1. Introduction and Architectural Classification
Carbohydrates are defined biochemically as polyhydroxy aldehydes or polyhydroxy ketones, or compounds that yield these structures upon hydrolysis. Historically referred to as "hydrates of carbon" due to the empirical formula Cn(H2O)n (or CnH2nOn), many physiological carbohydrates also contain nitrogen, phosphorus, or sulfur.
Carbohydrate Classification Architecture
2. Monosaccharides: Functional Groups, Chirality, and Stereochemistry
Monosaccharides are crystalline, colorless solids that are highly soluble in water but insoluble in non-polar organic solvents. Their backbone consists of an unbranched carbon chain where all carbon atoms except one are bonded to hydroxyl groups; the remaining carbon forms a carbonyl group.
2.1 Carbon Number and Functional Group Classification
Monosaccharides are named by combining their carbon count with the suffix -ose:
- Trioses: 3 carbons (e.g. glyceraldehyde, dihydroxyacetone)
- Tetroses: 4 carbons (e.g. erythrose, erythrulose)
- Pentoses: 5 carbons (e.g. ribose, ribulose, xylose)
- Hexoses: 6 carbons (e.g. glucose, fructose, galactose, mannose)
- Heptoses: 7 carbons (e.g. sedoheptulose)
They are further classified by the chemical nature of their carbonyl group:
- Aldoses: The carbonyl group is at the terminus of the carbon chain (an aldehyde group,
-CHO). - Ketoses: The carbonyl group is at an internal position, typically carbon-2 (a ketone group,
-C=O).
Glyceraldehyde is an aldotriose, whereas dihydroxyacetone is a ketotriose.
Chirality and Stereochemistry
An overview of chiral centers, enantiomers, and the D/L reference system using glyceraldehyde as the standard model.
2.2 Chirality, Enantiomers, and the D/L Reference System
All monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms and are thus optically active.
- Chiral Centre: A tetrahedral carbon atom bonded to four chemically distinct substituents.
- Enantiomers: Non-superimposable mirror images. The simplest aldose, glyceraldehyde, contains a single chiral centre (C2) and exists as a pair of enantiomers:
The absolute configuration of monosaccharides is designated using the Fischer-projection-based D/L system, which references glyceraldehyde:
- D-Isomer: The hydroxyl (
-OH) group attached to the penultimate carbon (the chiral carbon furthest from the carbonyl carbon) lies on the right side of the Fischer projection. - L-Isomer: The hydroxyl group on the penultimate carbon lies on the left side.
2.3 Epimers
Sugars that differ in stereochemistry around only one specific chiral carbon (excluding the anomeric carbon) are termed epimers.
- D-Glucose vs. D-Mannose: Epimers at C-2.
- D-Glucose vs. D-Galactose: Epimers at C-4.
- D-Mannose vs. D-Galactose: These are not epimers, as their configurations differ at two carbon positions (both C-2 and C-4).
3. Structural Trees of D-Aldoses and D-Ketoses
The structural lineages of D-aldoses and D-ketoses expand sequentially from their respective triose precursors by inserting chiral -CH(OH)- groups directly below the carbonyl carbon.
3.1 The D-Aldose Family
All D-aldoses share the same configuration at the highest-numbered chiral carbon as D-glyceraldehyde.
3.2 The D-Ketose Family
D-ketoses contain one fewer chiral centre than aldoses of the same carbon length because the carbonyl carbon (C-2) is not chiral.
4. Cyclisation, Ring Conformations, and Mutarotation
In aqueous solution, monosaccharides with five or more carbon atoms predominantly exist as cyclic (ring) structures. This cyclisation is thermodynamically driven by the low angle and eclipsing strain of five- and six-membered rings.
4.1 Hemiacetal and Hemiketal Chemistry
The cyclisation reaction occurs through an intramolecular nucleophilic attack:
- Hemiacetal Formation: An alcohol group reacts with an aldehyde group.
- Hemiketal Formation: An alcohol group reacts with a ketone group.
- Pyranose Rings: Six-membered ring structures (analogous to pyran). Formed when the C-5 hydroxyl group attacks the C-1 aldehyde in an aldohexose.
