biomass comprise of cellulose. The

most abundant organic material..• Cellulose has many uses as an anticake agent, emulsifier,

stabilizer, dispersing agent, thickener, and gelling agent but these are generally subsidiary to its most important use of holding on to water.

• Water cannot penetrate crystalline cellulose but dry amorphous cellulose absorbs water becoming soft and flexible.

• Purified cellulose is used as the base material for a number of water-soluble derivatives e.g. Methyl cellulose, carbomethycellulose

•Pair of unbranched cellulose molecules contain as many as 12,000 glucose units each.

•Held together by hydrogen bonding to form sheetlike strips called microfibril.

•Each bundle of fibrils contains approx. 40 of these pairs.

•The ability to digest cellulose is found only in microbes that possess enzyme cellulase.

•Certain animal species (termites and cows) use such organisms in their digestives tracts to digest cellulose.

•The breakdown of cellulose makes glucose available to both the microbes and their host.

•Many animals cannot digest cellulose however Cellulose play a vital role as dietary fibre.

Cellulose as polymer of β-D-glucose

Cellulose in 3D

CHITIN (Homosaccharide)

• Chitin is a polymer that can be found in anything from the shells of beetles to webs of spiders. It is present all around us, in plant and animal creatures.

• The structure and function is similar to cellulose, because the two are very molecularly similar.

• Cellulose contains a hydroxy group, and chitin contains acetamide.

• Chitin is unusual because it is a "natural polymer," or a combination of elements that exists naturally on earth.

• Usually, polymers are man-made. Crabs, beetles, worms and mushrooms contain large amount of chitin.

• Chitin is a very firm material, and it help protect an insect against harm and pressure

•The monomeric units is N-acetyl-glucosamine (amino sugar) linked in unbranched chains by β(1,4) glycosidic bonds.

•Microfibrils are formed from adjacent chitin molecules that are strongly hydrogen bonded together.

•Chitin occur in 3 types of microfibrils; α-chitin, β-chitin and ɣ-chitin.

Structure of the chitin molecule, showing two of the N-acetylglucosamine units that repeat to form long chains

in beta-1,4 linkage.

CHITOSAN• A spinoff of chitin that has been discovered by

the market is chitosan. This is a man-made molecule that is often used to dye shirts and jeans in the clothing industry.

• Chitosan can be used within the human body to regulate diet programs, and researchers are looking into ways in which it can sure diseases.

• Chitin, the polysaccharide polymer from which chitosan is derived, is a cellulose-like polymer consisting mainly of unbranched chains of N-acetyl-D-glucosamine. Deacetylated chitin, or chitosan, is comprised of chains of D-glucosamine. When ingested, chitosan can be considered a dietary fiber.

CHEMICAL STRUCTURE OF CHITOSANhttp://www.pdrhealth.com/drug_info/nmdrugprofiles/nutsupdrugs/chi_0067.shtml

HETEROPOLYSACCHARIDES

•Are high-molecular-weight carbohydrate polymers more than one kind of monosaccharide

•Important examples include glycosaminoglycans (GAGs) – the principle components of proteoglycans and murein, a major component of bacterial cell walls.

Glycoaaminoglycans (GAGs)•GAGs are linear polymers with disaccharides

repeating units. Many of their sugar residues are amino derivatives.

•The repeating units contain hexuronic acid (a uronic acid contain 6-C atoms) except for keratan sulphate – contains galactose.

•Usually N-acetylglucosamine sulphate is also present except in hyaluronic acid which contain N-acetylglucosamine.

•Many disacharide units contain both carboxyl nd sulfate functions groups.

•GAGs are classified according to their sugar residues, the linkages between residues and the presence and location of sulphate groups.

•5 classes has been distinguished: hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparin and keratan sulfate

THE SPECIFIC GAGs OF PHYSIOLOGICAL SIGNIFICANCE

Hyaluronic acid Occurence : synovial fluid, ECM of loose connective tissueHyaluronic acid is unique among the GAGs because it does not contain any sulfate and is not found covalently attached to proteins. It forms non-covalently linked complexes with proteoglycans in the ECM.Hyaluronic acid polymers are very large (100 - 10,000 kD) and can displace a large volume of water.

Hyaluronic acid (D-glucuronate + GlcNAc)

• (1,4)-O-β-D-Glucopyranosyluronic acid- (1,3)-2-acetamido-2-deoxy-β-D-glucopyranose

Dermatan sulfate (L-iduronate + GlcNAc sulfate)

Occurence : skin, blood vessels, heart valves

(1,4)-O-α-L-Idopyranosyluronic acid-(1,3)-2-acetamido-2- deoxy-4-O-sulfo-β-D-galactopyranose

Chondroitin sulfate (D-glucuronate + GalNAc sulfate)Occurence : cartilage, bone, heart valves ;

It is the most abundant GAG.

(1,4)-O-B-D-Glucopyranosyluronic acid-(1,3)-2-acetamido-2-deoxy-6-O-sulfo-B-D-galactopyranose

Heparin and heparan sulfate (D-glucuronate sulfate + N-sulfo-D-glucosamine)

Heparans have less sulfate groups than heparins Occurence : Heparin :component of intracellular granules of mast cells lining the arteries of the lungs, liver and skin Heparan sulfate : basement membranes, component of cell surfaces

Keratan sulfate ( Gal + GlcNAc sulfate) Occurence : cornea, bone, cartilage ; • Keratan sulfates are often aggregated with

chondroitin sulfates.

(1,3)-O-B-D-Galactopyranose-(1,4)-2-acetamido-2-deoxy-6-O-sulfo-B-D-glucopyranose.

MUREIN (Peptidoglycan)

• Peptidoglycan, also known as murein, is complex polymer that is the major structural feature of the cell walls of all bacteria.

• The sugar component consists of alternating residues of β-(1,4) linked N-acetylglucosamine and N-acetylmuramic acid residues.

• Attached to the N-acetylmuramic acid is a peptide chain of three to five amino acids.

• The peptide chain can be cross-linked to the peptide chain of another strand forming the 3D mesh-like layer.

• Some Archaea have a similar layer of pseudopeptidoglycan.

SUMMARY• Monosaccharides, the simplest carbohydrates,

are classified as aldoses or ketoses.• The cyclic hemiacetal and hemiketal forms of

monosacchs have either alfa or beta configuration at their anomeric carbon.

• Monosacch derivatives include aldonic acids, uronic acids, deoxy sugars, amino sugars, alfa & beta glycosides.

• Disaccharides simplest polysaccharides occuring as hydrolysis products of larger molecules e.g. Lactose,sucrose

• Oligosaccharides play important roles in determining protein structure and in cell-surface recognition phenomena. Oligosacchs with 3 or more sugar residues are mostly found in plants.

Summary contd. -1•POLOYSACCHARIDES consist of

monosacchs linked by glycosidic bonds.•Cellulose and chitin are structural

polysacchs with beta(1-4) linkages that adopt rigid and extended structures.

•The storage polysacchs starch and glycogen consist of alfa-glycosidically linked glucose residues

•Glycosaminoglycans are unbranched polysacchs containing uronic acids and amino sugars that are often sulfated

END NOTES

•The destiny of a nation depends on the manner in which it feeds itself.

•We eat to live, NOT, live to eat.

•Lower your carbohydrate consumption, but balance it with the right amount of protein and fat.