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Carbohydrates of biological importance

Homo-, and heteroglycans

Nature of Carbohydrates

• Carbohydrates are carbon compounds that contain large quantities of hydroxyl groups.

• The simplest carbohydrates also contain either an aldehydemoiety (these are termed

polyhydroxyaldehydes) or a ketonemoiety (polyhydroxyketones).

Carbohydrates classification

• Monosaccharides, either aldoses or ketoses, are often given more detailed generic names to describe both the important functional groups and the total number of carbon atoms.

• Thus, one can refer to aldotetroses and ketotetroses, aldopentoses and ketopentoses, aldohexoses and ketohexoses,and so on.

• Sometimes the ketone-containing monosaccharidesare named simply by inserting the letters-ul-into the simple generic terms, such as tetruloses, pentuloses, hexuloses, heptuloses, and so on.

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Carbohydrates

• Structurally, are polyhydroxy aldehydes or ketones.• The most abundant carbohydrate in nature is glucose.

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Monosaccharides consist typically of three to sevencarbon atoms and are described either as aldosesor ketoses, depending on whether the molecule contains an aldehyde or a ketone group

Aldohexoses

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Sedoheptulose (The Pentose Phosphate Pathway)

Stereoisomers that are mirror images of each other arecalledenantiomers, or sometimesenantiomeric pairs.

Pairs of isomers that have opposite configurations at one or more of the chiral centers but that are not mirror images of each other are called diastereomersor diastereomeric pairs.

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Carbohydrates –Stereochemistry

The configuration around every chiral carbon is dif ferent

Epimerism Sugar molecules that differ in configuration at only one several chiral centers are called epimers

Mannose and galactose are epimers of D-glucose (at C-2 and C-4, respectively).

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Mechanism for the base-catalyzed epimerization of a monosaccharide:

D or L refers to the configuration of the highest numbered asymmetric carbon (farthest from aldehyde or ketone groups)

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Cyclic Structures and Anomeric Forms

• Although Fischer projections are useful for presenting the structures of particular monosaccharides and their stereoisomers , they ignore one of the most interesting facts of sugar structure—the ability to form cyclic structures with formation of an additional asymmetric center.

• Pentoses and hexoses can cyclize, as the aldehyde or keto group reacts with a hydroxyl on one of the distal carbons.

• e.g., glucose forms an intramolecular hemiacetalby reaction of the aldehyde on C1with the hydroxyl on C5, forming a six-member pyranosering, named after the compound pyran.

• The representations of the cyclic sugars are called Haworth projections.

Formation of hemiacetals and hemiketalsAlcohols react readily with aldehydes and ketones to form hemiacetalsand hemiketals.

All monosaccharides have the ability to cyclize and form ringed structure

Aldoses cyclize to produce cyclic hemiacetals (forming a pyranose sugar) and ketoses cyclize to produce hemiketals (forming a furanose sugar)

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Formation of glucose hemiacetal in Fischer and Haworth projection

In the α-anomer the hemiacetal –OH group is on the same side of the Fischer projection as the oxygen at the configurational carbon

Fischer hemiacetal and Haworth projection of glucose

Formation of fructose hemiketal in Fischer and Haworth projection

The α anomer of fructose has the anomeric -OH group down, trans to the terminal -CH2OH group. The β anomer has the anomeric -OH group up, cis to the terminal -CH2OH.

Formation of the two cyclic forms of D-glucose:

anomeralpha (α) -OH is down in Haworth projection

beta (β) -OH is up in Haworth projection

The α and β anomes of D-glucoseinterconvert in aqueous solution bya process called mutarotation.

Isomeric forms of monosaccharides that differonly in the configuration about the hemiacetalor hemiketal carbon atom are called anomers.

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Anomerism

• Isomeric forms of monosaccharides differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers

• The hemiacetal or carbonyl carbon is called the anomeric carbon

Many derivatives of monosaccharides are found in nature.