- Furanose Rings: Five-membered ring structures (analogous to furan). Formed when the C-5 hydroxyl group attacks the C-2 ketone in a ketohexose, or when the C-4 hydroxyl group of an aldopentose (like ribose) attacks its C-1 aldehyde.
4.2 Anomers and the Anomeric Carbon
Cyclisation converts the pro-chiral carbonyl carbon into a new chiral centre, termed the anomeric carbon. This yields two diastereomeric forms called anomers, designated α and β:
- α-Anomer: The hydroxyl group on the anomeric carbon is on the opposite side of the ring from the
-CH2OHgroup at C-5 (below the ring in a standard Haworth projection). - β-Anomer: The hydroxyl group on the anomeric carbon is on the same side of the ring as the
-CH2OHgroup (above the ring in a Haworth projection).
4.3 Mutarotation Kinetics and Quantitative Equilibrium
When crystalline α-D-glucose is dissolved in water, its specific optical rotation undergoes a slow, spontaneous change until it reaches a stable equilibrium value. This physical phenomenon is called mutarotation, reflecting the reversible interconversion of the α and β anomers through the open-chain intermediate.
At equilibrium, the specific rotation stabilizes at +52.7°, corresponding to a mixture of approximately 37% α-D-glucose and 63% β-D-glucose (with less than 0.1% remaining in the open-chain form).
Worked Biophysical Problem: Anomeric Proportions
Scenario: An aqueous solution of D-galactose exhibits a specific optical rotation of [α]D25 = +80.2° at equilibrium. The specific rotations of pure, freshly dissolved α-D-galactose and β-D-galactose are +150.7° and +52.8°, respectively. Calculate the percentage of each anomer present in the equilibrium mixture.
Solution: Let x represent the fraction of the α-anomer, and (1 - x) represent the fraction of the β-anomer.
Thus, the equilibrium mixture consists of 28% α-D-galactose and 72% β-D-galactose.
5. Physiological and Biochemical Monosaccharide Derivatives
Chemical modifications of the hydroxyl or carbonyl groups on monosaccharides produce a diverse array of derivatives essential for cellular structure, regulation, and defence.
5.1 Glycosides
When a cyclic hemiacetal or hemiketal reacts with an alcohol, it forms an acetal or ketal. The covalent linkage formed between the anomeric carbon and the alkoxy oxygen is a glycosidic bond, and the resulting compound is a glycoside.
Glycosides are locked in their cyclic forms and cannot undergo mutarotation or act as reducing sugars unless the glycosidic bond is cleaved.
- Ouabain: A highly toxic plant-derived cardiac glycoside that binds to and potently inhibits the eukaryotic membrane
Na+/K+ATPase pump. - Streptomycin: An aminoglycoside antibiotic produced by Streptomyces griseus that binds to the 30S ribosomal subunit of bacteria, inhibiting protein translation.
5.2 Sugar Acids
Oxidation of monosaccharides can occur at different carbon positions to produce distinct families of acids:
- Aldonic Acids: Selective oxidation of the carbonyl carbon (C-1) of an aldose to a carboxyl group (
-COOH).- Example: D-glucose is oxidized to D-gluconic acid.
- Uronic Acids: Selective oxidation of the terminal primary alcohol carbon (C-6) to a carboxyl group, leaving the C-1 carbonyl intact.
- Example: D-glucose is oxidized to D-glucuronic acid (a major component of glycosaminoglycans and xenobiotic detoxification pathways).
- Aldaric Acids: Vigorous oxidation of both the C-1 carbonyl and the C-6 primary alcohol carbons to carboxyl groups.
- Example: D-glucose is oxidized to D-glucaric acid.
5.3 Sugar Alcohols (Alditols)
Reduction of the carbonyl group (aldehyde or ketone) of a monosaccharide yields a polyhydroxy linear alcohol called a sugar alcohol or alditol. Because they lack a functional carbonyl group, alditols cannot form cyclic hemiacetals or hemiketals.
- Examples: Sorbitol (from glucose reduction) and Xylitol (from xylose reduction), both widely used as non-cariogenic sweeteners.
5.4 Amino Sugars and Aminoglycosides
Substitution of a hydroxyl group (typically at C-2) with an amino group (-NH2) or an acetylated amino group (-NH-CO-CH3) produces amino sugars:
- β-D-Glucosamine: Glucosamine is a key structural precursor for chitin and glycosaminoglycans.
