Carbohydrates: Classification, Stereochemistry, and Reactions

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Topic 11 — Key Facts for Kenyatta University (Kenya) Core concept: Carbohydrates (sugars) are polyhydroxy aldehydes or ketones; they are classified as aldoses (aldehyde sugars) or ketoses (ketone sugars), and further as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), etc. High-yield point: D- and L- notation in sugars refers to the last chiral centre (farthest from the aldehyde/ketone); D-glucose has the –OH on the last chiral centre on the right (Fischer projection); all naturally occurring sugars in humans are D-sugars; epimers differ at only one chiral centre ⚡ Exam tip: Glucose is the most important carbohydrate — its open-chain Fischer projection shows CHO and CH₂OH at opposite ends with four chiral centres (C2, C3, C4, C5); D-(+)-glucose is the naturally occurring enantiomer; the ”(+)” refers to dextrorotatory optical rotation, not the D/L configuration

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Carbohydrates: Polyhydroxy Aldehydes and Ketones

Carbohydrates are the most abundant class of organic compounds on Earth, serving as the primary energy source for living organisms. They are defined as polyhydroxy aldehydes or polyhydroxy ketones, with the general formula (CH₂O)ₙ for simple sugars.

The name “carbohydrate” literally means “hydrate of carbon” (C·H₂O), though this is a historical misnomer — carbohydrates are not actually water of hydration of carbon.

Classification of Carbohydrates

1. Monosaccharides: Simple sugars that cannot be hydrolysed to smaller carbohydrate units. They are classified by:

• The number of carbon atoms (trioses, tetroses, pentoses, hexoses, heptoses)
• The functional group (aldoses have an aldehyde; ketoses have a ketone)

Sugar	Type	Carbon Atoms	Formula
Glyceraldehyde	Aldotriose	3	C₃H₆O₃
Dihydroxyacetone	Ketotriose	3	C₃H₆O₃
Ribose	Aldopentose	5	C₅H₁₀O₅
Glucose	Aldohexose	6	C₆H₁₂O₆
Fructose	Ketohexose	6	C₆H₁₂O₆
Deoxyribose	Aldopentose (deoxy)	5	C₅H₁₀O₄

2. Disaccharides: Composed of two monosaccharide units joined by a glycosidic bond:

• Maltose (glucose + glucose, α-1,4 linkage)
• Sucrose (glucose + fructose, α-1,2 linkage)
• Lactose (glucose + galactose, β-1,4 linkage)

3. Polysaccharides: Long chains of monosaccharide units:

• Starch (amylose + amylopectin): Plant storage polysaccharide; α-1,4 linked glucose
• Glycogen: Animal storage polysaccharide; highly branched α-1,4 and α-1,6 linked glucose
• Cellulose: Plant structural polysaccharide; β-1,4 linked glucose

D- and L- Notation (Fischer Projection)

Fischer Projection: A way of representing 3D molecules in 2D for sugars. Vertical lines go into the plane (away from viewer); horizontal lines come out (toward viewer).

Determining D/L Configuration:

• In the Fischer projection of a sugar, look at the last chiral centre (the carbon farthest from the aldehyde/ketone group)
• If the –OH group on this carbon is on the right → D configuration
• If the –OH group on this carbon is on the left → L configuration

D-Series Sugars: The naturally occurring sugars in mammals are predominantly D-sugars (e.g., D-glucose, D-fructose, D-ribose).

L-Series Sugars: Found in some plants and bacterial cell walls (e.g., L-arabinose, L-rhamnose).

⚡ Exam Tip: D/L notation tells you about the configuration at ONE specific centre. It does NOT tell you about the overall optical rotation. The sign of optical rotation (d or + for dextrorotatory, l or − for laevorotatory) is measured separately and is not related to D/L designation.

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Stereochemistry of Glucose and Other Aldoses

The Aldehyde Hexoses (Aldohexoses)

Glucose, galactose, mannose, and other aldohexoses have four chiral centres (C2, C3, C4, C5), giving 2⁴ = 16 possible stereoisomers. Only D-glucose and D-galactose (among the D-series) are nutritionally significant in human metabolism.

D-Glucose Configuration (Fischer projection):

    CHO
     |
    H–C–OH
     |
    H–C–OH
     |
    H–C–OH
     |
    H–C–OH
     |
   CH₂OH

Key Epimers of D-Glucose:

• D-Mannose: Epimer of glucose at C2 (mirror image at C2 only)
• D-Galactose: Epimer of glucose at C4 (mirror image at C4 only)
• D-Allose: Epimer of glucose at C3

Important Biological Sugar Structures:

Sugar	Relationship to Glucose	Significance
D-Glucose	Reference	Blood sugar; primary cellular fuel
D-Fructose	Ketose isomer of glucose	Fruit sugar; part of sucrose
D-Galactose	Epimer of glucose at C4	Part of lactose; galactose in brain
D-Mannose	Epimer of glucose at C2	Mannoproteins; N-linked glycoproteins

Mutarotation

When D-glucose is dissolved in water, it undergoes mutarotation — the specific optical rotation changes over time until it reaches an equilibrium value of +52.7°.

