Class Note for BIOC 460 at UA 3
Class Note for BIOC 460 at UA 3
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Bioc 460 Dr Miesfeld Fall 2008 Cellulose is a biofuel Lecture 31 Carbohydrate Structure Key Concepts Review of monosaccharide and disaccharide structures Structures of common complex carbohydrates Carbohydrates are often covalently attached to polypeptides Key Questions about carbohydrate structure and function Describe three ways in which carbohydrates contribute to cell structure and function Biochemical Applications Pigs and chickens cannot digest raffinose series oligosaccharides that are present in soybeanbased feed because they lack the enzyme cxgalactosidase To prevent dietary problems associated with the inability to digest these PS oligosaccharides soybean feed is pretreated with a commerciallyproduced 0L galactosidase enzyme This same cxgalactosidase enzyme preparation isolated from a fungus is marketed commercially as a dietary product for humans called BeanoTM Raf nose oligosaccharides are found in many types of vegetables eg beans broccoli and cabbage which helps make Beano a multimillion dollar product Review of monosaccharide and disaccharide structures Carbohydrates are the most abundant biomolecules on planet earth primarily because they are the major structural element of plants in the form of cellulose a repeating unit ofglucose found in plant cell walls The word quotcarbohydratequot comes from the term carbon hydrate which describes the empirical formula for carbohydrates CH20n in which n 3 3 However as we saw in the Calvin cycle carbon hydration is not an accurate description for carbohydrate biosynthesis since the formation ofglyceraldehyde3phosphate GAP is the result ofenzymatic reactions catalyzing 002 fixation not hydration We can divide carbohydrates into three major groups based on their basic structures 1 simple sugars consisting of monosaccharides and disaccharides that function as metabolic intermediates in energy conversion pathways 2 complex carbohydrates consisting of oligosaccharides several monosaccharide units or polysaccharides many monosaccharide units that serve structural roles or function as storage forms of glucose and 3 glycoconjugates consisting of carbohydrate units often modified forms ofglucose that are covalently linked to proteins giving rise to glycoproteins or attached to lipids forming glycolipids Simple sugars are de ned as monosaccharides and disaccharides The word saccharide comes from the Greek word sakcharon which means sugar Glucose is the most plentiful monosaccharide in nature and has the molecular formula of C6H1206 Glucose is a polyhydroxy aldehyde while fructose which has the same molecular formula as Figure 1 glucose is a polyhydroxy ketose The chemical structure of H OH CHZOH glucose is shown in gure 1 using either Fisherprojections H OH 0 that have flattened bond angles to represent linear 3 3H OH monosaccharides or Haworth perspectives which are used to HOCH2 OH OH OH OH illustrate the cyclic structure of monosaccharides Glucose can also be drawn using a conformational formula in which 1 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 glucose is represented by either the quotboatquot or quotchairquot conformations to reflect the nonplanar structure of pyranose rings Simple sugars are sweet to the taste and found in many types of fruits and vegetables they are also used commercially as additives to enhance the flavor of W CHQOHSucranse processed foods and beverages A 60000 C39 lH ho fog The sensation of sweetness is the Kl N W result of ligand activation of G quot quot proteincoupled receptors Aspartame expressed on the surface of NH 302 T gustatory cells taste cells in the 15900 HonLEM e3 Nigml g tongue The G proteincoupled Hm 0 H receptors in these cells bind Fructose sugars with differential affinities 175 39 Em M based on their chemical structure Re39ativesweetness Sucrose Sugar binding to taste receptors m human taStEteSts imif stimulates a neuronal signal that is 100 i 0 transmitted to the brain Human 75 H Glucose taste tests can be used to measure the relative sweetness of l u simple sugars and artificial 30 OHCH20HO H Gamma sweeteners figure 2 For M example fructose which is found H in high concentrations in many types of fruit is perceived to be about three times sweeter than glucose and two times sweeter than the disaccharide sucrose table sugar Honey is a natural product made by honeybees that consists of a mixture of glucose and fructose Interestingly it has been known for centuries that honey contains antibiotic properties which is why honey can be stored at room temperature without