- N-Acetyl-β-D-glucosamine (NAG): Formed by the N-acetylation of glucosamine; a fundamental monomer of fungal chitin and bacterial peptidoglycans.
- N-Acetylmuramic Acid (NAM): An ether linkage of lactic acid to the C-3 hydroxyl of NAG; unique to the peptidoglycan wall of eubacteria.
6. Disaccharides and the Thermodynamics of Glycosidic Bonds
Disaccharides are formed when the anomeric carbon hydroxyl group of one monosaccharide reacts with a hydroxyl group of another, eliminating a molecule of water to establish an O-glycosidic bond.
6.1 Key Disaccharides
The following table outlines the chemical composition, specific linkages, and biological roles of the most physiologically significant disaccharides:
| Disaccharide | Monomeric Components | Specific Glycosidic Linkage | Physiological Role and Occurrence | Reducing Status |
|---|---|---|---|---|
| Sucrose | D-Glucose + D-Fructose | α 1 ↔ 2β | Primary transport sugar in plants; major product of photosynthesis. | Non-reducing |
| Lactose | D-Galactose + D-Glucose | β 1 → 4 | Principal carbohydrate energy source in mammalian milk. | Reducing |
| Trehalose | D-Glucose + D-Glucose | α 1 ↔ 1α | Major circulatory sugar in insect hemolymph; stress protectant. | Non-reducing |
| Maltose | D-Glucose + D-Glucose | α 1 → 4 | Intermediate product of starch and glycogen digestion. | Reducing |
| Cellobiose | D-Glucose + D-Glucose | β 1 → 4 | Structural breakdown product of cellulose. | Reducing |
| Gentiobiose | D-Glucose + D-Glucose | β 1 → 6 | Constituent of plant glycosides and amygdalin. | Reducing |
6.2 Sucrose Hydrolysis and "Invert Sugar"
Sucrose is strongly dextrorotatory, with a specific optical rotation of +66.5°.
Hydrolysis of sucrose by hot dilute acid or the enzyme invertase (β-fructofuranosidase) yields an equimolar mixture of D-glucose and D-fructose. This hydrolysis alters the optical properties of the solution dramatically because:
- D-Glucose is dextrorotatory: [α]D20 = +52.7°
- D-Fructose is strongly levorotatory: [α]D20 = -92°
The net specific rotation of the resulting equimolar mixture (often called invert sugar) is the sum of their individual contributions:
Because the optical rotation inverts from positive (+66.5°) to negative (-39.3°), this enzymatic breakdown is called the inversion of sucrose.
Worked Biophysical Problem: Sucrose Inversion Kinetics
Scenario: The enzyme invertase is incubated with a solution of pure sucrose. The optical rotation of the solution is monitored continuously. The initial rotation is +66.5°. After complete hydrolysis, the rotation is -39.3°. If the optical rotation of a reaction aliquot is measured at exactly 0°, calculate the fraction of the sucrose that has been hydrolysed at that timepoint.
Solution: The total change in optical rotation upon complete hydrolysis (ΔRmax) is:
The change in optical rotation from the start of the reaction to the measured timepoint is:
The fraction of sucrose hydrolysed (f) is proportional to the ratio of these rotational shifts:
Thus, approximately 63% of the sucrose has been hydrolysed when the optical rotation reaches zero.
6.3 Combinatorial Complexity of Disaccharide Assembly
Unlike nucleic acids and proteins, which link linearly in a single direction, carbohydrates can branch and join through multiple distinct hydroxyl groups, yielding massive combinatorial diversity.
Worked Mathematical Problem: Disaccharide Isomers
Scenario: Calculate the total number of chemically distinct disaccharides that can be constructed using only one D-galactopyranose unit and one D-glucopyranose unit.
Solution: The monosaccharides can act as either the non-reducing donor (supplying the anomeric carbon) or the reducing acceptor (supplying a neutral hydroxyl group).
D-Galactopyranose as Donor, D-Glucopyranose as Acceptor (Galactosides):
- The anomeric carbon of galactose (C-1) can form a bond in either the α or β configuration.
- The glucose acceptor has 4 available hydroxyl groups (C-2, C-3, C-4, and C-6).
