Comprehensive Biochemistry Study Guide: Carbohydrates Chemistry

Learning Objectives

• Define and identify the biochemical importance and classification of carbohydrates.
• Define and classify stereoisomers.
• Study the optical activity and ring structure of monosaccharides.
• Understand mutarotation and its mechanistic explanation.
• Classify the physical and chemical properties of monosaccharides.
• Classify polysaccharides and identify their biomedical importance.

Definition and Chemical Nature of Carbohydrates

• General Definition: Carbohydrates are neutral organic compounds composed of carbon, hydrogen, and oxygen. In most carbohydrates, hydrogen and oxygen exist in the same 2:1 ratio as found in water, giving them the general empirical formula C_n(H_2O)_n$.\n- **Exceptions to the General Formula:**\n - **Non-carbohydrates sharing the empirical formula C_n(H_2O)_n$:**
  • Formaldehyde: $HCHO$
  • Acetic acid: $CH_3COOH$
  • Lactic acid: $CH_3\text{-}CHOH\text{-}COOH$
  • **Carbohydrates not fitting the empirical formula C_n(H_2O)_n$:**\n - Rhamnose: C_6H_{12}O_5 (where hydrogen and oxygen are not present in a 2:1 ratio).\n- **Chemical Definition:** Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones, or substances that yield polyhydroxy aldehydes or ketones upon hydrolysis. Chemically, they represent aldehyde or ketone derivatives of higher polyhydric alcohols.\n\n\n\n# Biomedical Importance of Carbohydrates\n\n- **Primary Energy Source:** Carbohydrates are the principal source of metabolic energy in the human body. Brain cells and red blood cells (RBCs) rely almost exclusively on carbohydrates for energy. The oxidation yield of carbohydrate combustion is 4\text{ kcal/g}.\n- **Energy Storage:** Carbohydrates function as energy reserves in plants (starch) and animals (glycogen). Excess ingested carbohydrates beyond daily requirements are converted into fat and stored in adipose tissue.\n- **Structural Components of Cell Membranes:** Glycoproteins and glycolipids are vital structural components of cellular membranes and function as cell-surface receptors.\n- **Structural Architecture of Organisms:**\n - Cellulose forms the structural framework of plant cell walls.\n - Chitin forms the exoskeleton of insects.\n - Peptidoglycan forms the cell walls of microorganisms.\n - Mucopolysaccharides (glycosaminoglycans) function as the ground substance in the extracellular matrix of higher organisms.\n- **Genetic Components:** Deoxyribose is an indispensable structural constituent of deoxyribonucleic acid (DNA), while ribose forms the sugar backbone of ribonucleic acid (RNA).\n- **Coenzyme Structure:** Ribose (a pentose sugar) enters into the structure of key cellular coenzymes, including Flavin Adenine Dinucleotide (FAD$), Nicotinamide Adenine Dinucleotide ($NAD^+$), and Coenzyme A ($CoA\n- **Physiological Sugar:** D-glucose serves as the chief physiological sugar circulating in human blood.\n\n# Classification of Carbohydrates\n\nCarbohydrates are broadly divided into three principal categories based on the degree of polymerization and hydrolytic products:\n1. Monosaccharides\n2. Oligosaccharides\n3. Polysaccharides\n\n\n\n## Monosaccharides\n\n- **Definition:** Monosaccharides (Greek *mono* = one; *saccharide* = sugar) are the simplest carbohydrate units that cannot be hydrolyzed into simpler sugar molecules.