Carbohydrates and Glycobiology Quiz
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Questions and Answers

What type of functional group does glucose have?

  • Ketone functional group
  • Carboxylic acid functional group
  • Aldehyde functional group (correct)
  • Alcohol functional group

    • What is the reason (R,S)-aspartame does not stimulate the sweet receptor?

  • All sites can interact with (R,S)-aspartame
  • It cannot bond with Site B due to lack of hydrogen
  • Site X cannot accommodate its hydrophobic benzene ring (correct)
  • It does not fit Site A due to steric hindrance

    • How many stereoisomers can a molecule with 3 chiral centers have?

  • 2 stereoisomers
  • 4 stereoisomers
  • 8 stereoisomers (correct)
  • 6 stereoisomers

    • How are carbons numbered in a sugar molecule?

    <p>Beginning at the end of the chain near the carbonyl group</p> Signup and view all the answers

    Which statement best describes D isomers of sugars?

    <p>They are configurations identical to D-glyceraldehyde</p> Signup and view all the answers

    What characteristic do enantiomers possess?

    <p>They are mirror images of each other</p> Signup and view all the answers

    In Fischer projection formulas of sugars, which bonds project out of the plane of the paper?

    <p>Bonds drawn horizontally</p> Signup and view all the answers

    What defines L isomers of sugars?

    <p>Configuration at the reference carbon matches with L-glyceraldehyde</p> Signup and view all the answers

    What determines the classification of a sugar as an aldose or ketose?

    <p>The position of the carbonyl carbon</p> Signup and view all the answers

    Which statement correctly describes trioses?

    <p>They are the simplest form of monosaccharides with three carbons.</p> Signup and view all the answers

    Which of the following monosaccharides has a carbonyl group positioned at the end of its carbon chain?

    <p>Glucose</p> Signup and view all the answers

    What is the chemical formula for a hexose?

    <p>C6H12O6</p> Signup and view all the answers

    Which of the following describes a ketose?

    <p>It has a carbonyl group in the middle of the chain.</p> Signup and view all the answers

    Which type of reaction most likely involves stereoisomer sugars?

    <p>Condensation reaction</p> Signup and view all the answers

    What does the term 'epimer' refer to in sugar chemistry?

    <p>Sugars that differ in configuration around one specific carbon</p> Signup and view all the answers

    Which of the following statements is true regarding enzymes that act on sugars?

    <p>They are stereospecific and only act on particular stereoisomers.</p> Signup and view all the answers

    What type of bond is formed when a second alcohol molecule is added to create an acetal or ketal?

    <p>Glycosidic bond</p> Signup and view all the answers

    Which of the following is NOT a product of the reaction between alcohols and aldehydes or ketones?

    <p>Ketones</p> Signup and view all the answers

    The reaction that produces a new chiral center in the formation of hemiacetals and hemiketals occurs at which carbon?

    <p>Carbonyl carbon</p> Signup and view all the answers

    In the context of monosaccharides, what distinguishes anomers from other types of isomers?

    <p>Differing in configuration about the hemiacetal carbon atom</p> Signup and view all the answers

    What effect does mutarotation have on sugar molecules?

    <p>Interconverts α and β anomers</p> Signup and view all the answers

    Which of the following statements is true about the cyclization of monosaccharides?

    <p>Is a reaction leading to hemiacetal or hemiketal formation</p> Signup and view all the answers

    Which configuration is known as trans in stereoisomers of sugar?

    <p>α</p> Signup and view all the answers

    Which of the following describes the structure of hemiacetals?

    <p>Require a single alcohol and an aldehyde or ketone</p> Signup and view all the answers

    What type of glycosidic bonds link glucose residues in cellulose?

    <p>(β1→4)</p> Signup and view all the answers

    Which of the following is a primary role of lectins?

    <p>Cell-cell recognition</p> Signup and view all the answers

    What is the primary challenge in glycobiology?

    <p>To decipher the sugar code of oligosaccharides</p> Signup and view all the answers

    Which characteristic is common in oligosaccharides but not in nucleic acids or proteins?

