Cellulose is a biofuel Lecture 31 - Carbohydrate Structure Key...

Bioc 460 - Dr. Miesfeld Fall 2008

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Figure 1.

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 soybean-based feed because they lack the enzyme α-galactosidase. To prevent dietary problems associated with the inability to digest these oligosaccharides, soybean feed is pre-treated with a commercially-produced α-galactosidase enzyme. This same α-galactosidase enzyme preparation (isolated from a fungus) is marketed commercially as a dietary product for humans called BeanoTM. Raffinose oligosaccharides are found in many types of vegetables, e.g., 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 of glucose found in plant cell walls. The word "carbohydrate" comes from the term carbon hydrate which describes the empirical formula for carbohydrates, (CH2O)n, in which n > 3. However, as we saw in the Calvin cycle, carbon hydration is not an accurate description for carbohydrate biosynthesis since the formation of glyceraldehyde-3-phosphate (GAP) is the result of enzymatic reactions catalyzing CO2 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 of glucose, that are covalently linked to proteins giving rise to glycoproteins, or attached to lipids forming glycolipids.

Simple sugars are defined 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 C6H12O6. Glucose is a polyhydroxy aldehyde, while fructose, which has the same molecular formula as glucose, is a polyhydroxy ketose. The chemical structure of glucose is shown in figure 1 using either Fisher projections that have flattened bond angles to represent linear monosaccharides, or Haworth perspectives which are used to illustrate the cyclic structure of monosaccharides. Glucose can also be drawn using a conformational formula in which


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Figure 2.

Figure 3.

glucose is represented by either the "boat" or "chair" 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 processed foods and beverages. The sensation of sweetness is the result of ligand activation of G protein-coupled receptors expressed on the surface of gustatory cells (taste cells in the tongue). The G protein-coupled receptors in these cells bind sugars with differential affinities based on their chemical structure. Sugar binding to taste receptors stimulates a neuronal signal that is transmitted to the brain. Human taste tests can be used to measure the relative sweetness of simple sugars and artificial sweeteners (figure 2). For example, fructose, which is found 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 the second carbon as in fructose (figure 3). Aldehyde-containing monosaccharides are called aldoses and ketone-containing monosaccharides are called ketoses. All monosaccharides have a CH2OH group on the end of the carbon chain opposite the aldehyde or ketone group, and each of the carbons in the middle have OH groups and function as chiral centers. The carbonyl atom (C=O) is either at the end of the carbon chain (aldose) or on the second carbon (ketose). The name of the monosaccharide reflects the number of carbons in the chain, for example, an aldose with three carbons is a triose, four carbons a tetrose, five carbons a pentose, and six carbons a hexose.


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Figure 4.

Figure 5.

The smallest monosaccharide is glyceraldehyde, a triose sugar with one chiral center. A carbon chiral center is an atom with four different functional groups. Chiral compounds lack a plane of symmetry and exist as two optical isomers, also called enantimers. Enantimers exist in nature as either right handed (D form) or left-handed (L form) and differentially reflect polarized light. The structures D-glyceraldehyde and L-glyceraldehyde are shown in figure 4 where it can be seen that these two isomers of glyceraldehyde are mirror images of each other. By convention, when the hydroxyl group in the chiral carbon is on the right side of a Fisher projection, it is the D isomer, and when it is on the left side, it is the L isomer. Monosaccharides of five, six or seven carbons are often more stable in aqueous solution as cyclic structures 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 C-5 hydroxyl group of D-glucose attacks the oxygen atom of the C-1 aldehyde group to form a cyclic hemiacetal. In this conformation, the C-1 carbon of D-glucose becomes the new chiral center, and as such, cyclic forms of glucose exists as either β-D-glucose with the hydroxyl group at C-1 above the plane of the ring, or as α-D-glucose with the hydroxyl group below the plane of the ring. Crystalline glucose exists in the cyclic α-D-glucose form, but once it is dissolved in an aqueous solution, an equilibrium is established between the α-D-glucose and β-D-glucose conformations at a ratio of about 40:60 for α-D-glucose:β-D-glucose. A very small amount of the monosaccharide is found in the open chain conformation (<0.05%).

