Exhaustive Biochemistry Study Notes: Chemistry of Carbohydrates

Definition of Carbohydrates

• Etymology: The term "saccharide" originates from the Greek word for sugar.

• Chemical Definition: A large biological macromolecule consisting of carbon ($C$), hydrogen ($H$), and oxygen ($O$).

• Empirical Formula: $(CH_2O)_n$

  • For every carbon atom, there is an equal proportion of hydrogen and oxygen atoms identical to the ratio in water ($H_2O$), earning them the name "hydrates of carbon".

• Abundance and Dietary Importance: Carbohydrates are the most abundant organic macromolecules on Earth and serve as the major source of dietary energy for living organisms.

  • Dietary intake of plant materials is the primary carbohydrate source for humans.

  • The average human diet should ideally consist of approximately two-thirds ($\frac{2}{3}$) carbohydrate by mass.

• Exceptions to the Rule:

  • Non-Carbohydrates possessing the $(CH_2O)_n$ formula: Acetic Acid ($C_2H_4O_2$) and Lactic Acid ($C_3H_6O_3$) match the empirical formula ratio but are organic acids, not carbohydrates.

  • True Carbohydrates deviating from $(CH_2O)_n$: Rhamnose ($C_6H_{12}O_5$) is a deoxysugar (modified carbohydrate) that does not match the standard hydrogen-to-oxygen ratio.

• Functional Definition: Polyhydroxylated compounds possessing multiple hydroxyl groups ($-OH$) with at least 3 carbon atoms that contain or yield a carbonyl group ($C=O$) upon hydrolysis.

  • Carbonyl Carbon: A carbon atom double-bonded to an oxygen atom ($C=O$).

• Subclassification by Carbonyl Location:

  • Aldose sugar: Aldehyde-containing sugar with the carbonyl carbon situated at position C1.

  • Ketose sugar: Ketone-containing sugar with the carbonyl carbon situated at position C2.

Functions of Carbohydrates

• Primary Metabolic Energy Source: Preferred energy source for cellular respiration and a major macronutrient in human nutrition.

• Short-Term Energy Reserve: Stored in human liver and skeletal muscle tissue as glycogen, serving as a rapidly accessible short-term energy reserve.

• Cell Membrane Components: Covalently attached to lipids (glycolipids) to form structural framework components of cell membranes.

• Biosynthetic Precursors: Supply carbon skeletons for the endogenous synthesis of other vital biochemical molecules, including proteins, lipids, and nucleic acids.

• Genetic Framework: Form structural backbone components of ribonucleic acid (RNA, via ribose) and deoxyribonucleic acid (DNA, via deoxyribose).

• Cellular Communication and Recognition: Covalently bound to membrane proteins (glycoproteins) to function in cell-to-cell signaling, tissue recognition, and molecular targeting.

Carbohydrate Structures

• Two Structural Representations:

  1. Fischer Projection: Linear open-chain representation.

  2. Haworth Projection: Cyclic closed-chain representation.

Fischer Projections (Open Chain)

• Carbon Orientation: Connects carbon atoms vertically, placing the most oxidized functional group (carbonyl carbon) at the top assigned with the lowest numerical locant (C1 for aldoses, C2 for ketoses).

• Bond Representation: Carbon-hydrogen and carbon-hydroxyl bonds are drawn horizontally.

• Chiral Centers: Intersections of vertical and horizontal line segments represent chiral (asymmetric) carbon atoms.

• Polyhydroxy Backbone: Demonstrates the presence of hydroxyl functional groups attached to each non-carbonyl carbon atom along the chain.

Haworth Projection (Cyclic Chain)

• Predominant Biological Form: Represents the prevalent, naturally occurring structural state of monosaccharides in aqueous solution ($>99\%$ exists in cyclic closed-chain form, while $<1\%$ remains in open-chain linear form).

• Thermodynamic Favorability: Ring closure produces the most thermodynamically stable conformation.

• Formation Mechanism: Results from an intramolecular nucleophilic reaction between an alcohol hydroxyl group ($-OH$) and the carbonyl carbon (aldehyde or ketone).

Conversion: Fischer to Haworth Projection

• Intramolecular Attack: The hydroxyl group attached to the penultimate carbon (C5 in hexoses) performs a nucleophilic attack on the electrophilic carbonyl carbon (C1 in aldoses, C2 in ketoses).

• Deprotonation and Cyclization: Deprotonation converts the C5 hydroxyl oxygen into a reactive nucleophile that bonds to the carbonyl carbon, forming a cyclic hemiacetal and generating a new asymmetric chiral center at C1 (the anomeric carbon).

