Comprehensive Carbohydrate Study Notes

Objectives

• Define carbohydrates chemically and classify them into major groups.
• Describe key physicochemical properties and biomedical importance.
• Outline functional roles and give an overview of carbohydrate metabolism.

Definition & Chemical Composition

• Empirical formula: $(C\cdot H<em>2O)</em>n$.
• Chemically: polyhydroxy aldehydes (aldoses) or ketones (ketoses) and compounds that yield them upon hydrolysis.
• Consist of three elements: C, H, O.
• "Sugar" specifically denotes monosaccharides and short‐chain oligosaccharides.

Abundance & Daily Requirement

• Most abundant class of bio-organic molecules on Earth (present in food, cotton, linen, bacterial walls, etc.).
• Recommended human diet: ≈ $\frac{2}{3}$ energy from carbohydrates.
• Average adult daily need: $225\text{–}325\ \text{g}$.

Energy Yield & Digestion

• Main digestion occurs in the small intestine.
• 1 g glucose → $\approx 4\,\text{kcal}$.
• Digestion: complex carbs → monosaccharides → absorbed as glucose → transported via insulin to cells → excess stored as glycogen or converted to fat.

Biological Functions

• Primary & storage energy source (glycogen, starch).
• Structural components (cellulose, chitin, bacterial peptidoglycan).
• Dietary fibre (non-digestible polysaccharides).
• Constituents of nucleotides (ribose, deoxyribose).
• Roles in cell recognition, signalling, lubrication, immunity.
• Detoxification (conjugation with glucuronic acid).

General Stereochemical Properties

• Asymmetric (chiral) carbon: C bonded to four distinct groups.
• Stereoisomers: identical formula, different 3-D arrangement.
  • Constitutional isomers – different connectivity.
  • Stereoisomers – same connectivity.
  – Enantiomers: non-superimposable mirror images.
  – Diastereomers: stereoisomers that are not mirror images.
• Optical activity: rotation of plane-polarised light.
  • Dextrorotatory (+, d) vs Levorotatory (–, l).
  • Racemic mixture: equal + and –.

Structural Formulae Conventions

• Fischer projection: linear, easy chirality reading.
• Haworth projection: cyclic ring form; right-side groups in Fischer appear down, left appear up.
• Conformational (chair/boat): true 3-D; relates to stability/reactivity.
• α / β anomers (cyclic):
  • α – anomeric OH on same side as ring O reference (down in D-sugars).
  • β – opposite side (up in D-sugars).

Chirality Examples

• Glyceraldehyde reference: D form has terminal OH to the right; L to left.
• Most naturally occurring sugars are D; natural amino acids are L.

Classification Scheme

By polymer length

• Monosaccharides – cannot be hydrolysed further.
• Oligosaccharides – 2–10 units (di-, tri-, tetra-, etc.).
• Polysaccharides – >10 units.

By carbon number (monosaccharides)

• Trioses $(\text{C}<em>3)$, Tetroses $(\text{C}</em>4)$, Pentoses $(\text{C}<em>5)$, Hexoses $(\text{C}</em>6)$.

By carbonyl group

• Aldoses (–CHO) vs Ketoses (>C=O).

Important Monosaccharides

• Glucose (D-glucose, aldohexose)
  • “Grape/blood sugar”, most abundant, primary cellular fuel.
  • Clinical use: 50 % IV hypertonic solution for hypoglycaemia.
• Galactose (aldohexose)
  • “Brain sugar”; component of glycoproteins, lactose, blood-group antigens.
  • Synthesised in mammary gland for lactose.
• Fructose (D-fructose, ketohexose)
  • “Levulose”; sweetest natural sugar; equal to glucose in honey; IV energy source in hepatic disease/diabetes.

Clinical notes

• Hyperglycaemia → glycosuria in diabetes mellitus.
• Hereditary fructose intolerance → fructose accumulation & hypoglycaemia.
• Galactosemia (GALT deficiency) → cataract, liver/kidney/brain damage.

Reducing vs Non-reducing Sugars & Tests

• Reducing sugar: free aldehyde/ketone capable of reduction (e.g., glucose, lactose, maltose).
• Non-reducing: no free carbonyl (e.g., sucrose).
• Detection: Benedict’s, Fehling’s tests (Cu²⁺ reduction → brick-red Cu₂O precipitate).

Disaccharides

• Two monosaccharides joined by glycosidic bond (condensation; water lost).
• Homo-oligosaccharides
  • Maltose: $\alpha\text{-D-Glcp}(1\rightarrow4)\alpha\text{-D-Glcp}$; reducing.
  • Isomaltose: $\alpha(1\rightarrow6)$ linkage.
  • Cellobiose: $\beta(1\rightarrow4)$; undigestible by humans.
• Hetero-oligosaccharides
  • Lactose: $\beta\text{-D-Galp}(1\rightarrow4)\beta\text{-/α-D-Glcp}$; reducing.
  • Sucrose: $\alpha\text{-D-Glcp}(1\rightarrow2)\beta\text{-D-Fruf}$; non-reducing.

Uses & Pathology

• Lactose intolerance: lactase deficiency → GI symptoms.
• Sucrose improves palatability of drugs; iron-sucrose treats iron deficiency.
• Maltose from icodextrin in dialysis fluids.