The hemiacetal C-1 carbon of cyclic D-glucose is called an anomeric carbon, and β-D-glucose and α-D-glucose are referred to as anomers because they only differ at the anomeric carbon. Cyclic conformations of hexose sugars are called pyranoses because the six-membered ring is similar to a pyran compound. Therefore cyclic forms of glucose are sometimes called α-D-glucopyranose and β-D-glucopyranose. Ketoses such as fructose can also form cyclic structures, but because the carbonyl is in the C-2 position of the open chain, the ring that forms contains only five carbons. These sugars are furanoses because they resemble a furan. Cyclic conformations of fructose are called α-D-fructofuranose and β-D-fructofuranose. Note that pyranose rings are much more stable in solution than furanose rings.


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Figure 6.

Figure 7.

Disaccharide sugars are formed by a condensation reaction between two monosaccharides. The covalent linkage is called an O-glycosidic bond and represents the formation of an acetal from a hemiacetal and an alcohol. The glycosidic bond in the disaccharide maltose is called an α1-4 linkage because the anomeric carbon is in the α conformation (figure 6). The glucose molecule on the right retains the hemiacetal structure at its C-1 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 Cu2+), 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 end because the C-1 carbon is part of the α1-4 linkage 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 α or β glycosidic bonds involving ring carbons, and therefore, it is convenient to 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. With this shorthand nomenclature, maltose can be described as a Glc(α1-4)Glc disaccharide in which the abbreviation "Glc" 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 β1-4 glycosidic bond linking a galactose (Gal) to a glucose to form Gal(β1-4)Glc, 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 form Glu(α1-β2)Fru, and 3) trehalose, a glucose disaccharide made in insects, contains a glycosidic bond between the two anomeric carbons to form the disaccharide Glc(α1-α1)Glc. Importantly, 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.


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Figure 8.

Figure 9.

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 are called oligosaccharides, a designation that could also include disaccharides, whereas, carbohydrates with ~10 or more monosaccharide units are called polysaccharides. Polysaccharides can either be homopolymeric (same repeating monosaccharide unit) or heteropolymeric (mixture of monosaccharides). We will first look at several oligosaccharides that share a common structure consisting of galactose units linked to 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 oligosaccharides found in many types of plants; the trisaccharide raffinose, the tetrasaccharide stachyose and the pentasaccharide verbascose. All three of these oligosaccharides contain 1-3 galactose units covalently attached to sucrose. These three oligosaccharides are sometimes called the raffinose series oligosaccharides because they are structurally related and produced by a set of enzymes that are found in similar types of plants. Humans and non-ruminating animals such as pigs and poultry cannot digest these galacto-oligosaccharides because they lack the necessary α-galactosidase enzyme needed to hydrolyze the α1-6 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 α-galactosidase, ferment the compounds to produce methane, carbon dioxide and hydrogen gases. The product BeanoTM is a commercial preparation of α-galactosidase that can be taken as a pill to aid in digestion of these oligosaccharides resulting in the release of free galactose. Sucrose is further metabolized to glucose and fructose by the enzyme sucrase found in the small intestine. The most abundant polysaccharide on earth is cellulose, a homopolymeric molecule consisting of thousands of repeating glucose units connected by β1-4 glycosidic bonds. Cellulose provides plants with a rigid cell wall consisting of layers of cellulose fibers that are held together by hydrogen bonds. Figure 9 shows the structure of the repeating Glc(β1-4)Glc unit in cellulose, called cellobiose, and a diagram depicting hydrogen bonding within and between cellulose strands. Most animals lack the enzyme cellulase which is required to hydrolyze the β1-4 glycosidic bonds in cellulose.


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Figure 10.

Figure 11.