• Spatial Orientation Rules (Fischer to Haworth Conversion):

  • Substituents written on the RIGHT side of a vertical Fischer projection point DOWNWARDS in the Haworth projection.

  • Substituents written on the LEFT side of a vertical Fischer projection point UPWARDS in the Haworth projection.

• Steric Considerations: Although any hydroxyl group along the chain can theoretically attack the carbonyl carbon, C5 attack is sterically favored, producing stable 5- or 6-membered rings.

• Chair and Boat Conformations:

  • Due to the tetrahedral bond angles of carbon ($109.5^\text{o}$), 6-membered pyranose rings exist in non-planar bent conformations known as "chair" or "boat" forms.

  • Chair Configuration: Highly stable and favored over the boat form because bulky substituent groups occupy equatorial positions further apart from each other, minimizing electron pair repulsion and steric hindrance.

• Ring Size Terminology:

  • Pyranose: A 6-membered cyclic ring system containing 5 carbon atoms and 1 oxygen atom (derived from pyran).

  • Furanose: A 5-membered cyclic ring system containing 4 carbon atoms and 1 oxygen atom (derived from furan).

Acetals and Hemiacetals

• Hemiacetal Definition: A functional group in which a single carbon atom is simultaneously bonded to one hydroxyl group ($-OH$) and one alkoxy group ($-OR$).

  • Formed via nucleophilic addition of an alcohol to an aldehyde or ketone.

  • Directly corresponds to the cyclic ring structure of monosaccharides.

• Acetal Definition: A functional group in which a single carbon atom is simultaneously bonded to two distinct alkoxy groups ($-OR$).

  • Formed when a hemiacetal reacts with a second alcohol molecule in the presence of an acid catalyst ($H^+$), liberating one molecule of water ($H_2O$).

• Cyclic Acetals: Glycosidic linkages connecting individual monosaccharide units into disaccharides and polysaccharides represent cyclic acetal linkages.

Classification of Carbohydrates

• Classification Criteria:

  • By Carbon Atom Count:

    • Trioses: 3 carbon atoms ($C_3H_6O_3$)

    • Tetroses: 4 carbon atoms ($C_4H_8O_4$)

    • Pentoses: 5 carbon atoms ($C_5H_{10}O_5$)

    • Hexoses: 6 carbon atoms ($C_6H_{12}O_6$)

    • Heptoses: 7 carbon atoms ($C_7H_{14}O_7$)

  • By Carbonyl Functional Group:

    • Aldose: Possesses an aldehyde group at C1.

    • Ketose: Possesses a ketone group at C2.

• Monosaccharide Definition: The simplest class of carbohydrates, consisting of a single polyhydroxy aldehyde or polyhydroxy ketone unit. Monosaccharides cannot be hydrolyzed into simpler carbohydrate compounds.

  • Physical Properties: Pure monosaccharides are water-soluble, white crystalline solids that taste sweet.

  • Carbon Range: Contain 3 to 7 carbon atoms (5- and 6-carbon monosaccharides are the most biologically common).

• Derived Carbohydrates: Modified carbohydrates resulting from structural chemical changes to basic monosaccharides:

  • Reduction: Carbonyl groups reduced to alcohols (forming sugar alcohols or polyols).

  • Oxidation: Carbonyl or hydroxyl groups oxidized to carboxylic acids (forming sugar acids).

  • Substitution: Hydroxyl groups substituted by amino groups (amino sugars) or hydrogen atoms (deoxysugars).

• Polysaccharides Subclassification: Macromolecules containing more than 10 monosaccharide units joined by glycosidic bonds.

  • Homopolysaccharides: Composed of a single repeating monosaccharide species (e.g., glycogen, starch, cellulose).

  • Heteropolysaccharides: Composed of two or more distinct repeating monosaccharide species (e.g., glycosaminoglycans).

Biochemically Important Monosaccharides

Glyceraldehyde and Dihydroxyacetone

• Biological Role: Triose monosaccharides that function as essential metabolic intermediates in glycolysis (converting glucose to pyruvate).

• Parent Molecules: Serve as the structural parent compounds for all aldoses and ketoses.

• Glyceraldehyde: Simplest aldotriose ($C_3H_6O_3$); chiral molecule containing one asymmetric carbon center (C2).

• Dihydroxyacetone: Simplest ketotriose ($C_3H_6O_3$); achiral molecule containing zero asymmetric carbon centers.