    <p>Branched structures</p> Signup and view all the answers

    What type of biological processes do selectins mediate?

    <p>Cell-cell recognition and adhesion</p> Signup and view all the answers

    Which of the following is NOT a function of lectins?

    <p>Nutrient absorption</p> Signup and view all the answers

    How do interchain and intrachain hydrogen bonds affect cellulose?

    <p>They create stable fibers.</p> Signup and view all the answers

    Which statement about selectins is false?

    <p>They are exclusively found in the cytoplasm.</p> Signup and view all the answers

    What defines homopolysaccharides?

    <p>They consist only of a single monomeric sugar species.</p> Signup and view all the answers

    Which of the following is NOT a function of homopolysaccharides?

    <p>Serving as extracellular matrix components.</p> Signup and view all the answers

    Which of the following polysaccharides serves as a structural element in animal exoskeletons?

    <p>Chitin</p> Signup and view all the answers

    What is a key characteristic of heteropolysaccharides?

    <p>They contain two or more kinds of monomeric units.</p> Signup and view all the answers

    Which statement correctly describes the biosynthesis of complex polysaccharides?

    <p>Their sequences are determined by the properties of biosynthetic enzymes.</p> Signup and view all the answers

    Which polysaccharide is primarily used for energy storage in animals?

    <p>Glycogen</p> Signup and view all the answers

    What type of support do heteropolysaccharides provide?

    <p>Extracellular support.</p> Signup and view all the answers

    Which of the following is an example of a heteropolysaccharide?

    <p>Pectin</p> Signup and view all the answers

    What type of sugar forms a six-membered ring upon the reaction of the hydroxyl group at C-6 with the keto group at C-2?

    <p>Pyranoses</p> Signup and view all the answers

    Which statement accurately describes reducing sugars?

    <p>They include sugars that contain free aldehyde groups.</p> Signup and view all the answers

    What is the end product of the oxidation of the carbonyl carbon of an aldose?

    <p>An aldonic acid</p> Signup and view all the answers

    Which type of sugar can form a five-membered ring structure?

    <p>Furanoses</p> Signup and view all the answers

    What is the primary reason phosphorylated sugars remain inside the cell?

    <p>Lack of membrane transporters for them</p> Signup and view all the answers

    Which reaction does glucose NOT typically undergo?

    <p>Oxidation by reducing sugars</p> Signup and view all the answers

    What is formed as a stable intramolecular ester from aldonic and uronic acids?

    <p>Lactones</p> Signup and view all the answers

    What does the reduction of Cu2+ to Cu+ during Fehling's test indicate?

    <p>Presence of reducing sugars</p> Signup and view all the answers

    Study Notes

    Carbohydrates and Glycobiology

    • Carbohydrates are aldehydes or ketones with at least two hydroxyl groups, or substances that yield such compounds on hydrolysis. Many carbohydrates have the empirical formula (CH₂O)ₙ.
    • Carbohydrates are produced from CO₂ and H₂O via photosynthesis in plants.
    • They range in size. Examples include glyceraldehyde (MW = 90 g/mol) and amylopectin (MW > 200,000,000 g/mol).
    • Carbohydrates can be covalently linked with proteins and lipids.
    • Carbohydrates fulfill many functions, including: (a list of functions is not provided).

    Classes of Carbohydrates

    • Monosaccharides: Simple sugars; consist of a single polyhydroxy aldehyde or ketone unit. e.g., D-glucose
    • Disaccharides: Oligosaccharides with two monosaccharide units; e.g., sucrose (D-glucose and D-fructose)
    • Oligosaccharides: Short chains of monosaccharide units, or residues, joined by glycosidic bonds.
    • Polysaccharides: Sugar polymers.

    Clicker Question 1

    • A disaccharide is a(n) oligosaccharide with two monosaccharide units.

    Clicker Question 2

    • The position of the carbonyl carbon determines if a sugar is an aldose or a ketose.