becoming contaminated Figure 2 also shows the structure of the artificial sweetener Sucralose which is a chlorinated sucrose molecule that is marketed under the brand name of SplendaTM Sucralose is currently the sweetest compound available commercially and is an amazing 600 times sweeter than sucrose because of its differential binding properties to taste receptor proteins on the tongue Another artificial sweetener aspartame marketed as NutraSweet is not a carbohydrate at all but rather a dipeptide derivative of aspartate and phenylalanine Monosaccharides have either an aldehyde group at the end of the molecule such as glucose or a ketone group on M H the second carbon as in fructose figure 3 Aldehyde H O containing monosaccharides are called aldoses and ketone CI FEE O containing monosaccharides are called ketoses All H C OH co monosaccharides have a CHZOH group on the end of the H0CH Ho H carbon chain opposite the aldehyde or ketone group and HCOH H OH each of the carbons in the middle have OH groups and I function as chiral centers The carbonyl atom 00 is either H C OH H f OH at the end of the carbon chain aldose or on the second CHZOH CHZOH carbon ketose The name of the monosaccharide reflects DGucose DFructose the number of carbons in the chain for example an aldose an aldohexose a ketohexose with three carbons is a triose four carbons a tetrose five carbons a pentose and six carbons a hexose 2 of 12 pages Bloc 460 Dr Miesfeld Fall 2008 The smallest monosaccharide is glyceraldehyde a triose sugar with one chiral center A carbon chiral center is I an atom with four different functional groups Chiral I compounds lack a plane of symmetry and exist as two optical I isomers also called enantimers Enantimers exist in nature I as either right handed D form or lefthanded L form and l differentially reflect polarized light The structures D I glyceraldehyde and Lglyceraldehyde are shown in figure 4 I I I I I Figure 4 where it can be seen that these two isomers of H0 glyceraldehyde are mirror images of each other By convention when the hydroxyl group in the chiral carbon is on H the right side of a Fisher projection it is the D isomer and i when it is on the left side it is the L isomer CH20H Monosaccharides of five six or seven carbons are DGycemldehyde LGwenNew often more stable in aqueous solution as cyclic structures 39 than they are as open chains Cyclic monosaccharides form spontaneously through a covalent linkage of the carbonyl carbon with a hydroxyl group in the carbon backbone This bond is the result of a reaction between an alcohol group and an aldehyde group of an aldose sugar to form a hemiacetal or between an alcohol group and the ketone group of a ketose sugar to form a hemiketal Figure 5 illustrates the cyclization reaction that occurs when the C5 hydroxyl group of Dglucose attacks the oxygen atom of the C1 aldehyde group to form a cyclic hemiacetal In this conformation the C1 carbon of Dglucose becomes the new chiral center and as such cyclic forms of glucose exists as either Dglucose with the hydroxyl group at C1 above the plane of the ring or as cxDglucose with the hydroxyl group below the plane of the ring Crystalline glucose exists in the cyclic cxDglucose form but once it is dissolved in an aqueous solution an equilibrium is established between the cxDglucose and Dglucose M conformations at a ratio of about 4060 for cxD quotc gucose3Dglucose A very small amount of the 1 monosaccharide is found in the open chain conformation lt005 n O I O IIIIIIIIIIII39 1Imrgt l n E O I llIirror H 2C OH H03CH DGlucose 4 The hemiacetal C1 carbon of cyclic Dglucose is 3 called an anomeric carbon and Dglucose and cxD GCHZOH glucose are referred to as anomers because they only differ at the anomeric carbon Cyclic conformations of soon hexose sugars are called pyranoses because the six SCI OH membered ring is similarto a pyran compound TJ quotJ H Therefore cyclic forms of glucose are sometimes called 4CIOH HC1 a Dgucopyranose and Dgucopyranose Ketoses H0 3clzcl such as fructose can also form cyclic structures but l l because the carbonyl is in the C2 position of the open chain the ring that forms contains only five carbons 6020 6020 These sugars are furanoses because they resemble a H 5c 0 5c 0 furan Cyclic conformations of fructose are called a D AIll AllJ fructofuranose and D fructofuranose Note that CM IWROH Clt3H III cxH pyranose rings are much more stable in solution than 3C zcl 0 3C ZCI furanose rings H OH H OH aDGlucopyranose BD Glucopyranose 3 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 Disaccharide Figure 6 sugars are formed by a condensation reaction H hemiacetal OH