• These include

• Oxidized forms in which the aldehyde and/or alcohol functional groups are oxidized to carboxylic acids

• Phosporylated forms in which phosphate is added by ATP to form phosphoester derivatives

• Amine derivatives such as glucosamine or galactosamine • Acetylated amine derivatives such as N-Acetyl-GlcNAc (GlcNAc) or

GalNAc • Lactone forms (intramolecular esters) in which an OH group attacks a

carbonyl C that was previously oxidized to a carboxylic acid • Condensation productsof sugar derivatives with lactate

(CH3CHOHCOO-) and pyruvate, (CH3COCOO- ), both from the glycolytic pathway, to form muramic acid and neuraminic acids, (also called sialic acids), respectively.

Reduction-oxidation reactions of carbohydrates

• When the aldehyde function of an aldose is oxidized to acarboxylic acid the product is calledan aldonic acid (a mildoxidizing agent such as bromine water must be used for thisconversion).

• When oxidation takes place only on carbon C6 the product iscalledan uronic acid

• If both ends of an aldose chain are oxidized to carboxylic acidsthe product is calledan aldaric acid.

Oxidation of Aldoses Using Bromine Water

Bromine water oxidizes the aldehyde group of an aldose to a carboxylic acid. Bromine water is used for this oxidation because it does not oxidize the alcohol groups of the sugar and it does not oxidize ketoses.

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Nitric Acid Oxidation.

Nitric acid is a stronger oxidizing agent than bromine water, oxidizing both the aldehyde group and the terminal -CH2OH group of an aldose to a carboxylic acid.

Sugar acids

Oxidation on C6 Aldonic and aldaric acids form lactones

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Lactone and gluconic acid are formed in pentose phosphate pathway Reduction

• either done catalytically (hydrogen and a catalyst) or enzymatically

• the resultant product is a polyol or sugar alcohol (alditol)• glucose forms sorbitol (glucitol)• mannose forms mannitol• fructose forms a mixture of mannitol and sorbitol• glyceraldehyde gives glycerol

Sugar alcohols are very useful intermediates

• Mannitol is used as an osmotic diuretic

• Glycerol, myoinositolare important in lipid metabolism

• Glycerol is used as a humectant and can be nitrated to nitroglycerin

• Ribitol constituent of riboflavin/flavin coenzymes

• Other important ones are xylitol and sorbitol

– used in food and pharmaceutical processing

– sugarless candies, gums have sorbitol in them

Reduction of Glucose

Sorbitol, also known as glucitol, is a sugar alcohol the body metabolises slowly. Sorbitol does not diffuse through cell membrane easily and therefore accumulates, causingosmotic damage.Diabetic retinopathyand neurophathy may be related to excess sorbitolin the cells of the eyesand nerves.

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ALDITOLS (ALCOHOLS)Derivatives of Monosaccharides

• Deoxy Sugars

– A hydrogen atom replaces a hydroxyl group

– Very important in DNA synthesis (e.g.,deoxyribose)

– L-Fucose (6-deoxy-L-galactose) is common in polysaccharides, blood groups

– L-rhamnose (6-deoxy-L-mannose) is used as a fermentative reagent in bacteriology

6-deoxy-L-mannose6-deoxy-L-galactose

Special monosaccharides: amino sugarsConstituents of glycosaminoglycans (GAG)

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SUGAR DERIVAIVES - Amino sugars

β D N-acetylglucosamine

• Muramic acid and neuraminic acid,which are components of the polysaccharides of cell

membranes of higher organisms and also bacterial cell walls, are glucosamines linked to three-carbon

acids at the C-1 or C-3 positions.

N-acetylneuraminic acid (NANA)

• N-acetylneuraminic acid, which is derived from N-acetylmannosamine and pyruvic acid, is an important constituent of glycoproteins

• Neuraminic acid(an amine isolated from neural tissue) forms a C-C bond between the C-1 of N- acetylmannosamine and the C-3 of pyruvic acid .

• N-acetylneuraminate, (also called sialic acid) is often found as a terminal residue of oligosaccharide chains of glycoproteins.

• Sialic acidimparts negative chargeto glycoproteins, because its carboxyl group tends to dissociate a proton at physiological pH.

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• In muramic acid(thus named as an amine isolated from bacterial cell wall polysaccharides; murus is Latin for “wall”), the hydroxyl group of a lactic acid moiety makes an ether linkage to the C-3 of glucosamine .