\n- **Classification by Functional Group:**\n - **Aldoses:** Monosaccharides containing an aldehyde functional group (-CHO).\n - **Ketoses:** Monosaccharides containing a keto functional group (-C=O).\n- **Classification by Carbon Chain Length:**\n - **Trioses:** Monosaccharides containing 3 carbon atoms.\n - **Tetroses:** Monosaccharides containing 4 carbon atoms.\n - **Pentoses:** Monosaccharides containing 5 carbon atoms.\n - **Hexoses:** Monosaccharides containing 6 carbon atoms.\n - **Heptoses:** Monosaccharides containing 7 carbon atoms.\n- **Combined Structural Classification:**\n - Aldotrioses and Ketotrioses\n - Aldotetroses and Ketotetroses\n - Aldopentoses and Ketopentoses\n - Aldohexoses and Ketohexoses\n - Aldoheptoses and Ketoheptoses\n\n| Number of Carbon Atoms | Generic Name | Aldoses (Aldehyde Group) | Ketoses (Keto Group) |\n| :--- | :--- | :--- | :--- |\n| 3 | Triose | Glyceraldehyde | Dihydroxyacetone |\n| 4 | Tetrose | Erythrose | Erythrulose |\n| 5 | Pentose | Arabinose, Xylose, Ribose | Xylulose, Ribulose |\n| 6 | Hexose | Glucose, Galactose, Mannose | Fructose |\n\n\n\n### Hexoses of Physiological Importance\n\n| Sugar | Physiological Importance |\n| :--- | :--- |\n| **D-Glucose** | Blood sugar; main source of cellular energy in the body. |\n| **D-Fructose** | Constituent of sucrose (table sugar); found in fruits and honey. |\n| **D-Galactose** | Constituent of lactose (milk sugar), glycolipids, and glycoproteins. |\n| **D-Mannose** | Constituent of serum globulins, mucoproteins, and glycoproteins. |\n\n# Stereoisomerism in Monosaccharides\n\n## Asymmetric Carbon Atoms and Stereoisomers\n\n- **Definition:** Stereoisomers are chemical compounds possessing the identical structural formula and chemical bonds, but differing in the three-dimensional spatial configuration of their atoms or functional groups.\n- **Asymmetric Carbon:** A carbon atom bonded to four completely different atoms or chemical groups. The presence of asymmetric carbon atoms imparts stereoisomerism.\n- **Reference Molecule:** Glyceraldehyde (glycerose) is the fundamental reference molecule because it contains a single asymmetric carbon atom (C-2).\n- **Calculation of Stereoisomers:** The total number of possible stereoisomers for a compound is given by the formula:\n  \text{Total Stereoisomers} = 2^n\n  where n represents the number of asymmetric carbon atoms.\n\n\n\n## D- and L- Isomerism\n\n- **Reference Carbon Atom:** The designation of D- or L- configuration depends on the spatial orientation of the -H$and$-OH groups on the reference asymmetric carbon—which is the highest-numbered asymmetric carbon atom (e.g., C-5 in glucose).\n- **D-Isomers:** The hydroxyl (-OH) group on the reference asymmetric carbon points to the right in the Fischer projection.\n- **L-Isomers:** The hydroxyl (-OH) group on the reference asymmetric carbon points to the left in the Fischer projection.\n- **Mirror Images:** D- and L- isomers are non-superimposable mirror images (enantiomers). Inverting D-glucose to L-glucose requires reversing the -H$and$-OH spatial orientations across all asymmetric carbons (C-2, C-3, C-4, and C-5).\n- **Biological Selectivity:** Naturally occurring sugars in human metabolism are almost exclusively D-sugars. Human enzymes are stereospecific and can metabolize only D-stereoisomers.