- Number of isomers = 2 (anomers) × 4 (hydroxyls) = 8.
D-Glucopyranose as Donor, D-Galactopyranose as Acceptor (Glucosides):
- The anomeric carbon of glucose (C-1) can form a bond in either the α or β configuration.
- The galactose acceptor has 4 available hydroxyl groups (C-2, C-3, C-4, and C-6).
- Number of isomers = 2 (anomers) × 4 (hydroxyls) = 8.
Non-reducing Disaccharides (Anomeric-Anomeric link):
- The linkage occurs directly between the anomeric carbon C-1 of D-galactopyranose and the anomeric carbon C-1 of D-glucopyranose.
- Both sugars can participate in either the α or β configuration at their respective C-1 carbons.
- Possible configurations: α-α, α-β, β-α, and β-β.
- Number of isomers = 4.
Total Isomers:
There are 20 chemically distinct disaccharide isomers possible for this single hexose pair.
7. Polysaccharides (Glycans) Overview
Polysaccharides, or glycans, are macromolecular polymers of monosaccharides. They are fundamentally divided into two major classes based on their monomeric composition:
7.1 Homopolysaccharides: Storage and Structural Blueprints
Homopolysaccharides serve either as highly mobilizable metabolic fuel storage or as rigid, inextensible structural scaffolds.
Starch: The primary carbohydrate storage form in plants, composed of a mixture of two discrete glucose polymers:
- Amylose: A linear, unbranched polymer of D-glucose residues strictly linked by α 1 → 4 glycosidic bonds. In aqueous environments, it spontaneously adopts a tight, left-handed helical conformation. This internal helical cavity is perfectly sized to coordinate iodine molecules (I2), resulting in the characteristic deep blue colour of the iodine-starch test.
- Amylopectin: A highly branched polymer. The main backbone is linked via α 1 → 4 bonds, while branch points are generated by α 1 → 6 glycosidic bonds occurring regularly every 25 to 30 residues.
Glycogen: The principal glucose storage analogue in animals, predominantly concentrated in the liver and skeletal muscle tissues. It is structurally identical to amylopectin but significantly more highly branched, with α 1 → 6 branch points inserted every 8 to 12 residues. This immense density of branching yields an extraordinarily high number of non-reducing ends, permitting incredibly rapid enzymatic cleavage by glycogen phosphorylase during periods of immediate metabolic demand.
Cellulose: A fundamental structural homopolysaccharide constituting the plant cell wall. Unlike starch, it is a linear, unbranched polymer of D-glucose residues linked by β 1 → 4 glycosidic bonds. This single, critical stereochemical difference forces the polysaccharide chain to adopt a fully extended, rigid conformation rather than a helix. Parallel cellulose chains pack tightly side-by-side, aggregating into microfibrils stabilized by an extensive, water-excluding network of intra- and inter-molecular hydrogen bonds. Because humans completely lack the enzyme cellulase (which specifically hydrolyses β 1 → 4 linkages), cellulose cannot be metabolically accessed and passes intact through the digestive tract as dietary fibre.
Chitin: The primary structural component of fungal cell walls and the robust exoskeletons of arthropods. It is a linear homopolymer of N-acetyl-D-glucosamine (NAG) linked strictly by β 1 → 4 bonds. The only structural difference distinguishing it from cellulose is the substitution of the C-2 hydroxyl group with an acetylated amino group. This modification significantly increases hydrophobic packing density, rendering chitin the second most abundant natural biopolymer on Earth.
Callose: A specialized plant structural polysaccharide composed of glucose monomers linked by β 1 → 3 bonds. It is synthesized very rapidly in response to mechanical wounding, pathogenic invasion, or during pollen development to temporarily isolate and protect susceptible cells.
7.2 Heteropolysaccharides: Glycosaminoglycans (GAGs)
Glycosaminoglycans are negatively charged, unbranched heteropolysaccharides characteristic of the animal extracellular matrix (ECM). They consist of repeating disaccharide units containing an acidic sugar (D-glucuronic acid or L-iduronic acid) and an amino sugar (NAG or N-acetylgalactosamine). High levels of sulfation and negative charge attract water molecules, creating hydrated, gel-like shock absorbers in joints.