Derivatives of Monosaccharides

• Sugar Phosphates

– Phosphate ester of one of the –OH groups

– Often on the terminal carbon

– Important in glucose metabolism (e.g., glucose 6-phosphate, glucose 1-phosphate)

Phosphate derivatives

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Phosphate derivativesGlycosidesare formed by elimination of water between

anomeric hydroxyl of sugar and hydroxyl of another

compound

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Carbohydrates - Glycosides

Glycosides (replace suffix –ose with –oside) → Reaction at C1

Hemiacetal/Hemiketal + Alcohol → O- Glycoside→ Polysaccharides

Hemiketal/Hemiketal + Amine → N-Glycoside

Condensation reactions: acetal and ketal formation

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Glycosidescan be with many types of molecules including another sugar (oligosaccharides). Disaccharides contain a glycosidic bond

Formation of

maltose

anomeric carbon

Glu( α1→4)Glu

The glycosidic bond protects theanomeric carbon from oxidation .

Maltose is reducing sugar

Lactose is reducing sugar

Lactose: Gal(β 1→4)GlcSince Glc is attached to Galthrough the OH on C4, itsanomeric carbon, C1, couldrevert to the noncyclicaldehyde form

Sucrose: Glc(α 1→2)Fru

• Sucrose: is composed of one molecule of glucose and fructose joined together through a α-1, β-2-glycoside linkage.

• Since both functional reducing groups are involved in the glycoside linkage, sucrose does not possess reducing properties.

Sucrose: Glc(α 1→2)Fru.

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Trehalose does not possess reducing properties.

Important to remember:

• The α in α-1→4- refers only to the fact that the first glucopyranose is the α anomer.

• Only anomeric carbons are designated α or β , not the other carbons in the carbohydrate.

• For sucrose, the α in α-1 indicates that the glucopyranose is the α anomer and β- 2 indicates that the fructofuranose is the β anomer.

• The correct specification of the configuration of the anomeric carbon is critical: an α(1→4) linkage is not the same thing as a β(1→4) linkage

Digestion of Carbohydrates

• Monosaccharides– Do not need hydrolysis before absorption

– Very little (if any) in most feeds

• Di- and polysaccharides– Relatively large molecules

– Must be hydrolyzed prior to absorption

– Hydrolyzed to monosaccharides

Only monosaccharides can be absorbed

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Carbohydrate Digestion

• Mouth

� Salivary amylase

� Breaks starches down to maltose� Plays only a small role in breakdown because of the short time food is in the mouth

Carbohydrate Digestion

• Pancreas

� Pancreatic amylase

� Hydrolyzes alpha 1-4 linkages� Produces monosaccharides, disaccharides,

and polysaccharides� Major importance in hydrolyzing starch and

glycogen to maltose

Polysaccharides DisaccharidesAmylase

Digestion in Small Intestine

• Digestion mediated by enzymes synthesized

by cells lining the small intestine (brush

border)

Disaccharides MonosaccharidesBrush Border Enzymes

* Exception is β-1,4 bonds in cellulose

Digestion in Small Intestine

Maltose Glucose + GlucoseMaltase

Lactose Lactase Galactose + Glucose

Sucrose Glucose + FructoseSucrase

* Ruminants do not have sucrase

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Polysaccharides are divided into two broad classes

1. Homoglycans, or homopolysaccharides, which are polymerscontaining only one type of monosaccharide residue

2. Heteroglycans, or heteropolysaccharides, which arepolymers containing more than one type of monosaccharideresidue

Polysaccharides - Classification

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Polysaccharides

Structural Polysaccharides

Storage Polysaccharides

Most polysaccharides can also be classified according to their biological roles

1. Storage polysaccharides: starch and glycogen2. Structural polysaccharides: cellulose and chitin

Starch- a very large glucan-is the plant energy reserve, consists of amylose and amylopectin

a. The repeating linear unit is two Glc residues in α-1→4 linkage

b. The helical structure of the non-branched component amylose

Amylopectin, a form of starch, haveα 1→4 glycosidic bondsand α 1→6 glycosidic bonds.

The actual distance between branch points averages 24 to 30 glucose residue.