\n\n\n\n## Epimers of Glucose\n\n- **Epimeric Carbon:** An asymmetric carbon atom other than the carbonyl carbon (C-1 in aldoses, C-2 in ketoses). In glucose, C-2, C-3, and C-4 are epimeric carbons.\n- **Definition of Epimers:** Epimers are stereoisomers that differ in the spatial configuration around only one specific asymmetric carbon atom.\n- **Relationships Among Glucose Epimers:**\n - **D-Galactose:** Epimer of D-glucose at C-4 (differs only in the orientation of -H$and$-OH at carbon 4).\n - **D-Mannose:** Epimer of D-glucose at C-2 (differs only in the orientation of -H$and$-OH at carbon 2).\n - D-Galactose and D-Mannose are not epimers of each other because they differ at two carbon centers (C-2 and C-4).\n\n\n\n## Optical Activity\n\n- **Mechanism:** The presence of asymmetric carbon atoms imparts optical activity to carbohydrate solutions. When plane-polarized light passes through a solution of an optically active sugar, the plane of polarization rotates.\n- **Direction of Rotation:**\n - **Dextrorotatory (+$or$d):** Rotates the plane of polarized light to the right (clockwise).\n - **Levorotatory (-$or$l):** Rotates the plane of polarized light to the left (counter-clockwise).\n- **Distinction Between Configuration and Rotation:** The structural prefix (D- or L-) indicates absolute spatial configuration on the reference carbon and has no direct relationship with optical rotation (d$- or$l$- /$+$or$-$). For example, D-glucose is dextrorotatory ($+$), whereas D-fructose is levorotatory ($-).\n- **Racemic Mixture:** An equimolar mixture of dextrorotatory and levorotatory enantiomers displays zero net optical rotation due to mutual cancellation.\n\n# Ring Structures and Anomers of Monosaccharides\n\n## Ring Formation (Hemiacetal and Hemiketal)\n\n- **Intramolecular Cyclization:** Monosaccharide molecules containing 4, 5, or 6 carbons possess flexible backbones. This flexibility allows the functional carbonyl group to bend close to hydroxyl groups located on distal carbons within the same molecule.\n- **Hemiacetal Formation:** Intramolecular reaction between an aldehyde group (-CHO$at C-1) and a hydroxyl group ($-OH at C-5) yields a cyclic hemiacetal.\n- **Hemiketal Formation:** Intramolecular reaction between a keto group (-C=O$at C-2) and a hydroxyl group ($-OH at C-5) yields a cyclic hemiketal.\n\n## Pyranose and Furanose Rings\n\n- **Pyranose Ring:** A 6-membered ring system consisting of 5 carbon atoms and 1 oxygen atom, derived from the heterocyclic ring *pyran*.\n- **Furanose Ring:** A 5-membered ring system consisting of 4 carbon atoms and 1 oxygen atom, derived from the heterocyclic ring *furan*.\n- **Predominant Forms:**\n - Glucose exists predominantly as a 6-membered pyranose ring (**Glucopyranose**).\n - Fructose exists predominantly as a 5-membered furanose ring (**Fructofuranose**).\n\n\n\n## Anomeric Carbon and Anomers\n\n- **Anomeric Carbon:** The newly created asymmetric carbon atom derived from the carbonyl carbon upon ring closure (C-1 in aldoses, C-2 in ketoses).\n- **Anomers:** Stereoisomers that differ specifically in the spatial orientation of the newly formed hydroxyl group attached to the anomeric carbon.\n- **Anomeric Orientations:**\n - **\alpha$-Anomer:** The hydroxyl group attached to the anomeric carbon points downward (trans to the$-CH_2OH terminal group in Haworth projections).