• D-Glyceraldehyde Family Tree: Gives rise to D-Erythrose and D-Threose $\rightarrow$ D-Ribose, D-Arabinose, D-Xylose, D-Lyxose $\rightarrow$ D-Allose, D-Altrose, D-Glucose, D-Mannose, D-Gulose, D-Idose, D-Galactose, and D-Talose.

• Dihydroxyacetone Family Tree: Gives rise to D-Erythrulose $\rightarrow$ D-Ribulose and D-Xylulose $\rightarrow$ D-Psicose, D-Fructose, D-Sorbose, and D-Tagatose.

Aldohexoses Overview

• Eight D-Aldohexoses Mnemonic: "All altruists gladly make gum in gallon tanks" (Allose, Altrose, Glucose, Mannose, Gulose, Idose, Galactose, Talose).

• Chiral Configuration Table for D-Aldohexoses:

• Note: The D-configuration is universally dictated by the C5 hydroxyl pointing to the Right ($R$) in Fischer projections.

Glucose and Fructose

• Glucose:

  • Most abundant monosaccharide in nature and primary blood fuel for human metabolic processes.

  • Aldohexose forming a 6-membered pyranose ring.

  • Also called dextrose (dextrorotatory optical activity) or grape sugar (grapefruit contains $20\text{--}30\%$ glucose by mass).

  • Precursor for synthesizing all body carbohydrates; absorbed directly into the blood following enzymatic hydrolysis of dietary starch and disaccharides.

• Fructose:

  • Biologically most important ketohexose; sweetest tasting natural sugar.

  • Forms a 5-membered furanose ring.

  • Also called levulose (levorotatory optical activity) or fruit sugar (found in fruits and honey).

  • Contains two carbon atoms ($CH_2OH$ at C1 and C6) situated outside its cyclic ring structure.

Galactose

• Aldohexose forming a 6-membered pyranose ring.

• Known as milk sugar or brain sugar (constituent of brain and nerve tissue glycosphingolipids).

• C4 Epimer of Glucose: Differs from D-glucose specifically in the spatial orientation of the C4 hydroxyl group.

• Determines ABO blood group specificity when attached as oligosaccharides on cell surfaces.

Ribose

• Aldopentose forming a 5-membered cyclic ring.

• Essential structural backbone component of nucleic acids (RNA), coenzymes, and energy carriers (ATP).

Properties of Monosaccharides

Chirality and Chiral Carbons

• Chiral Center Definition: A tetrahedral carbon atom bonded to four completely different atoms or substituent groups. Chiral molecules are non-superimposable on their mirror images.

• Rules for Identifying Achiral Carbons:

  • Carbons involved in multiple bonds ($C=O$, $C=C$).

  • Carbons bonded to two or more identical groups (e.g., $-CH_2-$ or $-CH_3$).

  • Carbons possessing internal planes of symmetry.

• Identifying Chiral Centers in Rings: A ring carbon is chiral if it is bonded to two different non-ring substituents and the two path halves of the ring originating from that center are non-identical.

• Chiral Carbon Counting in Monosaccharides:

  • Aldoses: C1 ($C=O$) is achiral; C2 through penultimate carbon are chiral; terminal $CH_2OH$ is achiral. Formula for total chiral centers: $n - 2$ (where $n$ = total carbon count). Aldohexoses (e.g., glucose) possess 4 chiral centers (C2, C3, C4, C5).

  • Ketoses: C1 ($CH_2OH$) is achiral; C2 ($C=O$) is achiral; C3 through penultimate carbon are chiral; terminal $CH_2OH$ is achiral. Formula for total chiral centers: $n - 3$. Ketohexoses (e.g., fructose) possess 3 chiral centers (C3, C4, C5).

  • Exception: Dihydroxyacetone possesses zero chiral carbon centers (symmetrical molecule).

Stereoisomerism

• Definition: Isomers possessing identical molecular formulas and atom connectivity that differ exclusively in the three-dimensional orientation of their atoms in space.

• $2^n$ Rule: The maximum number of possible stereoisomers is $2^n$, where $n$ represents the number of chiral (asymmetric) carbon atoms.

  • Glucose ($n=4$): $2^4 = 16$ stereoisomers.

  • Fructose ($n=3$): $2^3 = 8$ stereoisomers.

  • Aldopentose ($n=3$): $2^3 = 8$ stereoisomers.

• Enantiomers: Stereoisomers that are non-superimposable mirror images of each other.