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Starch digestion is sequential

•α-amylosesequentially digests disaccharides from non-reducing ends of polymers; products are maltotriose, maltose, and oligosaccharides with branch points (dextrins)

•α-glucosidase cleaves maltotriose and maltose to component glucose residues

•α-dextrinase (debranching enzyme) cleaves both α1→4 and α1→6 linkages to degrade dextrins to monomers

The major form of storage polysaccharide in animals is glycogen.

• Glycogen is found mainly in the liver (where it mayamount to as much as 10% of liver mass) and skeletalmuscle (where it accounts for 1 to 2% of musclemass).

Glycogen

• also known as animal starch• stored in muscle and liver• contains both α(1,4) links and α(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 α and β-amylases and by glycogen phosphorylase

Dextrans• If you change the main linkages between glucose from

alpha(1,4) to alpha(1,6), you get a new family ofpolysaccharides -dextrans

• Branches can be (1,2), (1,3), or (1,4)

• Dextrans formed by bacteria are components of dentalplaque

• Cross-linked dextrans are used as "Sephadex" gels incolumn chromatography

• These gels are up to 98% water!

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Cellulose

• Cellulose monomers (β-D-glucopyranose) arelinked together throughβ1→4 glycosidic bondsby condensation.

Cellulose

• Beta(1,4) linkages make all the difference!

• Strands of cellulose form extended ribbons

• H bonds are intrachain as wellas interchain

• Interchain H-bonding allows multi-chain interactions. Forms cable like structures.

• H-bonded sheets made up of linear polymer, where residues are turned 180o relative to their neighbors

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)

A short segment of chitin

Linear polymer of N-acetylglucosamine sugars linked by β 1→4 bonds. Each GlcNAc residue is rotated 180o relative to its neighbors

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Heteroglycans

• High molecular weight carbohydrate polymers that contain more than one kind of monosaccharides.

• Major classes found in animals are N and O-linked glycans attached to proteins.

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Heteropolysaccharides of the animal extracellular matrix (ECM)and cell surface, termed glycosaminoglycans(GAGs), play bothstructural and recognition roles

•Repeating disaccharidesmake up these polymers, andcomponent sugars are negatively charged derivatives

(uronic acids and sulfated sugars)and amino sugars(usuallyacetylated)

•Some GAGs have tissue-specific distribution, and all but heparin arefound more often in complex structures (proteoglycans) than inisolation

Structure of Glycosaminoglycans (GAGs)

• Glycosaminoglycans are long, unbranched, heteropolysaccharide chains generally composed of a repeating disaccharide unit [acidic sugar–amino sugar]n.

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Heteroglycans, or heteropolysaccharides,

• Common monosaccharidederivatives in heteropoly-saccharides include:

� the amino sugars:D-glucosamineand D-galactosamine

� their derivatives: N-acetylneuraminic acid andN-acetylmuramic acid

� and simple sugar acids:glucuronic and iduronic acids.

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Classification of the glycosaminoglycans(GAGs)

• The six major classes of glycosaminoglycans are dividedaccording to monomeric composition, type of glycosidiclinkages, and degree and location of sulfate units.

• The specific GAGs of physiological significance are:hyaluronic acid, dermatan sulfate, chondroitin sulfate,heparin, heparan sulfate, and keratan sulfate.

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Highly glycosylated proteins carry up to 99% carbohydrate by weight

Proteoglycansare components of the ECM, made up of GAGs, core protein, and link proteins in a complex structure

•Hyaluronate forms a backbone onto which large core proteins are attached via linker proteins; other GAGs are attached to core proteins via association with protein-linked glycans. •The end structures are millions of daltons, and can vary even in core and linker protein identity.

•Roles of proteoglycans include both support, as described for GAGs, and recognition.• Proteoglycans can bind to growth factors, allowing their presentation in the context of the ECM and limiting the target distance of the growth factors.

Hyaluronic acid is among the largest glycans

•The repeating disaccharide is Glucuronate β 1→3-GlcNAc, in β1→4 linkages so the structure of hyaluronate is an extended helix

•The numerous anionic groups result in a rigid and highly hydrated molecule- high viscosity in solution and structurally responsive to shear forces. •At low shear, high resistance as molecules are relatively disordered; at high shear, molecules line up with the direction of the force so little resistance. •This allows function as shock absorber and lubricant.