\n - **\beta$-Anomer:** The hydroxyl group attached to the anomeric carbon points upward (cis to the$-CH_2OH terminal group in Haworth projections).\n\n## Mutarotation\n\n- **Definition:** Mutarotation is the spontaneous change in the specific optical rotation of an optically active carbohydrate solution over time, reaching an equilibrium mixture of \alpha$- and$\beta-anomers alongside open-chain forms.\n- **Mechanism in Glucose:**\n - Freshly prepared crystalline \alpha$-D-glucose dissolved in water exhibits a specific rotation of$+112.2^\circ\n - Freshly prepared crystalline \beta$-D-glucose dissolved in water exhibits a specific rotation of$+18.7^\circ\n - Upon standing in solution, both forms open and re-cyclize through an equilibrium open-chain intermediate. Eventually, the mixture stabilizes at an equilibrium optical rotation of +52.7^\circ$(consisting of roughly two-thirds$\beta$-anomer, one-third$\alpha-anomer, and trace amounts of open-chain form).\n\n\n\n# Properties of Monosaccharides\n\n## Physical Properties\n\n- **Solubility:** All monosaccharides are readily soluble in water due to their extensive hydrogen-bonding capabilities.\n- **Taste:** Dissolved monosaccharides yield a sweet taste.\n- **Optical Activity:** All monosaccharides possess optical activity, with the exception of **dihydroxyacetone** (which lacks an asymmetric carbon).\n- **Anomeric State:** All monosaccharides with cyclic structures exist in \alpha$and$\beta anomeric forms.\n- **Mutarotation:** All monosaccharides capable of ring closure undergo mutarotation in aqueous solution.\n\n## Chemical Properties\n\nChemical properties are dictated by the two functional groups present in monosaccharides: hydroxyl (-OH$) groups and carbonyl ($-CHO$or$-C=O) groups.\n\n### Reactions of the Hydroxyl (OH) Group\n\n- **Esterification:** Hydroxyl groups react with organic or inorganic acids to form esters. Phosphoric acid esters (e.g., Glucose-6-phosphate) represent vital metabolic intermediates in carbohydrate utilization pathways.\n- **Oxidation of Primary Alcohol (C-6):** Selective oxidation of the terminal primary alcohol group (-CH_2OH) yields **uronic acids** without altering the aldehyde group:\n - Glucose \rightarrow Glucuronic acid\n - Galactose \rightarrow Galacturonic acid\n\n\n\n- **Formation of Glycosides:** A glycosidic bond forms between the hydroxyl group attached to the anomeric carbon (C-1 of aldoses or C-2 of ketoses) and a second compound.\n - **Carbohydrate Acceptor:** Forms disaccharides, oligosaccharides, or polysaccharides.\n - **Aglycone (Non-carbohydrate) Acceptor:** Forms specialized glycosides:\n - Glycolipids and Glycoproteins\n - Tannic acid (digallic acid attached to glucose)\n\n\n\n### Reactions of the Carbonyl Group\n\n- **Reduction to Sugar Alcohols:** The carbonyl group is reduced to yield polyhydric sugar alcohols:\n - Glyceraldehyde \rightarrow Glycerol\n - Ribose \rightarrow$Ribitol (constituent of Vitamin$B_2 / Riboflavin)\n - Glucose \rightarrow Sorbitol\n - Galactose \rightarrow Galactitol\n - Mannose \rightarrow Mannitol\n - Fructose \rightarrow Equal amounts of Sorbitol and Mannitol (reduction creates a new asymmetric center at C-2)\n- **Oxidation of Aldo-sugars:**\n - **Mild Oxidation (e.g., dilute