Polysaccharides – General Parameters

1. Identity of repeating unit (homo vs hetero).
2. Chain length (degree of polymerisation).
3. Glycosidic linkage type (α/β, (1→4), (1→6)…).
4. Degree & pattern of branching.

Homoglycans (single monomer)

• Starch (amylose + amylopectin)
  • Amylose: linear $\alpha(1\rightarrow4)$; forms helix—blue with I₂.
  • Amylopectin: branched $\alpha(1\rightarrow6)$ every ~20 units.
• Glycogen
  • “Animal starch”; $\alpha(1\rightarrow4)$ backbone with $\alpha(1\rightarrow6)$ branches every ~10 residues; red-violet with I₂; stored in liver & muscle.
• Cellulose
  • Linear $\beta(1\rightarrow4)$ D-glucose; most abundant biomolecule; H-bonded fibrils; no I₂ colour; indigestible due to lack of cellulase (except ruminant microflora).
• Dextran
  • Mainly $\alpha(1\rightarrow6)$ with $\alpha(1\rightarrow3)/(1\rightarrow4)$; MW 40–75 kDa; plasma expander & chromatography matrix.
• Dextrin
  • Partial starch hydrolysates (amylodextrins, etc.); used in infant formula.
• Chitin
  • $\beta(1\rightarrow4)$ N-acetyl-D-glucosamine; exoskeletons, fungal walls.
• Inulin
  • Fructosan (β-D-fructose polymer); renal clearance marker for GFR.

Heteroglycans

• Agar, peptidoglycans, glycosaminoglycans, etc.

Glycosaminoglycans (GAGs)

• Long unbranched repeating disaccharide chains (amino sugar + uronic acid, often sulfated).
• Usually bound to core proteins → proteoglycans; form extracellular matrix.

Major GAGs & Localisation

• Hyaluronic acid – synovial fluid, vitreous humour; shock absorber, unsulfated.
• Chondroitin sulfate – cartilage, bone; most abundant.
• Heparan sulfate – basement membranes; highly acetylated.
• Heparin – mast cell granules; strongest anticoagulant, highly sulfated.
• Keratan sulfate – cornea, cartilage; aggregates with chondroitin sulfate.

Functions: hydration, elasticity, cell signalling, angiogenesis, metastasis modulation.

Carbohydrates in Cell Walls & ECM

• Bacterial peptidoglycan: repeating N-acetylglucosamine + N-acetylmuramic acid, $\beta(1\rightarrow4)$.
  • Gram-positive: thick peptidoglycan layer.
  • Gram-negative: thin layer plus outer lipopolysaccharide membrane & periplasm.
• Plant cell wall: cellulose matrix.
• Animal extracellular matrix: proteoglycans + collagen.

Clinical & Metabolic Disorders

• Diabetes mellitus
  • Type 1: autoimmune β-cell destruction → no insulin.
  • Type 2: insulin resistance + relative deficiency; obesity, inactivity, genetics.
• Galactosemia: GALT deficiency → accumulation of galactose-1-P & galactitol → cataracts, liver & brain damage.

Laboratory Measurement of Glucose

1. Hexokinase/NAD⁺ method: $\text{Glucose} \xrightarrow[ATP]{\text{Hexokinase}} \text{G-6-P} \xrightarrow[\text{NAD}^+]{\text{G-6-PDH}} \text{6-phosphogluconate}+ \text{NADH} + H^+$ (A340 nm).
2. Glucose oxidase: $\text{Glucose} + O<em>2 \xrightarrow{\text{GOD}} \text{Gluconic acid} + H</em>2O_2$; H₂O₂ + phenol + 4-AAP $\xrightarrow{\text{POD}}$ quinoneimine (A505 nm).
3. Glucose dehydrogenase: NAD-dependent; NADH reduces MTT via diaphorase → blue formazan (A660/840 nm).
4. Folin-Wu: alkaline Cu²⁺ reduction → phosphomolybdate blue.
5. o-Toluidine: hot acid reaction → N-glycosylamine (green).

Chemical Reactions of Monosaccharides

• Oxidation → acidic sugars (aldonic/uronic acids).
• Reduction → sugar alcohols (alditols) e.g., glucose → sorbitol (humectant & sweetener).
• Esterification with phosphates → key metabolic intermediates (e.g., glucose-6-phosphate).

Concept Map Synopsis

• Monosaccharides connected via glycosidic linkages → di/oligo/polysaccharides → energy (glycolysis), storage (starch/glycogen), structure (cellulose/chitin), signalling (glycoproteins, blood group antigens).

Key Numbers & Facts (Quick Reference)

• Dietary recommendation: $\frac{2}{3}$ calories.
• Adult need: $225\text{–}325\,\text{g day}^{-1}$.
• Energy yield: $4\,\text{kcal g}^{-1}$ glucose.
• Branch frequency: glycogen every $\approx10$ residues; amylopectin $\approx20$.
• Iodine tests: amylose (blue), glycogen (red-violet), cellulose (negative).
• GAG sulfation: Heparin > Heparan sulfate > Chondroitin sulfate; Hyaluronate unsulfated.