Repeating disaccharide of hyaluronic acid

• GlcUA =D-glucuronate

• GlcNAc= N-acetylglucosamine

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�Heparan SO4 is a ubiquitous cell surface and ECM GAG, andmost variable in composition. The repeating disaccharide is

Iduronate SO4α1→4 Sulfo-glucosamine SO4 .

�Heparin is similar to heparan sulfate but is less variable.�This is the most highly charged GAG- there are on average 2.5SO4 groups per disaccharide- and is the most negatively chargedmolecule in animal tissues.�Heparin is synthesized and held within mast cells lining arteries, isreleased after injury and inhibits the blood clotting cascade byenhancing anti-thrombin activity.

HeparinHeparin, a soluble glycosaminoglycan found in granules of mast cells, has a

structure similar to that of heparan sulfates, but is relatively highly sulfated.

� made & released from mast cells in lungs & liver

� also associated with luminal surface of endothelium

� anticoagulant

– forms complex with antithrombin III

– this complex binds to thrombin, inactivating it

– as a result, clot growth is limited

– fast-acting, making it therapeutically useful

GAG Localization

• Hyaluronate: synovial fluid, vitreous humor, • Chondroitin sulfate: cartilage, bone, heart valves• Heparan sulfate: basement membranes,

components of cell surfaces contains higher acetylatedglucosamine than heparin

• Heparin: component of intracellular granules of mast cells lining the arteries of the lungs, liver

• Dermatan sulfate: skin, blood vessels, heart valves-• Keratan sulfate: cornea, bone, cartilage aggregated with

chondroitin sulfates

• Glycoconjugates = carbohydrate derivatives in which one or more carbohydrate chains are linked to a protein, peptide chain, or

lipid:

– Proteoglycans, peptidoglycans, glycoproteins, glycolipids

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Proteoglycans are glycosaminoglycansthat are covalently linked to serine residues of specific core proteins.

The linkage of GAGs to the protein core involves a specific trisaccharide composed of two galactose residues and a xylose residue (GAG-GalGalXyl-O-CH2-protein A typical tetrasaccharide linker (blue) connects a glycosaminoglycan—in

this case chondroitin 4-sulfate (orange)—to a Ser re sidue (pink) in the core protein. The xylose residue at the reducing end of the linker is joined by its anomeric carbon to the hydroxyl of the Ser residue.

Proteoglycans: cell surface or extracellular matrix

Proteoglycans

• The disaccharide units containeither of two modified sugarsN-acetylgalactosamine (GalNAc)

orN-acetylglucosamine(GlcNAc) anda uronic acid suchas glucuronate oriduronate.

Proteoglycan (the protein-carbohydrate) complex in cartilage

A major function of GAGs is the formation of a matr ix to hold together the protein components of the skin and connective tissu e

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Proteoglycan aggregate of cartilage Glycoprotein

• Glycoproteins are proteins that contain oligosaccharide chains (glycans) covalently attached to their polypeptide side-chains.

• The process of attaching the glycans is known as glycosalation.

• The sugar groups attached to glycoprotein can assist in protein folding or improve a proteins’ stability.

GLYCOPROTEIN FUNCTIONS

• Structural→proteoglycans • Transport proteins→transferrin • Enzymes • Antibodies • Antifreeze protein in Antarctic fish -carbohydrate forms

H-bonds with water, preventing growth of ice crystals • Recognition phenomena-cell to molecule, cell-virus and cell-

cell • Insulin receptor is a glycoprotein • CD4 receptor is site of HIV attachment is a glycoprotein • Immunoglobulins are examples of glycoproteins.

Chapter 8

Glycoproteins

• Proteins that contain covalently-bound oligosaccharides, either to serine (O-Glycosidic linkage) or asparagine (N-glycosidic linkage)

• Oligosaccharide chains exhibit great variability in sugar sequence and composition

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• Human blood groups are formed from complex groups of polysaccharides on the surfaces of erythrocytes.

• These carbohydrates are attached to glycoproteins and glycolipids on the surfaces of red blood cells where they act as specialized anitgenic determinants.

• The three most common blood types (A,B,O) are differentiated by the presence and type of only ONE monosaccharide!

