CARBOHYDRATES

Carbohydrate Structure, Classification, and Physiological Function: Medical Biochemistry Notes

1. Structure and Chemical Definition of Carbohydrates

  • Definition & General Formula: Carbohydrates are organic macromolecules composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio represented by the empirical formula \((\text{CH}_2\text{O})_n\) or \(\text{C}n\text{H}{2n}\text{O}_n\) (such as glucose, \(\text{C}6\text{H}{12}\text{O}_6\)). Chemically, they are defined as polyhydroxy aldehyde or ketone derivatives, or substances that yield these compounds upon hydrolysis.

  • Biomedical Importance:

    • Serve as the chief source of energy for cellular metabolism, yielding \(4\text{ cal/gram}\).

    • Function as the primary metabolic fuel transported in the bloodstream (predominantly glucose).

    • Act as the storage form of energy in animals (glycogen) and plants (starch).

    • Form essential structural components of cell membranes.

    • Provide precursors for synthesizing nucleic acids (ribose in RNA, deoxyribose in DNA) and milk sugar (galactose for lactose).

    • Serve structural and protective roles, such as lubricating skeletal joints.

    • Function pharmacologically as pharmacological agents and drugs (e.g., cardiac glycosides, antibiotics).

Structural Chemistry & Isomerism of Monosaccharides

Monosaccharides exhibit distinct chemical structures, stereoisomerism, and ring conformations:

  1. Functional Group Classification:

    • Aldoses: Contain an aldehyde group (\(-\text{CHO}\)) at C1 (e.g., glyceraldehyde, erythrose, ribose, xylose, glucose, galactose).

    • Ketoses: Contain a ketone group (\(\text{C=O}\)) at C2 (e.g., dihydroxyacetone, xylulose, fructose).

  2. Carbon Chain Length:

    • Classified by carbon count into trioses (\(3\text{C}\)), tetroses (\(4\text{C}\)), pentoses (\(5\text{C}\)), hexoses (\(6\text{C}\)), and heptoses (\(7\text{C}\)).

  3. Asymmetric Carbon & Stereoisomerism:

    • An asymmetric carbon atom is bound to four different atoms or functional groups, imparting stereoisomerism and optical activity to the molecule.

    • Enantiomers (D and L Isomers): D- and L-sugars are non-superimposable mirror images. The designation depends on the spatial arrangement of \(-\text{H}\) and \(-\text{OH}\) groups on the penultimate carbon atom (if \(-\text{OH}\) is on the right, it is a D-isomer; if on the left, an L-isomer). Human physiological pathways exclusively utilize D-sugars.

  4. Ring Formation & Anomerism:

    • In aqueous solution, monosaccharides cyclize into thermodynamically stable ring structures rather than remaining as open chains.

    • Aldoses primarily form pyranose rings (6-membered rings), while ketoses primarily form furanose rings (5-membered rings).

    • Ring closure converts C1 in aldoses or C2 in ketoses into an asymmetric center called the anomeric carbon atom. Sugars differing only in configuration at this anomeric carbon are termed anomers (\(\alpha\) and \(\beta\) forms, e.g., \(\alpha\text{-D-glucose}\) vs. \(\beta\text{-D-glucose}\)).

  5. Epimerism:

    • Isomers that differ in the configuration of \(-\text{H}\) and \(-\text{OH}\) around a single carbon atom.

    • D-Mannose is the C2-epimer of glucose.

    • D-Galactose is the C4-epimer of glucose.

  6. Optical Activity:

    • Compounds with asymmetric carbons rotate plane-polarized light. Rotations to the right are designated dextrorotatory (\(d\) or \(+\)), such as \(\text{D-(d)-glucose}\) (dextrose); rotations to the left are levorotatory (\(l\) or \(-\)), such as \(\text{D-(l)-fructose}\). A D-configured sugar can be dextrorotatory (\(\text{D+}\)) or levorotatory (\(\text{D-}\)).

2. Classification of Carbohydrates

Carbohydrates are categorized into four major classes based on the degree of polymerization:

                          CARBOHYDRATES
                                |
      +-------------------------+-------------------------+-------------------------+
      |                         |                         |                         |
Monosaccharides           Disaccharides            Oligosaccharides          Polysaccharides
(1 sugar unit)            (2 sugar units)          (3-10 sugar units)        (>10 sugar units)
[Glucose, Fructose]       [Lactose, Sucrose]       [Maltotriose]                    |
                                                                   +----------------+----------------+
                                                                   |                                 |
                                                         Homopolysaccharides               Heteropolysaccharides
                                                         (Same monomer unit)               (Different monomer units)
                                                         [Starch, Glycogen]                [GAGs / Heparin]
A. Monosaccharides

  • Simple base sugars that cannot be hydrolyzed further into simpler carbohydrates.