Therefore, plant material high in cellulose fiber is considered "roughage" 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 N-acetylglucosamine units (NAG, also abbreviated GlcNAc) linked by a β1-4 glycosidic bond. The only difference between glucose and N-acetylglucosamine is the replacement of the C-2 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 β1-4 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 α1-4 and α1-6 glycosidic bonds. The glucose homopolymer produced in 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 α1-4 glycosidic bonds, and amylopectin, a branched polysaccharide containing ~100,000 glucose units connected by α1-4 and α1-6 glycosidic bonds. About 20% of starch is in the linear amylose form and the rest is amylopectin. Amylose can form stable left-handed helical structures as a result of intramolecular hydrogen bonding (figure 11). Each turn of the helix contains six glucose molecules to allow efficient packing of the glucose polymer within starch granules. The presence of α1-6 bonds in amylopectin (and glycogen) creates branch points that greatly increase the number of free ends in the homopolymeric molecule. Unlike cellulose and chitin, starch is a terrific dietary source of glucose for animals because it can be readily hydrolyzed by the enzyme α-amylase which cleaves α1-4 glycosidic bonds.


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Figure 12.

Figure 13.

Both amylopectin and glycogen contain the same α1-4 and α1-6 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 "tree branch," whereas, glycogen lacks a free reducing end. This is because the glucose residue at the center of the glycogen "spiral" is covalently linked to a protein called glycogenin. 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 "gel" around tissues and in joints, and peptidoglycans which form the cell wall of bacteria. Glycoproteins Many types of membrane-bound 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


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Figure 14.

Figure 15.

Figure 16.

characterized of these glycoprotein interactions are 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 exploit these glycoprotein binding sites on immune 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

covalently linked to an asparagine residue in the CD2 polypeptide. Protein glycosylation is highly specific and requires the activity of glycosylating enzymes called glycosyltransferases. As shown in figure 15, carbohydrate linkage to glycoproteins occurs through either the amide nitrogen atom of asparagine (N-linked oligosaccharides), or through the oxygen atom of serine or threonine residues (O-linked oligosaccharides). Most all N-linked glycoproteins have a pentasaccharide core consisting of an N-linked N-acetylglucosamine (NAG), followed by a second NAG residue linked to three mannose 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 glycosyltransferases that attach the N-linked NAG residue to asparagine recognize the tripeptide Asn-X-Ser/Thr in the protein sequence where X is any amino acid except proline. O-linked glycoproteins often contain a mannose sugar linked directly to a threonine or serine residue (figure 15). Genetic differences in the expression and activity of glycosyltransferases accounts for immunological incompatibility between individuals. One of the best examples of this is the biochemical basis for the A, B and O blood groups in humans. As illustrated in figure 16, all three blood groups have a common core oligosaccharide linked to protein or lipid molecules on the surface of red blood cells. This oligosaccharide, called the H antigen, is further modified by the A/B glycosyltransferase (α1-3-N-acetylgalactosaminyltransferase) enzyme.


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Figure 17.

Figure 18.

Individuals who express the A variant of this 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 of the glycosyltransferase enzyme differ by four amino acids that function to specify the carbohydrate substrate used in the glycosylation reaction. A third type of A/B glycosyltransferase, the O variant, is a nonfunctional mutant form of the enzyme. Individuals with the O variant contain unmodified forms of the H antigen oligosaccharide on the surface of their red blood cells. Proteoglycans An important class of polysaccharides found in connective tissue and the extracellular matrix are the glycosaminoglycans. These unbranched polysaccharide chains consists of repeating disaccharide units made up of modified monosaccharides, one of which is either a derivative of NAG or GalNAc. The largest glycosaminoglycan is hyaluronic acid which has up to 50,000 disaccharide units of D-glucoronic acid linked to NAG through a β1-3 glycosidic bond as shown in figure 17. The disaccharides in hyaluronic acid are themselves linked together through β1-4 glycosidic bonds. Hyaluronic acid is highly hydrated and functions as a lubricant in joints (synovial fluid) and is also found in the vitreous humor of the eye (clear gel inside the eyeball). Other glycosaminoglycans include keratan sulfate, chondroiton 4-sulfate and heparin, each of which are negatively charged polysaccharides containing anionic sulfhydral and carboxyl groups on the disaccharide repeating unit. Heparin is an example of highly sulfated glycosaminoglycan that is present 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 negatively-charged heparin to proteins that initiate the clotting cascade. Purified heparin is used as an anti-coagulant in clinical laboratories that need to store and process blood 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.