    Monosaccharides

    • Monosaccharides have multiple chiral carbons. The configuration of groups around each carbon atom determines how the compound interacts with other biomolecules. Biological evolution selected the D-series for sugars.
    • Sugar stereoisomers arise because many of the carbon atoms to which hydroxyl groups are attached are chiral centers. Enzymes that act on sugars are stereospecific.
    • Monosaccharides have backbones of carbon chains, all linked by single bonds. One carbon is double-bonded to an oxygen atom. Other carbons are bonded to a hydroxyl group.
    • Aldoses have carbonyl groups at an end of the carbon chain (aldehyde group).
    • Ketoses have carbonyl groups at any other position (ketone group).

    Monosaccharide Carbon Backbone

    • 3C = triose
    • 4C = tetrose
    • 5C = pentose
    • 6C = hexose
    • 7C = heptose

    Trioses

    • Simplest monosaccharides; three-carbon backbone.
    • Examples: D-glyceraldehyde (aldose) and dihydroxyacetone (ketose)

    Tetroses & Pentoses

    • Tetroses = four carbon backbone
    • Pentoses = five carbon backbone
      • Examples = D-ribose (aldose) and 2-deoxy-D-ribose (aldose).

    Hexoses & Heptoses

    • Hexose = six carbon backbone
    • Heptose = seven carbon backbone

    Clicker Question 3

    • Glucose is a monosaccharide with a(n) aldehyde functional group and six carbons.

    What Makes Sugar Sweet?

    • TAS1R2 and TAS1R3 encode sweet-taste receptors.
    • Binding of a compatible molecule generates a "sweet" electrical signal in the brain. The interaction requires a steric match.

    Clicker Question 4

    • (R,S)-aspartame does NOT stimulate the sweet receptor because site X cannot accommodate the hydrophobic benzene ring.

    Monosaccharides Have Asymmetric Centers

    • All monosaccharides (except dihydroxyacetone) contain one or more chiral carbon atoms and occur in optically active isomeric forms.
    • Enantiomers are mirror images.
    • A molecule with n chiral centers can have 2n stereoisomers.

    Fischer Projection Formulas

    • Used to represent three-dimensional sugar structures on paper.
    • Horizontal bonds project out of the plane.
    • Vertical bonds project behind the plane.

    Enantiomers of Glyceraldehyde

    • Displayed as mirror images.

    Numbering Carbons of a Sugar

    • Numbering begins at the end of the chain near the carbonyl group.

    D Isomers and L Isomers

    • Reference carbon = chiral center most distant from the carbonyl carbon.
    • D-isomers have the configuration at the reference carbon the same as D-glyceraldehyde (on the right).
    • L-isomers have the configuration at the reference carbon the same as L-glyceraldehyde (on the left).

    Clicker Question 5

    • The false statement regarding the enantiomers of glyceraldehyde is that glyceraldehyde does not occur in optically active isomeric forms.

    D-Aldoses

    • (a list of structures is provided)*

    D-Ketoses

    • (a list of structures is provided)*

    Epimers

    • Epimers = two sugars that differ only in the configuration around one carbon atom.
    • Examples: D-mannose (epimer at C2) and D-galactose (epimer at C-4) are both epimers of D-glucose.

    Structures to Know

    • Glyceraldehyde & dihydroxyacetone
    • Ribose
    • Glucose
    • Galactose
    • Mannose
    • Fructose

    (a) D-Aldoses

    (structures provided)

    (b) D-Ketoses

    (structures provided)

    Clicker Question 6

    • A monosaccharide with a ketone functional group and seven carbons is a ketoheptose.

    Hemiacetals and Hemiketals

    • Hemiacetals or Hemiketals are derivatives from alcohols and aldehydes/ketones.
    • Five or six membered rings can form if the OH and carbonyl groups are on the same molecule.
    • Acetal or Ketal is the product from the second alcohol molecule addition to form a glycosidic bond.

    α and β Stereoisomeric Configurations

    • The reaction with the first alcohol molecule creates an additional chiral center (carbonyl carbon).
    • This produces either α (trans) or β (cis) configurations.
    • Anomers are isomeric forms of monosaccharides that are different in configuration around the hemiacetals or hemiketals carbon atom.

