Chapter 13

Identify the unique characteristics of cellulose and chitin as well as the characteristics common to both.

Cellulose and chitin are both structural polysaccharides. Cellulose is a glucose homopolymer with the monosaccharide residues linked by a ß-1,4 glycosidic bond and individual strands that hydrogen bond to generate a rigid structure as part of the plant cell wall. Chitin is also a a homopolymer but the repeating monosaccharide unit is N-acetylglucosamine. Like cellulose, the monosaccharide units are linked by a ß-1,4 glycosidic bond and individual chitin strands hydrogen bond to adjacent strands, providing strength to the exoskeletons of many invertebrates.

Glucose and galactose are components of many disaccharides, oliogsaccharides, and polysaccharides. Sort these common carbohydrates to indicate if they contain glucose, galactose, or both.

Glucose can be found in many different carbohydrate structures, including disaccharides like maltose and sucrose and polysaccharides like starch. Both maltose and starch are homopolymers and contain only glucose, whereas sucrose also contains a fructose residue. Galactose is a component of all milk sugars like lacto-N-tetraose. These carbohydrates can contain modified residues like N-acetylglucosamine, but do not contain unmodified glucose. Glucose and galactose can both be found in the disaccharide lactose. Raffinose is composed of a galactose residue and a sucrose unit, which means it contains both glucose and galactose.

Why is penicillin considered a suicide inhibitor?

It forms an irreversible covalent bond with its enzyme target. Penicillin forms a covalent bond with transpeptidase and inactivates the enzyme. Because this bond is irreversible, it prevents the enzyme from catalyzing any furthur reactions and the cell is unable to overcome this inhibition.

The three N-linked glycans in the illustration are all present on a single glycoprotein. When the protein is treated with peptide N-glycosidase F, all of the glycan groups are released from the protein and are now present in a mixed solution. What method(s) could be used to determine the structure and composition of each glycan group?

Mass spectrometry can be used to determine that there are three different glycan groups based on their different masses. The three different glycans could then be isolated and the purified samples could each be treated with a series of sugar-specific glycosylases. The resulting cleavage products can be analyzed by liquid chromatography to determine the arrangement of sugars. Mass spectrometry can again be used to confirm this identity by determining the mass of the cleavage products.

Identify these carbohydrates as simple sugars or polysaccharides.

Simple sugars can be mono-, di-, or oligosaccharides like glucose, lactose, and raffinose. Although some oligosaccharides can contain up to 22 residues, polysaccharides contain significantly more. Glycogen and starch are both glucose homopolymers, whereas chitin is a hexosamine. The number of residues in polysaccharides differs according to type and the cell in which they are found.

The structure and composition of glycoconjugates are often illustrated with symbols, as shown, to represent individual residues. The color and shape of these symbols has been specified by the Consortium for Functional Glycomics (CFG). Unmodified sugars are shown as circles, whereas modified sugars are represented by squares, diamonds, triangles, stars, and hexagons. Modified and unmodified sugars that differ only by the presence of a single modified position (e.g., glucose, N-acetylglucosamine, and glucaronic acid) would be represented by different shapes but all would share a common color. At which position(s) are modified sugars found in this glycan structure?

The blue squares and purple diamonds represent modified sugars, whereas the unmodified sugars are indicated by the yellow and green circles.

Starch and glycogen are both which of the following?

-Glucose homopolymers -Short-term forms of energy storage Glycogen and starch are both polysaccharides formed from glucose homopolymers. Glycogen is a short-term energy storage molecule for animals, whereas starch is also used an an energy source for plants.

Which of the following statements are true regarding individuals with type AB blood?

-AB individuals are universal plasma donors. -AB individuals are universal recipients of red blood cells. To receive donated red blood cells, the recipient must not express antibodies to the antigens on the donated cells. Because AB individuals have both A and B antigens on their red blood cells, they do not make antibodies to either antigen and so can receive blood from type A, B and O individuals. The lack of antibodies also allows these individuals to be universal plasma donors.

Some types of wheat have been genetically manipulated to change the starch biosynthesis pathway. As a result, the modified wheat synthesizes more amylose than the unmodified wheat. Amylose is known as a resistant starch, meaning it resists digestion in the stomach and small intestine. Resistant starch is thought to be beneficial to the digestive system, in the same way that dietary fiber is beneficial. Why is amylose a resistant starch, whereas amylopectin is not?

-Amylose contains only α-1,4 glycosidic bonds, whereas amylopectin contains both α-1,4 and α-1,6 glycosidic bonds. The primary difference in the structures of amylose and amylopectin is the presence of α-1,6 branch points in amylopectin. These branch points give amylopectin significantly more free ends than amylose. Because hydrolysis occurs at these nonreducing ends, the increased number of nonreducing ends allows the glucose residues to be released more readily from amylopectin than from amylose. In addition, the lack of branches in amylose allows the formation of helical structures that limit access to the nonreducing end and furthur increase the difficulty of digestion.