nitric acid or hypobromite):** Oxidizes the aldehyde group to a carboxyl group, forming **aldonic acids**:\n - Glucose \rightarrow Gluconic acid\n - Galactose \rightarrow Galactonic acid\n - **Strong Oxidation (e.g., hot concentrated nitric acid):** Oxidizes both the aldehyde group and the primary alcohol group (C-6) to carboxyl groups, forming dicarboxylic **aldaric acids**:\n - Glucose \rightarrow Glucaric acid (Saccharic acid)\n - Galactose \rightarrow Galactaric acid (Mucic acid)\n- **Reducing Ability:** Free aldehyde or ketone groups render monosaccharides potent reducing agents.\n - Reduce metal ions in reagents like **Fehling reagent** and **Benedict reagent**.\n - Benedict reagent is clinically preferred over Fehling reagent because it is more stable over extended storage, more sensitive, and more specific.\n\n# Derivatives of Monosaccharides\n\n## Amino Sugars\n\n- **Structure:** Compounds in which the hydroxyl group at C-2 is replaced by an amino group (-NH_2).\n- **Primary Examples:** Glucosamine, Galactosamine, and Mannosamine.\n- **Biological Occurrence:** They frequently occur as N$-acetylated derivatives (e.g.,$N-acetylglucosamine) in mucopolysaccharides and cell membrane components.\n\n\n\n## Deoxy Sugars\n\n- **Structure:** Sugars in which a hydroxyl group (-OH$) is replaced by a hydrogen atom ($-H).\n- **Important Examples:**\n - **2-Deoxyribose:** Lacks hydroxyl group at C-2; essential building block of DNA.\n - **L-Fucose (L-6-deoxygalactose):** Lacks hydroxyl group at C-6; essential component of glycoproteins and human ABO blood group antigens.\n\n\n\n## Sugar Acids\n\n- **Aldonic Acids:** Formed by oxidation of the aldehyde group (e.g., Gluconic acid).\n- **Uronic Acids:** Formed by oxidation of the terminal primary alcohol group (e.g., Glucuronic acid).\n- **Aldaric Acids:** Formed by dual oxidation of both aldehyde and primary alcohol groups (e.g., Glucaric acid).\n\n\n\n## Sugar Alcohols\n\n- Formed by carbonyl group reduction (e.g., Sorbitol from glucose, Ribitol from ribose).\n\n# Oligosaccharides\n\n## Classification of Oligosaccharides\n\n- **Definition:** Carbohydrates composed of 2 to 10 monosaccharide units linked by glycosidic bonds, released upon complete hydrolysis.\n- **Categories:**\n - **Disaccharides:** 2 monosaccharide units\n - **Trisaccharides:** 3 monosaccharide units\n - **Tetrasaccharides:** 4 monosaccharide units\n - **Pentasaccharides:** 5 monosaccharide units\n- Disaccharides represent the most abundant and biologically significant oligosaccharides.\n\n\n\n## Disaccharides\n\n- **Definition:** Formed by the condensation of 2 monosaccharide molecules with the elimination of water, connected via a glycosidic bond. General empirical formula: C_n(H_2O)_{n-1}.\n\n\n\n### Sucrose\n\n- **Structure:** \alpha$-D-glucose and$\beta$-D-fructose joined by an$\alpha(1\rightarrow \beta 2) glycosidic linkage.\n- **Sources:** Sugar cane, sugar beets, table sugar, pineapples, and carrots.\n- **Chemical Properties:**\n - Contains no free aldehyde or keto group because both anomeric carbons (C-1 of glucose and C-2 of fructose) participate in the glycosidic bond.\n - Non-reducing sugar (gives negative test with Benedict's reagent).\n - Cannot exist in \alpha$or$\beta anomeric forms.\n - Does not exhibit mutarotation.