The blood group antigensare important group of oligosaccharides

• On some cells these are attached as O-linked glycansto membrane proteins

• The antigens which determine blood types belong to glycoproteins and glycolipids.

There are three types of blood-group antigens: O, A, and B. They differ only slightly in the composition of carbohydrates

• The A and B antigens are formed by addition of GalNAc or Gal, respectively to the O-oligosaccharide

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Blood type is determined by the nature of the sugar bound to the protein on the surface of red blood cells:

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Peptidoglycans

• The cell of many bacteria are made of peptydoglycans,which are heteroglycan chains linked

to peptides

• Its polysaccharide componentconsists of linear chains of alternating β(1→4)-linked

N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (NAM)

PeptidoglycanPeptidoglycan

• glycan backbone– muramic acid (MurNAc)– N-acetylglucosamine (GlcNAc)

• peptide side chain• peptide cross-bridge

– D- and L- amino acids

Bacterial Cell Walls

Composed of 1 or 2 bilayers and peptidoglycan shell

• Gram-positive: One bilayer andthick peptydoglycan outer shelllouter shell

• Gram-negative: Two bilayerswith thin peptydoglycan shell inbetweenhell in between

• Gram-positive: pentaglycinebridge connects tetrapeptides

• Gram-negative: direct amidebond between tetrapeptidesetween

Gram-positive

Gram-negative

Peptide cross-bridges

Gram-negative:

Amino acids directly joined via cross-bridge

Gram-positive:

Glycine pentapeptide bridge joins amino acids

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• Polymer of disaccharideN-acetylglucosamine (NAG) & N-acetylmuramic acid (NAM)

• Linked by polypeptides

Peptidoglycan Peptidoglycan: Polymer of disaccharideN-acetylglucosamine (NAG) & N-acetylmuramic acid (NAM)Linked by polypeptides

Gram-Negative Outer MembraneBilayer membrane outside the peptidoglycan contains phospholipids,

proteins, and lipopolysaccharide (LPS) Lipopolysaccharide (LPS)

• Lipopolysaccharide occurs on the outer membrane of gram-negative bacteria such as E. coli and Salmonella typhimurium

• Also known as endotoxin• The lipid A portion of the lipopolysaccharides of some bacteria is toxic to

humans (e.g. it is responsible for the dangerously lowered blood pressure that occurs with toxic shock syndrome in G(-) bacterial infections in humans

– Dead cells release lipid A when cell wall disintegrates– May trigger fever, vasodilation, inflammation, shock, and blood clotting

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• Gram-positive cell walls contain teichoic acids• Teichoic acids:

– Lipoteichoic acid links to plasma membrane

– Wall teichoic acid links to peptidoglycan

• May regulate movement of cations

• Polysaccharides provide antigenic variation

Gram-Positive cell walls

�Teichoic acidsare polymers of glycerol or ribitol (linked by phosphodiester bonds) with GlcNAc and D-Ala side chains

�These anionic molecules on the surface of Gram-positive bacteria- up to 50% dry weight of the cell wall- can be linked to peptidoglycan at NAcGlc or NAcMur or can be linked to membrane lipid. �They play structural role(s) and can be important to survival and virulence.

Bacterial cell walls

Comparison of Gram-Positive and Gram-Negative Structures of the following compounds are obligatory for the control test of carbohydrates

• General scheme of hemiacetal and hemiketal formation.• Structures of D-glucose and D-fructose presented in open chain, hemiacetal Fischer

projection formulas and Haworth projection formulas (use whole chemical name for each monosaccharide describing type of anomer and type of ring).

• Aldonic, uronic and aldaric acids formed by different oxidation types of monosaccharides (glucose and galactose).

• Reduction products of glucose and mannose (alditols)• Reaction of lactone formation (gluconolactone)• Derivatives of monosaccharides: glucosamine, galactosamine, mannosamine,

N-acetylglucosamine and N-acetylgalactosamine• Phosphate ester of sugars: glyceraldehyde -3-phosphate, dihydroxyacetone-3-

phosphate; glucose-6-phosphate, fructose 1,6 bisphosphate• Disaccharides: maltose and lactose and name them correctly using whole chemical

name. (You should be able to give the name of each disaccharide given its Haworth structure).

• Polysaccharides: fragment of starch and glycogen (type of bonds)