  • D and L Handedness: Determined by the orientation of the hydroxyl group attached to the penultimate carbon atom (the chiral center farthest from the carbonyl C1).

    • D-Sugar: Penultimate hydroxyl group points to the RIGHT (in Fischer projection) or the primary alcohol group ($CH_2OH$) points UPWARDS (in Haworth projection).

    • L-Sugar: Penultimate hydroxyl group points to the LEFT (in Fischer projection) or the primary alcohol group ($CH_2OH$) points DOWNWARDS (in Haworth projection).

  • Complete Mirror Image Inversion: Converting a D-sugar into its L-enantiomer requires reversing the hydroxyl orientations at ALL chiral carbon centers along the carbon chain.

• Biological Specificity of Handedness:

  • Monosaccharides in living organisms exist almost exclusively as D-enantiomers.

  • Amino acids in proteins exist almost exclusively as L-enantiomers.

  • Receptor binding sites are highly stereospecific: D-Epinephrine is 20 times ($20\times$) more physiologically active than L-Epinephrine due to precise active site fit.

• Diastereomers: Stereoisomers that are not mirror images of each other (differ at one or more, but not all, chiral centers).

• Epimers: Diastereomers that differ in absolute configuration at exactly ONE specific chiral carbon center, excluding the penultimate carbon and carbonyl carbon.

  • D-Glucose and D-Galactose: C4 epimers.

  • D-Glucose and D-Mannose: C2 epimers.

  • D-Galactose and D-Mannose: Diastereomers, but NOT epimers (differ at two carbon positions: C2 and C4).

• Anomers (Alpha and Beta Isomers):

  • Stereoisomers formed upon cyclization at the newly created chiral center (the carbonyl carbon, now termed the anomeric carbon: C1 in aldoses, C2 in ketoses).

  • Alpha ($\alpha$) Anomer: The anomeric hydroxyl group points DOWNWARDS (in Haworth projection) or OPPOSITE/AWAY from the primary alcohol group ($CH_2OH$).

  • Beta ($\beta$) Anomer: The anomeric hydroxyl group points UPWARDS (in Haworth projection) or on the SAME PLANE as the primary alcohol group ($CH_2OH$).

Aldose-Ketose Isomerism (Tautomerization)

• Constitutional isomers sharing identical molecular formulas that differ in functional group connectivity.

• Aldoses and ketoses interconvert in rapid solution equilibrium through tautomerization via an enediol intermediate:

  1. Aldose state: Carbonyl oxygen located at C1.

  2. Enediol intermediate: Hydrogen shifts from C2 to C1, forming a transient $C=C$ double bond between C1 and C2.

  3. Ketose state: Carbonyl oxygen relocates to C2.

Optical Activity

• Definition: The intrinsic ability of chiral compounds to rotate the plane of plane-polarized light.

  • Dextrorotatory ($d$ or $+$): Rotates plane-polarized light CLOCKWISE (to the right).

  • Levorotatory ($l$ or $-$): Rotates plane-polarized light COUNTERCLOCKWISE (to the left).

• Independence from Handedness: Structural handedness ($D$/$L$) does NOT dictate optical rotation direction ($+$/$-$).

  • D-Glucose is dextrorotatory ($D\text{(+)}-glucose$ / dextrose).

  • D-Fructose is levorotatory ($D\text{(-)}-fructose$ / levulose).

• Polarimeter Components: Light Source $\rightarrow$ Fixed Polarizer Lens $\rightarrow$ Sample Tube containing chiral solution ($10\,cm$ length) $\rightarrow$ Rotatable Analyzer Lens $\rightarrow$ Visual Detector.

Mutarotation

• Definition: The gradual change in optical rotation observed when a pure crystalline anomer of a monosaccharide dissolves in water and reaches an equilibrium mixture of $\alpha$, linear, and $\beta$ forms.

• D-Glucose Mutarotation Dynamics:

  • Pure $\alpha\text{-D-glucopyranose}$ initial specific rotation: $+112^\text{o}$.

  • Pure $\beta\text{-D-glucopyranose}$ initial specific rotation: $+19^\text{o}$.

  • Equilibrium mixture specific rotation: $+52.5^\text{o}$ (consisting of $\sim 38\%$ $\alpha$-anomer, $\sim 62\%$ $\beta$-anomer, and $<1\%$ open-chain linear form).

Reducing Property

• Mechanistic Basis: Dictated by the presence of a free, unattached hydroxyl group on the anomeric carbon.