\n - Dextrorotatory (+20^\circ\n- **Hydrolysis Pathways:**\n - Acid Hydrolysis: \text{Sucrose} + H_2O \xrightarrow{\text{Acid}} \text{Glucose} + \text{Fructose}\n - Enzymatic Hydrolysis: \text{Sucrose} + H_2O \xrightarrow{\text{Invertase / Sucrase}} \text{Glucose} + \text{Fructose}\n\n### Invert Sugar\n\n- **Structure:** An equimolar mixture of free D-glucose and D-fructose generated by sucrose hydrolysis.\n- **Mechanism of Inversion:** Sucrose is dextrorotatory (+20^\circ$). Upon hydrolysis, it splits into equal parts D-glucose ($+52.5^\circ$dextrorotatory) and D-fructose ($-92^\circ levorotatory). Because the levorotation magnitude of fructose outweighs the dextrorotation magnitude of glucose, the overall optical rotation flips from positive to negative (levorotatory). This optical reversal is termed *inversion*, and the product is called *invert sugar*.\n- **Natural Sources:** Bee honey; enzymatic digestion of sucrose via sucrase/invertase.\n\n### Lactose\n\n- **Structure:** \beta$-D-galactose and D-glucose joined by a$\beta(1\rightarrow 4) galactosidic linkage.\n- **Source:** The principal carbohydrate found in mammalian milk. Human milk contains 7\text{ g}\% lactose.\n- **Chemical Properties:**\n - Gives a positive Molisch test.\n - Retains a free C-1 aldehyde group on the glucose moiety, making it a reducing sugar that reduces Benedict and Fehling reagents.\n - Indigestible by intestinal yeast due to absence of fungal lactase.\n- **Nutritional Advantages for Infants:**\n - Non-fermentable in gut, preventing gas production and gastrointestinal distress.\n - Less sweet than sucrose, preventing sensory aversion and allowing adequate nutritional intake.\n - Serves as the sole natural source of D-galactose, required for galactolipid synthesis in infant brain and neural tissues.\n - Exerts a mild laxative effect promoting healthy stool elimination.\n\n### Maltose\n\n- **Structure:** Two \alpha$-D-glucose molecules joined by an$\alpha(1\rightarrow 4) glucosidic linkage.\n- **Sources:** Malt sugar; intermediate product formed during starch digestion by salivary and pancreatic amylase.\n- **Properties:** Positive Molisch test; possesses a free anomeric aldehyde group, acting as a reducing sugar.\n\n### Isomaltose\n\n- **Structure:** Two \alpha$-D-glucose molecules joined by an$\alpha(1\rightarrow 6) glucosidic linkage.\n- **Sources:** Produced during enzymatic digestion of starch amylopectin at branch points by amylase.\n- **Digestion:** Hydrolyzed into individual glucose units by the enzyme isomaltase.\n\n### Summary of Key Disaccharides\n\n| Disaccharide | Monosaccharide Composition | Glycosidic Bond Type |\n| :--- | :--- | :--- |\n| **Maltose** | \alpha$-Glucose +$\alpha$-Glucose |$\alpha(1\rightarrow 4) |\n| **Isomaltose** | \alpha$-Glucose +$\alpha$-Glucose |$\alpha(1\rightarrow 6) |\n| **Lactose** | \beta$-Galactose +$\beta$-Glucose |$\beta(1\rightarrow 4) |\n| **Sucrose** | \alpha$-Glucose +$\beta$-Fructose |$\alpha(1\rightarrow \beta 2) |\n| **Cellobiose** | \beta$-Glucose +$\beta$-Glucose |$\beta(1\rightarrow 4) |\n| **Trehalose** | \alpha$-Glucose +$\alpha$-Glucose |$\alpha(1\rightarrow 1) |\n\n# Polysaccharides\n\n- **Definition:** High-molecular-weight polymers made of numerous repeating monosaccharide units or derivatives joined by glycosidic bonds.\n- **Primary Biological Roles:** Energy storage and structural architecture.