The following table summarizes the structural biochemistry of the major physiological GAGs:
| GAG | Acidic / Non-Acidic Sugar A | Amino Sugar B | Sulfate Groups | Link to Protein | Physiological Role |
|---|---|---|---|---|---|
| Hyaluronic Acid | D-Glucuronic acid | N-Acetylglucosamine | None | None | Lubricant in synovial fluid; structural scaffold in vitreous humor. |
| Chondroitin Sulfate | D-Glucuronic acid | N-Acetylgalactosamine | Yes | Yes | Major component of cartilage; provides resistance to compression. |
| Dermatan Sulfate | D-Glucuronic or L-Iduronic acid | N-Acetylgalactosamine | Yes | Yes | Found in skin, blood vessels, and heart valves. |
| Heparan Sulfate | D-Glucuronic or L-Iduronic acid | N-Acetylglucosamine | Yes | Yes | Cell surface receptor; regulates growth factor binding. |
| Heparin | D-Glucuronic or L-Iduronic acid | N-Acetylglucosamine | Yes (High) | Yes | Intracellular anticoagulant released by mast cells. |
| Keratan Sulfate | D-Galactose (Non-acidic) | N-Acetylglucosamine | Yes | Yes | Found in cornea, cartilage, and bone. |
7.3 Bacterial Peptidoglycan (Murein)
Peptidoglycan is a rigid structural heteropolysaccharide of the eubacterial cell wall. It consists of alternating residues of NAG and NAM joined by β 1 → 4 glycosidic bonds.
These polysaccharide chains are cross-linked by a short stem peptide attached to the carboxyl group of NAM. In Gram-negative bacteria, the typical stem peptide sequence is:
In Gram-positive bacteria, a L-Lysine residue replaces the DAP, and the stem peptides are typically cross-linked via a pentaglycine bridge.
7.3 Bacterial Peptidoglycan (Murein)
Peptidoglycan is a rigid structural heteropolysaccharide of the eubacterial cell wall. It consists of alternating residues of NAG and NAM joined by β 1 → 4 glycosidic bonds.
These polysaccharide chains are cross-linked by a short stem peptide attached to the carboxyl group of NAM. In Gram-negative bacteria, the typical stem peptide sequence is:
In Gram-positive bacteria, a L-Lysine residue replaces the DAP, and the stem peptides are typically cross-linked via a pentaglycine bridge.
8. Glycoconjugates: Proteoglycans, Glycoproteins, and Glycolipids
Carbohydrates are frequently linked to proteins or lipids to form glycoconjugates, which carry out critical structural and informational roles at the cell surface and in the extracellular matrix.
8.1 Proteoglycans
Proteoglycans are a major subclass of glycoconjugates where the carbohydrate component is dominant (often representing up to 95% of the total mass). They consist of a core protein covalently bound to one or more GAG chains.
The covalent attachment of the GAG chain to a specific Serine residue of the core protein occurs via a conserved linker tetrasaccharide:
This covalent linkage is initiated in the endoplasmic reticulum and Golgi complex by specific glycosyltransferases.
8.2 Glycoproteins
Glycoproteins are protein-dominated glycoconjugates where oligosaccharide chains are covalently attached to specific amino acid side chains. Unlike proteoglycans, the carbohydrate chains are typically branched and lack repeating disaccharide motifs.
The carbohydrate-protein linkage occurs through two primary configurations:
- O-linked Glycosidic Bonds: The carbohydrate is attached via its anomeric carbon to the oxygen atom of a Serine or Threonine side-chain hydroxyl group.
- N-linked Glycosidic Bonds: The carbohydrate is attached via its anomeric carbon to the amide nitrogen atom of an Asparagine side chain. This linkage occurs only within the specific consensus sequence:
where X can be any amino acid except Proline.
9. Reducing vs. Non-Reducing Sugars
The chemical reactivity of a carbohydrate towards mild oxidizing agents (such as Cu2+ in Fehling’s or Benedict's reagent, or Ag+ in Tollens' reagent) determines its status as a reducing or non-reducing sugar:
- Reducing Sugars: Any carbohydrate that contains a free, unmasked carbonyl group (or a cyclic hemiacetal/hemiketal capable of opening in alkaline solution to yield an active aldehyde or ketone). All monosaccharides are reducing sugars. Disaccharides formed via "head-to-tail" condensation, where one sugar preserves a free anomeric hemiacetal carbon, are also reducing (e.g., maltose, lactose).