Reason: In aqueous solution, D-glucose exists in equilibrium between:

• α-D-glucose (specific rotation +112°): The cyclic hemiacetal form where the –OH at C1 is on the opposite side to the –CH₂OH at C6
• β-D-glucose (specific rotation +18.7°): The –OH at C1 is on the same side as –CH₂OH at C6
• The open-chain form (negligible amount): Trace amounts, but necessary for reactions

Equilibrium mixture: ~36% α-D-glucose, ~64% β-D-glucose, ~0.1% open-chain

⚡ Exam Tip: The cyclic forms (α and β) are anomers — they differ only at the anomeric carbon (C1 for aldoses). The anomeric carbon is the carbon that was the carbonyl carbon in the open chain and became a new chiral centre in the cyclic form.

Cyclic Forms: Pyranose and Furanose

Pyranose rings: Six-membered rings containing one oxygen (like pyran). Formed by intramolecular nucleophilic attack of the C5 –OH on the carbonyl carbon:

• α-D-glucopyranose: The anomeric –OH (at C1) is on the opposite side to the –CH₂OH at C6
• β-D-glucopyranose: The anomeric –OH is on the same side as the –CH₂OH at C6

Haworth Projection: A 2D representation of cyclic sugars showing the ring as a flat pentagon/hexagon with substituents drawn above or below the plane.

Furanose rings: Five-membered rings containing one oxygen (like furan). Formed by intramolecular attack of the C4 –OH on the carbonyl carbon (in ketoses and some aldoses).

Glycosidic Bonds

When a sugar’s anomeric carbon reacts with an –OH or –NH₂ group of another molecule, a glycosidic bond is formed:

• O-glycosidic bond: Anomeric carbon bonded to oxygen of another group
• N-glycosidic bond: Anomeric carbon bonded to nitrogen (as in DNA/RNA nucleosides)

Examples:

• Maltose: Two glucose units linked α-1,4 (from amylose)
• Sucrose: Glucose linked α-1,2 to fructose (non-reducing sugar)
• Lactose: Glucose linked β-1,4 to galactose (reducing sugar — has free –OH at anomeric carbon of glucose)
• Cellobiose: Two glucose units linked β-1,4 (from cellulose)

Reducing and Non-Reducing Sugars

Reducing sugars have a free anomeric carbon that can act as a reducing agent:

• All monosaccharides are reducing sugars
• Disaccharides with a free anomeric carbon are reducing (maltose, lactose)
• Non-reducing disaccharides: Sucrose (anomeric carbons of both sugars are involved in the glycosidic bond), trehalose

Benedict’s Test for Reducing Sugars: Reducing sugar + Cu²⁺ (blue) → Cu₂O↓ (brick-red precipitate) + sugar acid

Fehling’s Test: Reducing sugar + Cu²⁺ → brick-red Cu₂O precipitate (quantitative reduction test)

⚡ Biological Note: In diabetes mellitus, elevated blood glucose is detected using glucose oxidase test strips (specific for β-D-glucose, giving a colorimetric response) or Benedict’s test (for general reducing sugars). Standard urine glucose test strips in clinical practice use the glucose oxidase method.

Important Carbohydrate Reactions

1. Oxidation:

• Aldonic acid formation: Aldehyde end oxidised to carboxylic acid → aldonic acid (e.g., gluconic acid from glucose)
• Uronic acid formation: Aldehyde group stays intact, but the CH₂OH end is oxidised to COOH → uronic acid (e.g., glucuronic acid)
• Sugar acids: D-glucose → D-gluconic acid (oxidised at C1); D-glucose → D-glucuronic acid (oxidised at C6)

2. Reduction: Sugar alcohols (alditols) are formed by reduction:

• D-glucose → D-sorbitol (used as sweetener and humectant)
• D-fructose → D-sorbitol
• D-xylose → xylitol (sweetener in sugar-free gum)

3. Esterification: Phosphate esters are important in metabolism:

• Glucose-6-phosphate (first step of glycolysis)
• Fructose-1,6-bisphosphate (regulated step of glycolysis)
• Glyceraldehyde-3-phosphate (intermediate in glycolysis and gluconeogenesis)

4. Osazone Formation: Aldoses and ketoses react with phenylhydrazine to form osazone crystals, each with a characteristic shape:

• Glucose osazone: Sunflower-shaped crystals
• Fructose osazone: Spindle-shaped crystals
• Maltose osazone: Powdery precipitate

This test was historically used to identify sugars by their crystalline osazone form (less used today with modern chromatography and spectroscopy).

Biological Significance of Carbohydrates

Carbohydrate	Structure	Function
Glucose	Aldohexose	Primary cellular fuel; blood sugar
Fructose	Ketohexose	Fruit sugar; metabolised in liver
Sucrose	Glucose + Fructose	Table sugar
Lactose	Glucose + Galactose	Milk sugar
Glycogen	Polymer of α-D-glucose	Animal energy storage
Starch (amylose)	Linear α-D-glucose	Plant energy storage
Cellulose	Linear β-D-glucose	Plant cell wall structure
Chitin	N-acetylglucosamine polymer	Insect exoskeleton, fungal cell walls
Peptidoglycan	N-acetylglucosamine + N-acetylmuramic acid	Bacterial cell wall

⚡ Critical Distinction: Humans cannot digest cellulose (β-1,4 linkages) because we lack the cellulase enzyme. Ruminants and termites host symbiotic microorganisms that can hydrolyse cellulose. The β-1,4 linkage of cellulose makes it a linear, hydrogen-bonded structural polymer (like a rope), while the α-1,4 linkage of amylose creates a helical structure used for storage.

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