between two Nonreducing end 4 Reducing end monosacchandes The Ho H covalent linkage is called an Oglycosidic bond Maltose and represents the aDglucopyranosyl1 gt4Dgucopyranose formation of an acetal from a hemiacetal and an alcohol The glycosidic bond in the disaccharide maltose is called an 11 4 linkage because the anomeric carbon is in the oc conformation figure 6 The glucose molecule on the right retains the hemiacetal structure at its C1 anomeric carbon and can convert to the aldehyde open chain form in a reaction involving the reduction of Cu2 to form Cu Using this functional definition the reduction of Cu the glucose on the right is designated as the reducing end of the disaccharide molecule because it can participate in a reduction reaction In contrast the glucose on the left represents the nonreducing Figure 7 end because the 01 carbon is part of the d14 linkage 60420 5CH20H and cannot form the open chain structure in a Cu2 reduction reaction Since maltose contains one reducing end it is called a reducing sugar Disaccharides can contain different monosaccharide units connected through 0c or 5 glycosidic bonds Lactose involving ring carbons and therefore it is convenient to Gall 131 gt4GIc name disaccharides using a descriptive nomenclature Using standard conventions the disaccharide is named by first listing the nonreducing monosaccharide on the left followed by the glycosidic linkage between the two monosaccharides and then the monosaccharide on the right V th this shorthand nomenclature maltose can be described as a GlC0L1 4Glc disaccharide in which the abbreviation quotGlcquot is used for glucose Figure 7 shows the structures of three common disaccharides found in nature 1 lactose also called milk sugar which contains a 514 glycosidic bond linking a galactose Gal to a glucose to form Gal51 4Glc 2 sucrose made in plants and used as table sugar in its crystalline form contains fructose Fru linked to glucose through the two anomeric carbons to Trehalose form Gluoc152Fru and 3 trehalose a glucose Gca391egt1aGlc disaccharide made in insects contains a glycosidic bond between the two anomeric carbons to form the disaccharide GlC0L10L1GIC lmportantly of these three disaccharides only lactose is a reducing sugar because like maltose it contains a free anomeric carbon that can interconvert the hemiacetal to an aldehyde Both sucrose and trehalose are nonreducing sugars because they lack a reducing end Sucrose Gca391lt gt28Fru 4 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 Structures of common complex carbohydrates The most abundant carbohydrate molecules found in nature are actually large complex structures consisting of mixtures of monosaccharide derivatives Carbohydrates consisting of several monosaccharides M are called oligosaccharides a designation that could also include disaccharides whereas carbohydrates with 10 or more monosaccharide units are called l quot polysaccharides Polysaccharides can either be l u quotl homopolymeric same repeating monosaccharide unit l or heteropolymeric mixture of monosaccharides We if will first look at several oligosaccharides that share a 39 common structure consisting of galactose units linked to 39 w sucrose and then briefly describe the structure and function of the four major polysaccharides in nature cellulose chitin starch and glycogen Figure 8 shows three representative 39 oligosaccharides found in many types of plants the trisaccharide raffinose the tetrasaccharide stachyose and the pentasaccharide verbascose All three of these x oligosaccharides contain 13 galactose units covalently attached to sucrose These three oligosaccharides are sometimes called the raf nose series oligosaccharides because they are structurally related and produced by a set of enzymes that are found in similar types of plants 39quot quot Humans and nonruminating animals such as pigs and poultry cannot digest these galactooligosaccharides because they lack the necessary 0c galactosidase enzyme needed to hydrolyze the 116 glycosidic bond Eating foods high in raffinose series oligosaccharides can lead to gastrointestinal discomfort flatulence because the undigested carbohydrates end up in the lower intestine where bacteria which do contain ocgalactosidase ferment M the compounds to produce methane carbon dioxide and hydrogen gases The product BeanoT39VI is a commercial 0 HO OH preparation of ocgalactosidase that can be taken as a pill 0WON quot to aid in digestion of these oligosaccharides resulting in H OH O the release of free galactose Sucrose is further Sucrose M111 139quotquot Cellobiose metabolized to glucose and fructose by the enzyme 5 HO ceggljsg