\n- **Classification:**\n - **Homopolysaccharides:** Polymers composed of a single repeating monosaccharide type.\n - **Heteropolysaccharides:** Polymers composed of two or more distinct monosaccharide types or derivatives.\n\n\n\n## Homopolysaccharides\n\n### Starch\n\n- **Structural Architecture:** Plant energy reserve formed of two distinct structural layers within granules:\n 1. **Amylose (Inner Layer):** Constitutes 15\text{--}20\%$of the starch granule. Unbranched helical structure of D-glucose units linked by$\alpha(1\rightarrow 4) glycosidic bonds. Forms a deep blue complex with iodine solution.\n 2. **Amylopectin (Outer Layer):** Constitutes 80\text{--}85\%$of the starch granule. Highly branched structure consisting of$\alpha(1\rightarrow 4)$linked glucose chains with$\alpha(1\rightarrow 6) glycosidic linkages at branch points. Forms a red/purple complex with iodine. Whole starch granules yield a characteristic blue color with iodine.\n\n\n\n- **Sources:** Primary dietary carbohydrate (60\% of daily intake). Rich sources include cereals (rice, barley, maize, wheat) and tubers (potatoes, sweet potatoes). Completely absent in animal tissues.\n- **Hydrolysis:**\n - **Acid Hydrolysis:** Heating starch with dilute inorganic acids breaks all glycosidic bonds to yield free D-glucose.\n - **Enzymatic Digestion Sequence:**\n    \text{Starch} \rightarrow \text{Amylodextrin} \rightarrow \text{Erythrodextrin} \rightarrow \text{Achrodextrin} \rightarrow \text{Maltose} + \text{Isomaltose}\n\n### Dextrins\n\n- Intermediate breakdown products formed during starch digestion by salivary or pancreatic \alpha-amylase, categorized sequentially into amylodextrin, erythrodextrin, and achrodextrin.\n\n### Dextrans\n\n- **Structure:** Branched homopolysaccharides of glucose linked by \alpha(1\rightarrow 3)$bonds in linear chains and$\alpha(1\rightarrow 6) bonds at branch points.\n- **Source:** Synthesized from sucrose by bacterial cultures.\n- **Medical Application:** Used clinically as plasma volume expanders in shock.\n\n### Glycogen\n\n- **Structure:** Known as *animal starch*. Highly branched homopolysaccharide of \alpha$-D-glucose. Linear chains utilize$\alpha(1\rightarrow 4)$linkages, with$\alpha(1\rightarrow 6) linkages at branch points occurring every 8–12 glucose residues (significantly more compact and frequently branched than amylopectin).\n- **Tissue Location:** Stored predominantly in liver parenchymal cells and skeletal muscle fibers.\n\n\n\n### Comparison between Liver and Muscle Glycogen\n\n| Parameter | Liver Glycogen | Muscle Glycogen |\n| :--- | :--- | :--- |\n| **Tissue Concentration** | 8\text{--}10\%$of liver wet weight |$2\% of muscle wet weight | | *Primary Function* | Maintains blood glucose during early fasting | Local energy source for muscle contraction | | Effect of Fasting | Depleted after 12–18 hours | Little to no effect | | Effect of Strenuous Exercise | Minimal immediate change | Rapidly depleted | | Hormonal Control - Insulin | Stimulates glycogenesis | Stimulates glycogenesis | | Hormonal Control - Glucagon | Inhibits glycogenesis / stimulates glycogenolysis | No effect (lacks glucagon receptors) | | Hormonal Control - Adrenalin | Inhibits glycogenesis / stimulates glycogenolysis | Inhibits glycogenesis / stimulates glycogenolysis |