  • Serve as immediate metabolic fuels.

  • Classification Examples:

    • Trioses (\(3\text{C}\)): Glyceraldehyde (aldotriose), Dihydroxyacetone (ketotriose).

    • Tetroses (\(4\text{C}\)): Erythrose (aldotetrose).

    • Pentoses (\(5\text{C}\)): Ribose (aldopentose), Xylose (aldopentose), Xylulose (ketopentose).

    • Hexoses (\(6\text{C}\)): Glucose (aldohexose), Galactose (aldohexose), Fructose (ketohexose).

B. Disaccharides

  • Formed by a condensation (dehydration synthesis) reaction joining two monosaccharide units via a glycosidic bond, with the loss of one water molecule (\(\text{C}6\text{H}{12}\text{O}6 + \text{C}6\text{H}{12}\text{O}6 \rightarrow \text{C}{12}\text{H}{22}\text{O}_{11} + \text{H}_2\text{O}\)).

  • Glycosidic Linkage Characteristics:

    • \(\alpha\)-Glycosidic bonds: Monomers are oriented in the same direction, forming helical, digestible polymers.

    • \(\beta\)-Glycosidic bonds: Monomers alternate in orientation, forming linear, non-digestible structural polymers.

  • Major Disaccharides:

    • Maltose: \(\text{Glucose} + \text{Glucose}\) joined by an \(\alpha\text{ 1-4 glycosidic linkage}\) (source: malt; starch breakdown).

    • Lactose: \(\text{Galactose} + \text{Glucose}\) joined by a \(\beta\text{ 1-4 glycosidic linkage}\) (source: milk).

    • Sucrose: \(\text{Glucose} + \text{Fructose}\) joined by an \(\alpha\text{ 1-2 glycosidic linkage}\) (source: sugar cane).

    • Isomaltose: \(\text{Glucose} + \text{Glucose}\) joined by an \(\alpha\text{ 1-6 glycosidic linkage}\) (source: digestion of amylopectin).

C. Oligosaccharides

  • Contain 3 to 10 monosaccharide units linked together.

  • Example: Maltotriose (\(\text{glucose} + \text{glucose} + \text{glucose}\)).

D. Polysaccharides (Glycans)

  • High-molecular-weight polymers containing more than 10 monosaccharide units.

  • 1. Homopolysaccharides: Polymers composed of a single repeating monosaccharide unit.

    • Starch: Plant storage polymer composed of glucose units.

    • Glycogen: Primary storage form of glucose in liver and muscle tissue.

    • Cellulose: Structural plant fiber composed of glucose units.

    • Dextrin: Intermediate product formed during partial hydrolysis of starch.

    • Inulin: Fructose polymer derived from dahlia roots.

    • Chitin: Polymer of \(N\)-acetylglucosamine found in fungal cell walls and insect/arthropod exoskeletons.

  • 2. Heteropolysaccharides: Polymers composed of different monosaccharide units or sugar derivatives.

    • Mucopolysaccharides (MPS) / Glycosaminoglycans (GAGs): Complex carbohydrates that bind proteins to form proteoglycans in the extracellular matrix. Examples include hyaluronic acid, heparin, chondroitin sulfate, keratan sulfate, heparan sulfate, and dermatan sulfate.

    • Blood group substances.

3. Detailed Structural Comparison: Amylose vs. Amylopectin

Starch is a plant storage homopolysaccharide composed of two distinct glucose components: amylose and amylopectin.

Structural Parameter

Amylose

Amylopectin

Proportion in Starch

15–20% of starch content.

80–85% of starch content.

Basic Architecture

Unbranched, linear chain that twists into a helical conformation.

Highly branched chain network.

Molecular Weight

Approximately \(60\text{ kDa}\).

Approximately \(500\text{ kDa}\).

Glycosidic Linkages

Contains only \(\alpha\text{ 1-4 glycosidic linkages}\) joining \(250\text{ to }300\) glucose residues.

Contains \(\alpha\text{ 1-4 linkages}\) along main chains, with \(\alpha\text{ 1-6 linkages}\) forming branch points.

Reaction with Iodine

Forms a deep blue color as iodine molecules trap inside its helical structure (color vanishes on heating and reappears on cooling).

Forms a reddish-violet color with iodine solution.

4. Carbohydrates of Major Physiological and Clinical Significance

1. Monosaccharides

  • Glucose: The primary blood sugar and principal metabolic fuel for animal tissues. Synthesized via photosynthesis in plants, it serves as the essential precursor for glycogen storage, galactose production, and pentose synthesis.