WHO OH sucrase found in the small intestine a 0 The most abundant polysaccharide on earth is cellulose a homopolymeric molecule consisting of g 39 ll thousands of repeating glucose units connected by 514 quotquotquot010wtgHoxkgggo vo quotEH07 glycosidic bonds Cellulose provides plants with a rigid HO OHT527 O HO mm 0 cell wall consisting of layers of cellulose fibers that are 39 ll OH held together by hydrogen bonds Figure 9 shows the quotquot39010Avquotquot5Hoyt gf3 5H0 structure of the repeating Glc514Glc unit in cellulose 0H0 0 called cellobiose and a diagram depicting hydrogen Pf if OH OH bonding Within and between cellulose strands lVlost quotSOXXOwCHO Mxo animals lack the enzyme cellulase which IS required to OHHHBJ o HO H nag hydrolyze the 514 glycosidic bonds in cellulose 5 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 Therefore plant material high in cellulose fiber is considered quotroughagequot in the diet because it passes through the digestive system without being degraded Some animals have evolved symbiotic relationships with microorganisms that inhabit their digestive tracts and secrete cellulase Ruminating herbivores plant eating organisms such as cows and goats have an unusual stomach that permits them to regurgitate their food and thereby maximize mechanical and enzymatic breakdown of cellulose with the help of bacteria The released glucose is absorbed by the intestine and used as the primary source of metabolic energy for the animal Termites and moths also depend on the help of symbiotic microorganisms to help digest cellulose in their diets which can unfortunately include houses and sweaters Another abundant linear polysaccharide in nature is chitin the structural component of invertebrate exoskeletons found in insects and crustaceans As seen in figure 10 chitin consists of repeating Nacetylglucosamine units NAG also abbreviated GlcNAc linked by a 314 glycosidic bond The only difference w Chitin between glucose and Nacetylglucosamine is the replacement of the C2 hydroxyl with an acetylated amino group Chitin like cellulose provides the organism with an excellent biomaterial for building a strong body frame by virtue of hydrogen bonding contacts within and between polysaccharide strands Moreover because of the 314 glycosidic bond chitin can only be used as a source of carbohydrate fuel by microorganisms that contain the enzyme chitinase It is estimated that over 1012 metric tons of cellulose and chitin are synthesized each year by plants and invertebrates Plants and animals store glucose in the form of very large polysaccharide glucose homopolymers that contain both oc14 and oc16 glycosidic bonds The glucose homopolymer produced in 7 plants is called starch while the glucose homopolymer produced in animal cells is called glycogen Plants synthesize two forms of starch amylose a linear polysaccharide containing about 100 glucose units linked by oc14 glycosidic bonds and amylopectin a branched polysaccharide containing 100000 glucose units connected by oc14 and oc16 glycosidic bonds About 20 of starch is in the linear amylose form and the rest is M amylopectin Amylose can form stable lefthanded helical structures Ho OH o as a result of intramolecular hydrogen bonding figure 11 Each CHZOH turn of the helix contains six glucose molecules to allow efficient packing of the glucose polymer within starch granules The presence of oc16 H0 bonds in amylopectin and glycogen creates branch points that a1 gt4Iinked greatly increase the number of free 0 Dglucose units ends in the homopolymeric HO molecule Unlike cellulose and chitin starch is a terrific dietary source of glucose for animals because it can be readily hydrolyzed by the enzyme ocamylase which cleaves oc14 glycosidic bonds Starch polymer am ylose CH20H 6 of 12 pages BiOC 460 Dr Miesfeld Fall 2008 Both amylopectin and glycogen contain the same 114 and d1 6 glycosidic bonds however glycogen which is the carbohydrate energy store for animals is much more highly branched and contains up to 106 glucose units per granule As shown in figure 12 amylopectin contains a branch point about once every 25 glucose units forming a sort of tree branch arrangement whereas glycogen has a branch point every 10 glucose residues resulting in a much more compact spiraling macromolecule Since glucose units can only be added and removed from the non reducing ends of amylopectin and glycogen the more branch points there are the more ends that are available for glucose retrieval and storage A second difference between the macromolecular structures of amylopectin and glycogen is that amylopectin contains one free glucose at the