Agar-agar

• Structure: Galactose homopolysaccharide derived from seaweeds.
• Application: Solidifying agent used to prepare bacterial culture media in microbiology.

Cellulose

• Structure: Unbranched linear homopolysaccharide of \beta$-D-glucose units joined by$\beta(1\rightarrow 4) glycosidic bonds.
• Occurrence: Chief structural material in plant cell walls.
• Digestibility: Insoluble in water; gives no color with iodine. Humans and non-ruminant mammals cannot digest cellulose because they lack hydrolase enzymes capable of cleaving \beta(1\rightarrow 4) linkages.
• Physiological Importance: Forms indigestible dietary fiber that increases stool bulk, enhances intestinal peristalsis, and prevents constipation.

Inulin

• Structure: Fructosan polymer composed of repeating D-fructose units linked by \beta(1\rightarrow 2) glycosidic bonds.
• Clinical Application: Administered intravenously to measure Glomerular Filtration Rate (GFR) in renal function testing.

Heteropolysaccharides

Gums

• Complex heteropolysaccharides (e.g., Gum Arabic) composed of distinct monosaccharides such as arabinose and galactose.

Pectins

• Structural heteropolysaccharides found in fruits; form gels used commercially in jam production and medically in managing infantile diarrhea.

Mucopolysaccharides (Glycosaminoglycans / GAGs)

• Structure: Unbranched heteropolysaccharide chains composed of repeating disaccharide units: [Acidic Sugar – Amino Sugar].
  • Intra-disaccharide Linkage: \beta(1\rightarrow 3)
  • Inter-disaccharide Linkage: \beta(1\rightarrow 4)
  • Acidic Sugar Component: D-Glucuronic acid or its C-5 epimer, L-Iduronic acid.
  • Amino Sugar Component: D-Glucosamine or D-Galactosamine (frequently N-acetylated and sulfated at C-4 or C-6).
• Charge Properties: High density of sulfate and carboxylate groups yields a strongly negative charge across the polymer.
• Localization: Extracellular matrix components of connective tissue (bone, cartilage, collagen, elastin), with the sole exception of heparin (which is intracellular).
• Biomechanical Function: Act as tissue lubricants and shock absorbers due to high water-binding capacity. Physical compression forces water out of the GAG matrix; releasing compression causes hydrated volume recovery due to electrostatic repulsion between negative charges. This property accounts for the compressibility of joint synovial fluid and ocular vitreous humor.

Individual Glycosaminoglycans

• Mucoitin Sulfate: Complexed with proteins in mucin secreted by digestive and respiratory mucosa.
• Chondroitin Sulfate: The most abundant GAG in the human body. Located in cartilage, tendons, ligaments, bone, aorta, skin, cornea, umbilical cord, and neurons. Used as a dietary supplement for osteoarthritis treatment.
• Hyaluronic Acid: Highly viscous GAG abundant in subcutaneous tissue, synovial fluid, vitreous humor, umbilical cord, cartilage, and around the ovum. Functions as a joint lubricant and facilitates cell migration during tissue repair and embryonic morphogenesis. Hydrolyzed by the tissue-permeability enzyme hyaluronidase.
• Heparin: Intracellular GAG found in mast cells along blood vessel walls in the liver, lungs, skin, heart, kidneys, and spleen. Serves as a potent biological anticoagulant.
• Other GAGs: Heparan sulfate, Keratan sulfate, and Dermatan sulfate.

Clinical Correlations and Pharmacology

Carbohydrate-Based Pharmaceutical Drugs

• Antiviral Drugs: Carbohydrate derivatives target viral enzymes to block replication or cell attachment. Example: Oseltamivir (Tamiflu) used in treating influenza.
• Anti-inflammatory Drugs: Carbohydrate derivatives modulate immune signaling to reduce systemic or local inflammation. Example: Mesalazine (5-aminosalicylic acid) used in managing inflammatory bowel disease (IBD).
• Vaccines: Purified or synthetic carbohydrate antigens stimulate protective adaptive immunity. Example: Haemophilus influenzae type b (Hib) vaccine containing capsular polysaccharide antigens.

Health Benefits of Dietary Fibers

1. Prevents constipation and promotes normal gastrointestinal motility.
2. Binds and eliminates bacterial toxins and harmful metabolites by absorbing fluid and toxic residues.
3. Increases fecal mass and decreases intestinal transit time.

Questions & Discussion

• Question 1: Which of the following glycosidic bonds prevents humans from completely digesting fiber?

  • a. Glucose \alpha(1\rightarrow 4) glucose
  • b. Glucose \alpha(1\rightarrow 6) glucose
  • c. Glucose \beta(1\rightarrow 4) glucose
  • d. Glucose \alpha(1\rightarrow 2) fructose
  • e. Galactose \beta(1\rightarrow 4) glucose
  • Answer: c. Glucose \beta(1\rightarrow 4)$glucose.</strong> Humans lack the hydrolytic enzymes capable of cleaving the$\beta(1\rightarrow 4)$$ glycosidic linkages found in cellulose.

• Question 2: Which of the following is NOT a disaccharide?

  • a. Galactose
  • b. Sucrose
  • c. Lactose
  • d. Maltose
  • Answer: a. Galactose. Galactose is a monosaccharide hexose sugar, whereas sucrose, lactose, and maltose are disaccharides.