The structure and composition of glycoconjugates are often illustrated with symbols, as shown, to represent individual residues. The color and shape of these symbols has been specified by the Consortium for Functional Glycomics (CFG). Unmodified sugars are shown as circles, whereas modified sugars are represented by squares, diamonds, triangles, stars, and hexagons. Modified and unmodified sugars that differ only by the presence of a single modified position (e.g., glucose, N-acetylglucosamine, and glucaronic acid) would be represented by different shapes but all would share a common color. The structure shown above is an N-linked glycan. What characteristic(s) of the structure confirm it as an N-linked glycan rather than an O-linked glycan?

-The oligosaccharide is linked to the glycoprotein by a glycosidic bond with an asparagine residue. -The monosaccharide linked directly to the amino acid is N-acetylglucosamine. -The structure contains a core group composed of N-acetylglucosamine and mannose in a specific arrangement characteristic of N-linked glycans. The identification of this structure as an N-linked an can first be made by the identity of the amino acid in the glycosidic bond. N-linked oligosaccharides are generally linked to asparagine residues, whereas O-linked glycans are formed through glycosidic bonds with serine or threonine residues. Next, the composition of the glycan can be used to confirm identity. N-linked glycoproteins all contain a five-sugar core glycan group consisting of GlcNAc2Man3, whereas the core of an O-linked oligosaccharide is more varible but generally contains an N-acetylgalactosamine.

Which of the following statements correctly describe both amylopectin and glycogen?

-They are homopolymers of glucose. -α-1,6 glycosidic bonds are found at branch points. Glycogen and amylopectin are both oligosaccharides composed of multiple glucose residues. In both structures, branches contain α-1,6 glycosidic bonds, with branches in glycogen occuring every 8-12 residues, whereas amylopectin branches occur less frequently. The reducing end differs, with amlyopectin containing a single glucose residue, while glycogenin serves as a protein anchor for glycogen.

Which of the following are functions of proteoglycans?

-facilitating cell migration -sequestering cytokines -regulating the permeability of some membranes Proteoglycans can serve many distinct functions, both on the cell surface and in the extracellular matrix. Their varied structure influences their properties. The protein-binding ability of proteoglycans allows them to bind and sequester cytokines as well as facilitate cell migration and regulate membrane permeability.

An individual has type B blood. Which type of blood products can the individual safely receive?

-whole blood from a donor with type B blood -red cells from a donor with type O or type B blood -plasma from a donor with type AB or type B blood An individual with type B blood could receive whole blood from another type B individual. These individuals would have the B antigen on the surface of their red blood cells and the anti-A antibody in their plasma. Therefore, they could receive red cells only from individuals without the A antigen (i.e., B or O) and plasma from individuals without the anti-B antibody (i.e., B or AB).

If one individual has two copies of GTA and one has one copy of GTB, what are the possible blood types of their offspring?

AB or A Blood type is determined by the expression of glycosyltransferases. Individuals with the GTA gene express the glycoslytransferase necessary for generating the A antigen on the surface of red blood cells. Likewise, individuals with the GTB gene express the enzyme necessary for generating the B antigen. An individual with two copies of GTA would pass one along to the offspring. In this case, the blood type is determined by inheritance from the heterozygous parent. If the GTB gene is inherited along with the GTA gene from the homozygous parent, both glycosyltransferases will be expressed and the child will have type AB blood. However, if the type B parent does not pass along the GTB gene, the child will inherit only GTA and will have type A blood.

An individual with type A blood donates whole blood. After the donation, the red cells are separated from the plasma before the blood products are used. How does this step affect the number of recipients who can receive blood products from this individual?

After separation, the red cells can be donated to recipients of two different blood types and the plasma can be used with individuals of three different blood types. When the red cells are separated from the plasma, the number of possible compatible recipients increases. As the red cells carry the A antigen, they can be donated to type A or AB individuals. The plasma would carry the anti-B antibody, so it could be donated to type A, AB, or O individuals.

Linear hexosamine polysaccharides such as heparan sulfate, keratan sulfate, and chondroitin sulfate are classified as __________. These particular hexosamines are distinct from other polysaccharides such as chitin due to their ability to form __________ through covalent attachment to proteins.

Glycosaminoglycans Proteoglycans Glycosaminoglycans are a class of linear hexosamine polysaccharides with structural and functional roles within cells. Heparan sulfate, keratan sulfate, and chondroitin sulfate can all form covalent attachments to proteins to form glycoconjugates known as proteoglycans.

The human gut contains a wide variety of microbial organisms known as the microbiome. Many of these organisms are beneficial and contribute to a number of cellular processes, as well as providing protection against pathogenic organisms. Although there are similarities in the gut flora of different individuals, researchers are interested in determining why differences in the microbiome occur and why some organisms are retained in the gut, whereas others are attacked by the immune system of the host. When an immune response occurs, it is thought to be due to an interaction between surface glycans on the organism and host antibodies. Humans are not the only animals to host these microbes, and, as you might expect, there do appear to be differences in the gut flora of different animals. The surface glycan antigens from a variety of microbes can be immobilized on an array. These arrays can then incubated with antibodies isolated from a variety of animals. What type of information could be determined from this experiment?