reducing end of the quottree branchquot whereas glycogen lacks a free reducing end This is because the glucose residue at the center of the glycogen quotspiralquot is covalently linked to a protein called glycogenin Figure 12 Nonreducing ends Am ylopectin Nonreducing ends Carbohydrates are often covalently attached to polypeptides The simplest type of protein glycoconjugate is a glycoprotein which consists of a small number of carbohydrate units oligosaccharides covalently attached to a core protein Oligosaccharide modification of proteins takes place within the lumen of the endoplasmic reticulum compartment of the cell A second general type of protein glycoconjugate is one in which the majority of the macromolecule consists of carbohydrate units with only a small contribution coming from protein Two classes of glycoconjugates are proteoglycans which are found in the extracellular matrix and serve as the quotgelquot around tissues and in joints and peptidoglycans which form the cell wall of bacteria Glycoproteins Many types of membranebound proteins and most secreted proteins are glycoproteins containing covalently linked carbohydrate residues that serve as high affinity binding sites for extracellular proteins figure 13 The best Oligosaccharide Figure 13 Virus chain Plasma membrane protein Glycolipidl it a l Enzyme b quot Glycogen Bacterium d Mannose 6 phosphate x receptorllectin f Mannose 6 phosphate residue on newly synthesized protein Trans Golgi Reducing 39 end Nonreducing ends Lymph Pselectin r l Enzyme Lysosome 7 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 characterized of these glycoprotein interactions are 39L in the immune system where receptors on certain types of immune cells bind to glycoproteins on the surface of target cells In fact viruses and bacteria Extracellulardomain exploit these glycoprotein binding sites on immune mum cells to gain entry into the cells or to kill the cells though the binding of high affinity toxins Figure 14 shows the molecular structure of the extracellular portion of the human CD2 protein that is expressed on the surface of T cells This glycoprotein contains five sugar residues two N acetylglucosamines and three mannoses that are Asparagine oligosaccharide covalently linked to an asparagine residue in Figure 15 the CD2 polypeptide Protein gycosyation is aloIinked blNIinked highly specific and requires the activity of We f gycosyating enzymes called ch f HOCHz o c gycosytransferases H0 H H 0 H H NH g CH2 CH As shown in figure 15 carbohydrate H on H 0 CHZ CH 0 mi 4 H quotquot linkage to glycoproteins occurs through either H NH NH J H NH the amide nitrogen atom of asparagine N o o linked oligosaccharides or through the GaINAEHs Ser GkNACCHa As oxygen atom of serine or threonine residues Examples Examples Olinked oligosaccharides Most all N 5 linked glycoproteins have a pentasaccharide l 5 geolg GlcNAc core consisting of an Nlinked N 325quot acetylglucosamine NAG followed by a E ONeuSAc second NAG residue linked to three mannose WE gmk residues as seen in the CD2 protein Additional carbohydrate residues are often attached to these the branched mannose sugars to generate a variety of glycoprotein structures It has been found that gycosytransferases that attach the Nlinked NAG residue to asparagine recognize the tripeptide AsnXSerThr in the protein sequence where X is any amino acid except proline Olinked glycoproteins often contain a Figure 16 mannose sugar linked directly to a threonine O or serine residue figure 15 a 6 s5 0 3 5 g 0 Genetic differences in the expression 0 of and activity of gycosytransferases accounts g lTl is 7 7 for immunological incompatibility between Q a a individuals One of the best examples of this 0 is the biochemical basis forthe A B and O U C q i O blood groups in humans As illustrated in trof 0 O figure 16 all three blood rou s have a 23 V 39 k 9 l0 395 IV LIV common core oligosaccharide linked to protein or lipid molecules on the surface of red blood cells This oligosaccharide called the H Nacclylgalaclosamine Fumst antigen is further modified by the AIB Legend Rammed gycosytransferase a13N e Nacetylglucosamine Galaclosc acetylgalactosaminyltransferase enzyme 8 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 Individuals who express the A variant ofthis glycosyltransferase are able to add an extra GalNAc residue to the terminal galactose of the pentasaccharide whereas individuals with the B variant add an extra galactose residue in this same position The A and B variants ofthe Figure 17 glycosyltransferase enzyme differ by four amino Glywsaminoglycan Repeating disaccharide acids that function to specify