  • Galactose: Synthesized in mammary glands to form lactose in milk; C4-epimer of glucose.

  • Fructose: Ketohexose structural isomer of glucose found in dietary fruits and sucrose.

  • Ribose & Deoxyribose: Essential structural pentoses forming the backbone of RNA and DNA, respectively.

2. Disaccharides & Clinical Relevance

  • Lactose (Milk Sugar): Major disaccharide in milk composed of galactose and glucose linked by a \(\beta\text{ 1-4}\) glycosidic bond.

    • Clinical Focus — Lactose Intolerance:

      • Etiology: Caused by a deficiency of the intestinal brush border enzyme lactase.

      • Pathophysiology: Uncleaved lactose cannot be absorbed in the small intestine and passes into the large intestine. Anaerobic colonic bacteria ferment the sugar into short-chain organic acids and gases (\(\text{H}_2\) and \(\text{CO}_2\)).

      • Clinical Presentation: Abdominal cramps, diarrhea, and flatulence appearing shortly after consuming dairy products.

      • Diagnosis: Confirmed by a positive hydrogen breath test (detecting increased exhaled \(\text{H}_2\) gas produced by bacterial fermentation).

      • Management: Dietary restriction of dairy products or oral administration of lactase enzyme tablets.

      • Excretion: Excretion of lactose in urine is termed lactosuria.

  • Sucrose: Non-reducing table sugar made of glucose and fructose joined via an \(\alpha\text{ 1-2}\) bond. Because both functional groups (aldehyde of glucose and ketone of fructose) are tied up in the bond, it yields a negative Benedict's test unless hydrolyzed first.

  • Maltose & Isomaltose: Reducing disaccharides formed as intermediates during the enzymatic digestion of starch and amylopectin.

3. Storage and Structural Polysaccharides

  • Glycogen ("Animal Starch"):

    • Primary storage carbohydrate in vertebrates, concentrated in liver and muscle cells.

    • Polymer of glucose units containing \(\alpha\text{ 1-4}\) chains with \(\alpha\text{ 1-6}\) branches.

    • Structurally similar to amylopectin, but significantly more compact and highly branched, having only 11 to 18 glucose residues between branch points. Hydrolyzed rapidly to liberate free glucose during energetic demand.

  • Cellulose:

    • Structural plant homopolymer composed of \(\beta\text{-D-glucose}\) units connected by \(\beta\text{ 1-4 glycosidic bonds}\), forming rigid linear chains stabilized by intermolecular hydrogen bonds.

    • Indigestible in humans due to the absence of the cellulase enzyme (present in certain herbivorous animals).

    • Functions as vital dietary fiber, providing stool bulk and stimulating intestinal peristalsis.

  • Inulin: Fructose polymer joined by \(\beta\text{ 2-1}\) linkages found in dahlia roots. Clinically utilized to measure the Glomerular Filtration Rate (GFR) to evaluate kidney function.

  • Chitin: Polymer of \(N\)-acetylglucosamine with \(\beta\)-glycosidic linkages that forms the structural exoskeleton of arthropods and insects, as well as fungal cell walls.

4. Mucopolysaccharides (GAGs) & Proteoglycans

  • Hyaluronic Acid: Component of the extracellular matrix ground substance that acts as a physical barrier against tissue penetration by bacteria.

  • Heparin: Heteropolysaccharide that acts as an anticoagulant both in vitro and in vivo by inhibiting thrombin.

5. Sugar Derivatives & Clinical Pharmacology

  • Sugar Alcohols (Alditols):

    • D-Sorbitol & D-Dulcitol: Intracellular accumulation exerts strong osmotic effects, causing cellular swelling implicated in cataracts, peripheral neuropathy, and nephropathy.

    • D-Mannitol: Administered clinically as an osmotic diuretic to reduce elevated intracranial pressure.

    • Xylitol: Used as a non-cariogenic sugar sweetener.

  • Glycosides in Medicine:

    • Cardiac Glycosides (Digitoxin / Digoxin): Molecules containing a carbohydrate bound to a non-carbohydrate residue; act as cardiac stimulants by increasing heart muscle contraction.

    • Streptomycin: Glycosidic antibiotic utilized in the treatment of tuberculosis.

6. Diagnostic Chemical Tests

  • Reducing Sugars: Sugars containing a free aldehyde or ketone group (all monosaccharides, maltose, lactose) act as reducing agents that convert cupric ions (\(\text{Cu}^{2+}\)) to cuprous ions (\(\text{Cu}^+\)). This forms the chemical basis of Benedict's test for detecting reducing sugars in clinical diagnostic assays.