- Non-Reducing Sugars: Carbohydrates in which the anomeric carbons of both participating monosaccharide units are locked within a glycosidic bond (e.g., sucrose, trehalose). Because they lack a free hemiacetal or hemiketal group, they cannot open into linear forms to react with oxidizing agents unless they are first hydrolysed.
10. Analytical Reference Index: Polysaccharide Topography
The following table serves as an analytical reference mapping the monomeric composition, primary glycosidic linkages, branch points, and structural patterns of the major biologically significant polysaccharides:
| Polysaccharide Name | Primary Monomer | Primary Glycosidic Linkages | Branch Linkages and Frequency | Structural and Conformational Shape | Biological Function |
|---|---|---|---|---|---|
| Murein (Peptidoglycan) | NAG + NAM | β 1 → 4 | None (linked via stem peptides) | Rigid, cross-linked envelope sheet. | Bacterial cell wall structural integrity. |
| Dextran | D-Glucose | α 1 → 6 | α 1 → 2, α 1 → 3, or α 1 → 4 | Highly branched, amorphous network. | Extracellular bacterial plaque; plasma volume expander. |
| Cellulose | D-Glucose | β 1 → 4 | None | Fully extended, rigid linear ribbons forming microfibrils. | Primary structural component of plant cell walls. |
| Amylose | D-Glucose | α 1 → 4 | None | Left-handed, compact helical coil. | Energy storage in plant plastids. |
| Amylopectin | D-Glucose | α 1 → 4 | α 1 → 6 (every 25–30 residues) | Branched, clustered arborized tree. | Principal energy storage component of starch. |
| Inulin | D-Fructose | β 2 → 1 | None | Linear, flexible polymer. | Energy storage in tubers (e.g., dahlias). |
| Chitin | N-Acetylglucosamine | β 1 → 4 | None | Rigid, extended linear sheets stabilized by hydrogen bonds. | Structural scaffold in fungi and arthropod shells. |
| Glycogen | D-Glucose | α 1 → 4 | α 1 → 6 (every 8–12 residues) | Densely branched, spherical macromolecular granules. | Fast-mobilising glucose reserve in animals. |
| Callose | D-Glucose | β 1 → 3 | None | Helical polymer; forms dense plugs. | Defense and developmental isolation barrier in plants. |
Q. Which of the following carbohydrates will fail to yield a brick-red precipitate when heated with Benedict's reagent?
View Answer & Explanation
Correct Answer: C) Sucrose
Explanation: Benedict's reagent relies on the presence of a free, unmasked carbonyl group (reducing sugar) to reduce Cu2+ to Cu+. Sucrose is a non-reducing sugar because the anomeric carbons of both its monosaccharide units (C-1 of glucose and C-2 of fructose) are locked in the α 1 ↔ 2β glycosidic bond. It cannot ring-open in an alkaline solution, thus failing to react.
Advanced Carbohydrate Biochemistry Question Bank
Question 1
View Answer & Explanation
Correct Answer: B) The open-chain form remains at less than 0.1% because thermodynamic stability heavily favors cyclic pyranose ring conformations.
Explanation: Although the open-chain aldehyde form is an obligate transient intermediate required to break and reform the covalent bond for mutarotation, its thermodynamic equilibrium concentration is extremely low (<0.1%). Pyranose ring forms are heavily favored due to minimal strain and favorable orbital overlaps.
Question 2
View Answer & Explanation
Correct Answer: B) 0.720
Explanation: Let x be the fraction of α-anomer and (1-x) be the fraction of β-anomer. Setting up the linear mixture equation: 150.7x + 52.8(1-x) = 80.2 ⇒ 97.9x = 27.4 ⇒ x ≈ 0.280 (fraction of α). Therefore, the fraction of the β-anomer is 1 - 0.280 = 0.720 (72%).
Question 3
View Answer & Explanation
Correct Answer: C) 0.629
Explanation: The total maximal rotational shift from start to completion is ΔRmax = +66.5° - (-39.3°) = 105.8°. The shift at the zero-rotation timepoint is ΔRcurrent = +66.5° - 0° = 66.5°. The hydrolyzed fraction f = ΔRcurrent / ΔRmax = 66.5 / 105.8 ≈ 0.629 (62.9%).