the carbohydrate Egmg zgges substrate used in the glycosylation reaction A third per chain CH20H type of N8 glycosyltransferase the O variant is a H H 0 nonfunctional mutant form of the enzyme 0390 Ho H H Individuals with the O variant contain unmodi ed Hyalurona e H H 0 H NH 1 7 4 forms of the H antigen oligosaccharide on the quot5 39 0 H H CI0 surface of their red blood cells H OH 5 33 GIcA GlcNAc Proteoglycans An important class of polysaccharides found in 2 connective tissue and the extracellular matrix are 39 35 H o the glycosaminoglycans These unbranched Chondmnm 0 0 H H H polysaccharide chains consists of repeating 4sulfate H o H M quot1 4 disaccharide units made up of modi ed 20 OH H m 393 clo monosaccharides one of which is either a H OH le derivative of NAG or GalNAc The largest 6ch GaINAc4S glycosaminoglycan is hyaluronic acid which has up to 50000 disaccharide units of Dglucoronic 020 quot205003 0 acid linked to NAG through a 513 glycosidic bond Kama H0 H o H H H as shown in gure 17 The disaccharides in 5quotquot6 9 H 0 114 hyaluronic acid are themselves linked together 25 H Him 4 if through 514 glycosidic bonds Hyaluronic acid is quot i0 highly hydrated and functions as a lubricant in joints Ga GlcNAcc synovial fluid and is also found in the vitreous humor ofthe eye clear gel inside the eyeball Other glycosaminoglycans include keratan sulfate 3 H2050 chondroiton 4sulfate and heparin each ofwhich H H H 0 H are negatively charged polysaccharides containing Heparin H cooo 050 quotquot quot l o o anIonIc sulfhydral and carboxyl groups on the 1590 H H mo disaccharide repeating unit Heparin is an example H 050 awn of highly sulfated glycosaminoglycan that is present GIcAZS or IdoAZS G39ltN53565 in granules of special cells in the circulatory system called mast cells Release of heparin from mast cells prevents blood clotting by the binding of the negativelycharged heparin to proteins that initiate the clotting cascade Puri ed heparin is used as an anticoagulant in clinical laboratories that need to store and process blood M products Figure 18 shows the molecular structure of a portion of heparin where the high density of sulfate groups in the repeating disaccharide can be seen as yellow atoms 9 of 12 pages BiOC 460 Dr Miesfeld Fall 2008 Glycosaminoglycans are covalently or noncovalently linked to proteins to form glycoconjugates called proteoglycans Some types of proteoglycans are membrane bound proteins with glycosaminoglycans attached to the extracellular domain of the protein as shown in figure 19 In this example a modified version of heparin called heparin sulfate is covalently attached to the protein along with the glycosaminoglycan chondroiton sulfate Glycosaminoglycans are often attached to membrane proteins via a trisaccharide linker that has an Olinked glycosidic bond to a serine or threonine residue in the protein The glycosaminoglycan portion of proteoglycans serve as binding sites for a variety of extracellular molecules primarily Figure 19 Heparan V NH3 v sulfa ng s ma a te M Chondroitin 39 sulfate 1 Gly X Hyaluronate l up to 50000 3 1 gt3 164 B1 gt3ll6 1 8 B 1 gt4 Gly repeat disaccharides l l Wquot 215 l 39 Gal gt Gal gt Xyl gt Ser Keratan Chondroitin sulfate sulfate Core protein gt Chondroitin sulfate 4 7 Link Amino terminus 9 proteins Aggrecan proteins in the extracellular matrix but also receptors on other cells Proteoglycans can be secreted directly into the extracellular matrix where they form large aggregates often associated with hyaluronic acid Proteins noncovalently attached to hyaluronic acid serve as anchors for covalently bound oligosaccharides and glycosaminoglycans The glycosaminoglycans keratan sulfate and chondroiton sulfate can be covalently attached to the core protein through an oligosaccharide linker that is Olinked to the core protein at a serine or threonine residue Peptidoglycans Bacterial cell walls are rigid structures that give 9F39 Ure 20 NAcetylglucosamine bacteria their shape and serve as the physical GkNAC boundary for bacterial membranes thereby protecting the cell from osmotic pressure and lysis N Acetylmuramic acid Mur2Ac The bacterial cell wall consists of multiple strands of Site 1 a linear polysaccharide made up of repeating units cleavage by of a 314 linked disaccharide containing NAG and Ivsozvrne Nacetylmuramic acid NAM These NAGNAM polysaccharide strands are tethered together by Reducing peptide linkages to form a type of glycoconjugate 9quot called a peptidoglycan The structure of the Pentaglycine peptidoglycan cell wall in the bacteria strain crossnnk Staphylococcus aureus is shown in figure 20 Tetrapeptide linkers