    Clicker Question 7

    • Anomers are monosaccharides that differ in configuration around the hemiacetal or hemiketal carbon atom.

    Formation of Cyclic Forms of D-Glucose

    • A reaction between the aldehyde group at C-1 and the hydroxyl group at C-5 forms a hemiacetal linkage.
    • Mutarotation = interconversion of α and β anomers.

    Clicker Question 8

    • Cyclization of monosaccharides is the reaction of hemiketal or hemiacetal formation.

    Pyranoses and Furanoses

    • Pyranoses: Six-membered ring compounds where the hydroxyl group at C-6 reacts with the keto group at C-2.
    • Furanoses: Five-membered ring compounds where the hydroxyl group at C-5 reacts with the keto group at C-2.

    Aldonic and Uronic Acids

    • Aldonic acids form following oxidation of the carbonyl carbon of aldoses.
    • Uronic acids form following oxidation at C-6.
    • Both form stable intramolecular esters called lactones.

    Phosphorylated Derivatives

    • Some sugar intermediates are phosphate esters (e.g., glucose-6-phosphate).
    • These are stable at neutral pH and bear a negative charge.
    • This helps to trap sugars inside cells.

    Sugars that are, or Can Form, Aldehydes are Reducing Sugars

    • Reducing sugars undergo a redox reaction with Cu²⁺ under alkaline conditions. The reduction of Cu²⁺ to Cu⁺ forms a brick-red precipitate.
    • Ketoses that can tautomerize to form aldehydes are also reducing sugars.

    Chain-Ring Equilibrium & Reducing Sugars

    • Rings are in equilibrium with open-chain forms.
    • Reducing sugars have a free anomeric carbon. The aldehyde can reduce Cu²⁺ to Cu⁺ (Fehling's test) and Ag+ to Ag⁰ (Tollens' test).
    • These tests allow detection of reducing sugars.

    Clicker Question 10

    • Glucose does not undergo oxidation by reducing sugars.

    Blood Glucose Measurements in the Diagnosis and Treatment of Diabetes

    • Untreated diabetes has long-term consequences like kidney failure, cardiovascular disease, blindness, etc.
    • Average blood glucose over days can be measured due to a nonenzymatic reaction between glucose and primary amino groups in hemoglobin. This is reflected in glycated hemoglobin (GHb).
    • Normal levels are ~5% of hemoglobin as GHb. Diabetic levels may reach 13%.

    O-Glycosidic Bonds

    • O-glycosidic bond = covalent linkage joining two monosaccharides
    • The hydroxyl group of one sugar molecule reacts with the anomeric carbon of the other resulting in a glycoside.

    The Reducing End

    • Formation of a glycosidic bond makes a sugar nonreducing.
    • The reducing end of a disaccharide or polysaccharide chain has a free anomeric carbon.

    Clicker Question 11

    • Non-reducing sugars refer to carbohydrates linked by their anomeric carbons.

    Naming Reducing Oligosaccharides

    Clicker Question 12

    • The name of lactose is β-D-galactopyranosyl-(1→4)-β-D-glucopyranose where the non-reducing end is located on the left, and the configuration of the anomeric carbon is β.

    Symbols and Abbreviations for Monosaccharides

    • (Table provided explaining common abbreviations for specific monosaccharides)*

    Three Common Disaccharides

    • Lactose is a reducing disaccharide. Sucrose and Trehalose are non-reducing.

    Clicker Question 13

    • Trehalose is a non-reducing sugar

    Clicker Question 14

    • Disaccharides do not always lack free anomeric carbons.

    Polysaccharides

    • Most carbohydrates in nature occur as polysaccharides (MW > 20,000), also called glycans.

    Homopolysaccharides and Heteropolysaccharides

    • Homopolysaccharides: Contain only one type of monosaccharide monomer. Serve as storage forms (starch, glycogen) or structural elements (cellulose, chitin).
    • Heteropolysaccharides: Contain two or more types of monosaccharide monomers. Provide extracellular support.

    Clicker Question 15

    • Heteropolysaccharides are not the common form of storage for monosaccharides for energy.