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Figure 19.

Figure 20.

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 O-linked 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

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 O-linked to the core protein at a serine or threonine residue. Peptidoglycans Bacterial cell walls are rigid structures that give bacteria their shape and serve as the physical boundary for bacterial membranes, thereby protecting the cell from osmotic pressure and lysis. The bacterial cell wall consists of multiple strands of a linear polysaccharide made up of repeating units of a β1-4 linked disaccharide containing NAG and N-acetylmuramic acid (NAM). These NAG-NAM polysaccharide strands are tethered together by peptide linkages to form a type of glycoconjugate called a peptidoglycan. The structure of the peptidoglycan cell wall in the bacteria strain 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 NAG-NAM strands are interconnected by pentaglycine bridges which further strengthens the proteoglycan structure.


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Figure 21.

Figure 22.

Figure 23.

Based on differences in the higher order organization of the NAG-NAM polysaccharide strands and on the presence or absence of an outer membrane, bacteria can be divided into two groups, 1) gram-positive bacteria which have a thick peptidoglycan cell wall (250 Å) but no outer membrane, and 2) gram-negative bacteria which have a thin peptidoglycan cell wall (25 Å) surrounded by a protective outer membrane called a capsule (figure 21). The terms gram-positive and gram-negative refer to a laboratory assay first described in 1884 by Christian Gram, a Danish bacteriologist, who developed a simple test to differentiate between non-pathogenic bacteria (gram-positive), and the pathogenic bacteria Klebsiella pneumonia (gram-negative) which causes clinical pneumonia. The biochemical basis for the Gram test is the differential ability of heat-treated bacteria to retain an indicator dye (crystal violet) after washing the cells with ethanol or acetone. Gram-positive bacteria, which lack an outer membrane, are color-stained by this procedure, whereas, gram-negative 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 his Staphylococcus aureus bacterial plates was killing the bacteria. The mold was identified as Penicillium notatum and the antibacterial agent it secreted was named penicillin (figure 22). Penicillin inhibits bacterial enzymes called transpeptidases that are required for peptidoglycan synthesis, and thereby kills bacteria that depend on high rates of cell wall synthesis for cell division. Since animal cells don't have a cell wall or 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 β-lactamase that hydrolyzes the β lactam ring in penicillin to inactivate it. This type of penicillin-resistance has been overcome by developing synthetic compounds such as methicillin that block transpeptidase activity without being substrates for β-lactamase. However, because of the wide-spread use of methicillin, particularly in hospitals where bacterial infections are treated aggressively, a methicillin-resistant strain of S. aureus, called MRSA, has emerged that expresses a variant form of the transpeptidase enzyme. This transpeptidase does not recognize methicillin as a substrate and it is ineffective in blocking peptidoglycan synthesis and bacteria survive. A MRSA infection can be very serious because it is resistant to treatment (figure 23).


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ANSWER TO KEY QUESTIons 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) highly-branched glucose polymers in the form of starch and glycogen particles function to store metabolic energy for the cell, 2) modified carbohydrate residues serve as intercellular signaling moieties when covalently-linked 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 (non-reducing ends) that are subject to rapid degradation and synthesis through highly-regulated 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 β1-4 glycosidic bonds, whereas, bacterial cell walls contain multiple strands of a linear polysaccharide tethered together by peptide linkages to form a glycoconjugate called a peptidoglycan.

Cellulose is a biofuel Lecture 31 - Carbohydrate Structure Key...

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