Question 4
View Answer & Explanation
Correct Answer: D) 20
Explanation: Galactose as donor to glucose acceptor gives 2 (anomers) × 4 (hydroxyls: C2, C3, C4, C6) = 8 isomers. Glucose as donor to galactose acceptor gives another 2 × 4 = 8 isomers. Non-reducing disaccharides involve linking C-1 of galactose to C-1 of glucose, permitting αα, αβ, βα, and ββ combinations (4 isomers). Summing them up yields 8 + 8 + 4 = 20 unique isomers.
Question 5
View Answer & Explanation
Correct Answer: A) Cellulose possesses β 1 → 4 linkages forcing an extended ribbon conformation stabilized by extensive hydrogen networks, whereas amylose utilizes α 1 → 4 linkages that induce bending and a tight left-handed helical twist.
Explanation: The distinct stereochemistry of the glycosidic oxygen (β vs α) alters the dihedral angles between adjacent glucose units. The β 1 → 4 configuration rotates each consecutive glucose ring by 180 degrees, producing a straight, rigid, extended ribbon that packs into water-excluding microfibrils. Conversely, α 1 → 4 angles force a curved trajectory, naturally curling into a left-handed helix.
Question 6
View Answer & Explanation
Correct Answer: B) High density of carboxylate and sulfate ester groups conferring a massive net negative charge density that electrostatically sequesters water molecules and cations.
Explanation: GAGs are unbranched heteropolysaccharides featuring acidic sugars and amino sugars carrying high loads of sulfate and carboxylate groups. This clustered negative charge forces adjacent chains to repel one another and attract heavy shells of hydration water, forming a resilient, slippery, gel-like matrix capable of withstanding high compressive loads.
Question 7
View Answer & Explanation
Correct Answer: A) L-Alanine → D-Glutamic acid → meso-Diaminopimelic acid (DAP) → D-Alanine
Explanation: The classic stem peptide sequence found in Gram-negative eubacterial peptidoglycan consists of alternating L- and D-amino acids: L-Alanine → D-Glutamic acid → meso-Diaminopimelic acid (DAP) → D-Alanine. Gram-positive organisms typically substitute DAP with L-Lysine and incorporate a pentaglycine cross-bridge.
Question 8
View Answer & Explanation
Correct Answer: B) Carbohydrate-dominated (up to 95% mass); core protein covalently bound to heavy GAG chains via a conserved linkage tetrasaccharide (Xylose → Galactose → Galactose → GlcUA → Serine).
Explanation: Proteoglycans are heavily carbohydrate-biased glycoconjugates where long, linear repeating glycosaminoglycan chains link covalently to specific Serine residues on a core protein via the invariant tetrasaccharide bridge: Core Protein—Ser → Xylose → Galactose → Galactose → GlcUA → [GAG]n.
Question 9
View Answer & Explanation
Correct Answer: A) Asn — X — Ser/Thr (where X can be any amino acid except Proline)
Explanation: N-linked glycosylation targets the amide nitrogen of an Asparagine (Asn) residue embedded strictly within the consensus triplet Asn—X—Ser/Thr, where X represents any amino acid variant except Proline. O-linked glycosylation attaches to the hydroxyl group of Serine or Threonine without a strict sequence motif.
Question 10
1. It produces a stable brick-red precipitate when treated with alkaline copper tartrate (Benedict's reagent).
2. Acid hydrolysis yields an equimolar mixture of D-galactose and D-glucose.
3. It readily undergoes mutarotation.
Identify this carbohydrate derivative:
View Answer & Explanation
Correct Answer: B) Lactose
Explanation: Lactose is a disaccharide composed of D-galactose and D-glucose joined by a β 1 → 4 linkage. Because the anomeric carbon of the glucose residue remains free (unmasked hemiacetal), lactose is a reducing sugar (positive Benedict's test), exhibits mutarotation, and can open into an active aldehyde form in solution. Sucrose and trehalose are non-reducing, while cellulose is a high-molecular-weight polysaccharide.
In this lesson
LessonStep 11 of 61