consisting of both L and D amino acid stereoisomers connect NAM residues in adjacent strands In S aureus the NAGNAM strands are interconnected by pentaglycine bridges which further strengthens the proteoglycan structure 10 of 12 pages Bioc 460 Dr Miesfeld Fall 2008 Based on differences in the higher order organization of the NAGNAM polysaccharide strands and on the presence or absence of an outer membrane bacteria can be divided into two groups 1 grampositive bacteria which have a thick peptidoglycan cell wall 250 A but no outer membrane and 2 gramnegative bacteria which have a thin peptidoglycan cell wall 25 A surrounded by a protective outer membrane called a capsule figure 21 The terms grampositive and gramnegative refer to a laboratory assay Figure 21 first described in 1884 by Christian Gram a Danish bacteriologist who developed a simple test to differentiate between nonpathogenic bacteria grampositive and the pathogenic bacteria Kebsiela pneumonia gramnegative which causes clinical pneumonia The biochemical basis for the Gram test is the differential ability of heattreated bacteria to retain an indicator dye crystal violet after washing the cells with ethanol or acetone Grampositive bacteria which lack an outer membrane are colorstained by this procedure whereas gramnegative bacteria remain colorless because the dye cannot penetrate the outer cell membrane The Scottish bacteriologist Alexander Fleming discovered the antibiotic penicillin in 1929 when he noticed that a mold on one of Outer membrane Peptidoglyclan 39 cell wall Gram negative Gram positive his Staphylococcus aureus bacterial M plates was killing the bacteria The mold R H S was identified as Penicilium notatum and I the antibacterial agent it secreted was Penicillin o g named penicillin figure 22 Penicillin o0H inhibits bacterial enzymes called ow transpeptidases that are required for H H peptidoglycan synthesis and thereby kills N 33 I bacteria that depend on high rates of cell 60 o Jlt wall synthesis for cell division Since 0 r a Methicimn HOFO Methicillinresistant S aureus MRSA transpeptidase penicillin is an ideal antibiotic because it is highly selective for its bacterial target and has few side effects Some bacteria are resistant to penicillin because they produce an enzyme called Blactamase that hydrolyzes the 3 lactam ring in penicillin to inactivate it This type of penicillinresistance has been overcome by developing synthetic compounds such as methicillin that block transpeptidase activity without being substrates for Blactamase However because of the widespread M use of methicillin particularly in hospitals where bacterial infections are treated aggressively a methicillinresistant strain of aureus 3 called MRSA has emerged that expresses a variant form of the i i i transpeptidase enzyme This transpeptidase does not recognize I methicillin as a substrate and it is ineffective in blocking Sf39 j peptidoglycan synthesis and bacteria survive A MRSA infection can L 39 be very serious because it is resistant to treatment figure 23 11 of 12 pages Bloc 460 Dr Miesfeld Fall 2008 ANSWER TO KEY QUEsrrons about carbohydrate structure and function Describe three ways in which carbohydrates contribute to cell structure and function Carbohydrates contribute to cell structure and function in three major ways 1 highlybranched glucose polymers in the form of starch and glycogen particles function to store metabolic energy for the cell 2 modi ed carbohydrate residues serve as intercellular signaling moieties when covalentlylinked to cell surface glycoproteins 3 bacterial and plant cell walls consist of cross linked carbohydrates that form a strong structural barrier preventing cell lysis due to osmotic stress Branchpoints in glucose polymers contained in plant and animal cells provide a means to generate large numbers of free ends nonreducing ends that are subject to rapid degradation and synthesis through highlyregulated enzymatic processes Many types of complex intercellular signaling interactions are mediated by glycoproteins which consists of a small number of carbohydrate units oligosaccharides covalently attached to proteins Glycoproteins are either inserted into the plasma membrane where they function as extracellular signaling molecules ligands or receptors or they are secreted Plant cell walls contain large amounts of cellulose consisting of glucose polymers with 514 glycosidic bonds whereas bacterial cell walls contain multiple strands ofa linear polysaccharide tethered together by peptide linkages to form a glycoconjugate called a peptidoglycan 12 of 12 pages
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