    Principle 4

    • The order of complex polysaccharides is directed by the biosynthetic enzymes that add each monomeric unit to the growing polymer, contrasted with DNA, RNA, and proteins, which use templates for synthesis.

    Polysaccharides Generally Do Not Have Defined Lengths

    • Length is not defined; this contrasts with the defined lengths of proteins or DNA sequences. The synthesis program is intrinsic to the enzymes that catalyze the polymerization of monomer units.

    Some Homopolysaccharides Are Storage Forms of Fuel

    • Storage polysaccharides (starch in plants, glycogen in animals).
    • The molecules are heavily hydrated because they have many exposed hydroxyl groups available to hydrogen bond with water.

    Starch and Glycogen

    • Starch: Contains two glucose polymers, amylose and amylopectin.
      • Amylose: Long, unbranched chains of α(1→4)-linked glucose residues.
      • Amylopectin: Larger than amylose, with α(1→4) linkages and highly branched due to α(1→6) linkages.
    • Glycogen: A polymer of α(1→4)-linked glucose subunits, with α(1→6)-linked branches (more extensively branched than amylopectin).

    Structure of Starch and Glycogen

    (Structures are provided)

    Clicker Question 16

    • The α(1→4) O-glycosidic bond is common in amylose, amylopectin and glycogen.

    Clicker Question 17

    • A glycogen molecule with 28 branches has 29 non-reducing ends and 1 reducing end.

    Principle 3

    • Storage of low molecular weight metabolites as polymers avoids very high osmolarity. This would cause cells to swell and potentially lyse from excess water entry.

    Storage of Glucose as Polymers Avoids High Osmolarity

    • Hepatocytes in the fed state store glycogen equivalent to 0.4 M glucose.
    • 0.4M glucose in the cytosol would elevate the osmolarity causing the cell to swell and potentially lyse.

    Clicker Question 18

    • Glycogen molecules with many nonreducing ends allow the rapid glucose storage and release.

    Some Homopolysaccharides Serve Structural Roles

    • Cellulose: Tough, fibrous, water-insoluble substance.
      • Linear, unbranched homopolysaccharide of 10,000-15,000 β(1→4)-linked D-glucose residues.

    Clicker Question 19

    • The β-glycosidic linkage of glucose molecules in cellulose produces straight, stable fibers that exclude water.

    The Challenge of Glycobiology

    • Glycobiology is the study of the structure and function of glycoconjugates. The challenge is understanding how cells use specific oligosaccharides to encode information, intracellular targeting of proteins, cell-cell interactions, cell differentiation, and extracellular signals.

    Oligosaccharide Structures Are Information-Dense

    • Branched structures, not found in nucleic acids or proteins, are common in oligosaccharides.
    • Almost limitless variety due to stereochemistry, position of glycosidic bonds, types and orientations of substituent groups, number and type of branches.

    Lectins Are Proteins That Read the Sugar Code

    • Lectins bind carbohydrates with high specificity and high affinity.
    • Functions: cell-cell recognition, signaling, adhesion, and the intracellular targeting of proteins.

    Selectins

    • Selectins are a family of plasma membrane lectins mediating cell-cell recognition and adhesion in diverse cellular processes.
    • they move immune cells through the capillary wall, mediate inflammation, and rejection of transplanted organs.

    Clicker Question 30

    • Selectins are not intracellular.

    Role of Lectin-Ligand Interactions in Leukocyte Movement

    (Diagram provided)

    Binding Site on Influenza Neuraminidase

    (Diagram provided)

    Lectin-Carbohydrate Interactions Are Highly Specific

    • Subtle molecular complementarity allows interaction only with the lectin's correct carbohydrate binding partners.

    Lectin Multivalency

    • Single lectin molecules have multiple carbohydrate-binding domains, which increases the effective affinity of the interaction.

    Interactions of Sugar Residues Due to the Hydrophobic Effect

    (Diagram provided)

    Clicker Question 31

    • Lectins often bind their ligands via multiple weak interactions.

    Biological Interactions Mediated by the Sugar Code

    (Diagram provided)

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