Identification of microbes that have caused an immune response in each animal Incubation of a glycan array derived from microbe surface antigens with antibodies isolated from different animals would identify any glycan-antibody interactions. The presence of an antibody to a specific glycan indicates that an immune response has been raised previously to that glycan. Performing this same experiment with antibodies isolated from different animals can be used to determine which glycans have led to an immune response. This information can then be used to determine the microbes responsible.

Which of the following are modified monosaccharides frequently found in glycoconjugates?

N-acetylglucosamine and N-acetylmuramic acid are examples of modified monosaccharides frequently found in glycoconjugates. Glucose is a primary fuel source and so is not frequently used as a componant of a glycoconjugate. Fucose can be found in glycoconjugates but is not as abundant as N-acetylglucosamine and N-acetylmuramic acid.

Sort the characteristics of N-linked and O-linked glycans.

N-linked and O-linked glycans have several differences in their core structure and in the common monosaccharides found within each. N-linked glycans share more structural similarity, as all have a core GlcNAc2Man3 group. By contrast, O-linked glycans only have a core GalNAc group. They also differ in the protein attachments, with N-linked glycans conjugated at asparagines, while O-linked are found at serine and threonine residues.

Would penicillin be expected to have an equal ability to kill both Gram-positive and Gram-negative bacteria? Why or why not?

No; the lipopolysaccharide layer surrounding Gram-negative bacteria can inhibit penicillin uptake by these bacteria. Penicillin is not easily able to diffuse across the outer membrane of Gram-negative bacteria. Because Gram-positive bacteria do not have an outer membrane, penicillin is able to more easily enter these cells and block transpeptidase activity.

Individuals with which blood type are the exact opposite in terms of their ability to donate and receive blood products as compared to those with AB blood?

O Individuals with type O blood do not express either the A or B antigen and so have antibodies to both. Because their red cells do not cause an immune response in individuals with any type of red blood cells, they can act as universal red blood cell donors. The lack of antigens on their red blood cells also allows these individuals to be universal plasma recipients.

Label the figure to illustrate the glyconjugate synthesis pathway.

The process of synthesizing a glycoconjugate begins with free monosaccharides that are transported into the cell. After activation, these monosaccharides can now be added to a protein or glycan chain. Once a protein is glycosylated, it can be transported to the cell surface. These glycoconjugates are then able to bind to glycan-specific receptors on other cells.

Beano is a dietary supplement that allows humans to digest oligosaccharides such as stachyose by supplying the enzyme necessary to hydrolyze the α-1,6 glycosidic bond in these carbohydrates. However, other dietary carbohydrates such as amylopectin contain α-1,6 glycosidic bonds and these polysaccharides are easily digested by humans. How is this possible?

The residues connected by the α-1,6 glycosidic bond are different in stachyose and amylopectin. Amylopectin is a glucose homopolymer and the α-1,6 bond can be cleaved by intestinal enzymes with specificity for glucose. Stachyose contains three galactose residues linked by α-1,6 glycosidic bonds, and these bonds cannot be hydrolyzed by the same enzyme.

Which bacterial enzyme is inhibited by penicillin?

Transpeptidase The bacterial cell wall contains a number of peptidoglycans with cross-linked polysaccharide strands. The transpeptidase enzyme catalyzes the cross-linking of these strands. Penicillin inhibits transpeptidase activity and therefore prevents correct formation of the peptidoglycan layer.

Penicillin resistance has been overcome by developing antibiotics like methicillin. However, because of the widespread use of methicillin, particularly in hospitals where bacterial infections are treated aggressively, a methicillin-resistant strain of Staphylococcus aureus called MRSA has emerged. Methicillin-resistant bacteria can be killed with the antibiotic vancomycin, which binds to the terminal D-alanine residues in the terminal peptides of bacterial cell walls and prevents cross-linking. Which of the following best explains why vancomycin can also be used to kill penicillin-resistant bacteria?

Vancomycin would inhibit transpeptidase activity by blocking access to substrates. Vancomycin binds to and blocks the terminal D-alanine residues in the terminal peptides attached to the polysaccharide chains that form the layer. Transpeptidase would normally catalyze cross-linking at these residues to form the peptiodgycan layer. By blocking these residues, vancomycin inhibits the activity of transpeptidase.

Could a bacterium be resistant to both penicillin and methicillin?

Yes; it is possible for a bacterium to express both ß-lactamase and the variant transpeptidase. Bacteria can share resistance genes through lateral gene transfer. Therefore, it is possible for a single bacterium to express both proteins and be resistant to both penicillin and methicillin.

The difference in human blood groups is due to expression of different glycosyltransferases. Individuals with type A blood express _____, whereas those with type B blood express ______. Expression of both glycosyltransferases results in an individual with type ______ blood.

α-1,3-N-acetylgalactosaminyltransferase α-1,3-galactosyltransferase AB Determination of blood type is the result of inheriting a combination of glycosyltransferases. Individuals with type A blood express α-1,3-N-acetylgalactosaminyltransferase, whereas those with type B express α-1,3-galactosyltransferase. Inheritance of genes that code for both enzymes results in a codominant effect in which both glycosyltransferases are expressed and active. These individuals have type AB blood. Individuals with type O blood did not inherit the genes for either glycosyltransferase.