• Reducing Sugars: Monosaccharides with free anomeric hydroxyls can open into linear forms, exposing reactive aldehyde or ketone functional groups that donate electrons to reduce mild oxidizing agents (e.g., alkaline copper solutions such as Benedict's or Fehling's reagents).

• Clinical Application: Used in diagnostic urine tests to detect glycosuria for monitoring uncontrolled Diabetes Mellitus.

Monosaccharide Derivatives

Amino Sugars

• Definition: Derivatives in which a hydroxyl group (typically at carbon C2) is replaced by an amino group ($-NH_2$), which is frequently N-acetylated.

• Glucosamine: C2 amino derivative of glucose; marketed as a therapeutic supplement for osteoarthritis to support cartilage glycosaminoglycan synthesis.

• N-Acetylglucosamine (GlcNAc) and N-Acetylgalactosamine (GalNAc): Acetylated derivatives that form essential structural subunits of chitin, hyaluronic acid, bacterial cell walls, and blood group antigens.

Sugar Alcohols (Polyols)

• Definition: Organic polyols produced by reducing the carbonyl group ($C=O$) of an aldose or ketose to a hydroxyl group using hydrogen. They completely lack aldehyde or ketone functional groups.

• D-Ribitol: Reduced derivative of D-ribose; structural component of gram-positive bacterial cell walls.

• Sorbitol (Glucitol): Reduced derivative of D-glucose aldehyde group. Accumulates in lens tissue during chronic hyperglycemia, causing osmotic swelling and cataract formation in diabetic patients.

• Mannitol: Reduced derivative of D-mannose; administered intravenously as an osmotic diuretic to reduce intracranial pressure in cerebral edema or hemorrhagic stroke.

• Xylitol: Reduced pentose derivative used as a non-cariogenic artificial sweetener.

Deoxysugars

• Definition: Sugars possessing a hydroxyl group replaced by a hydrogen atom ($-H$) via selective reduction.

• 2-Deoxy-D-ribose: Ribose derivative missing the C2 hydroxyl group; forms the structural backbone of DNA, increasing resistance against hydrolytic cleavage compared to ribose in RNA.

• Rhamnose ($C_6H_{12}O_5$): Naturally occurring L-deoxysugar.

Sugar Acids

• Definition: Carboxylic acid derivatives of monosaccharides produced through oxidation.

• Four Major Sugar Acid Classes:

  1. Aldonic Acid: Oxidation occurs exclusively at the C1 aldehyde group, converting it into a carboxylic acid ($-COOH$) (e.g., Gluconic acid, Ascorbic acid / Vitamin C).

  2. Uronic Acid: Oxidation occurs exclusively at the primary alcohol group ($CH_2OH$) on the terminal carbon, preserving the C1 aldehyde intact (e.g., D-Glucuronic acid, L-Iduronic acid).

  3. Aldaric Acid: Oxidation occurs at BOTH the C1 aldehyde and the terminal primary alcohol group when exposed to strong oxidizing agents, forming dicarboxylic acids (e.g., Saccharic acid, Mucic acid, Tartaric acid).

  4. Ulosonic Acid: Formed when the first hydroxyl group of a 2-ketose undergoes oxidation to create an $\alpha$-ketoacid structure (e.g., Neuraminic acid, Ketodeoxyoctulosonic acid / KDO).

Sugar Phosphates

• Definition: Phosphate esters formed when a monosaccharide hydroxyl group reacts with inorganic phosphate derived from high-energy donors (ATP).

• Biological Importance: Essential metabolic intermediates in glycolysis and the pentose phosphate pathway (e.g., Glucose-6-phosphate, Ribose-5-phosphate). Phosphorylation traps glucose inside cells by imparting a negative charge that prevents passage across lipid membranes.

Disaccharides

• Definition: Carbohydrates consisting of two monosaccharide units joined covalently by a glycosidic bond, releasing water ($H_2O$).

• Glycosidic Bond Chemistry: Condensation reaction joining the anomeric hydroxyl ($R-OH$) of one sugar with a hydroxyl group ($HO-R'$) of another sugar, producing an acetal linkage ($R-O-R'$).

• Alpha vs. Beta Bond Orientations:

  • $\alpha$ Glycosidic Bond: Anomeric hydroxyl points DOWNWARDS / OPPOSITE side relative to the primary alcohol group ($CH_2OH$).

  • $\beta$ Glycosidic Bond: Anomeric hydroxyl points UPWARDS / SAME side relative to the primary alcohol group ($CH_2OH$).

Comprehensive Disaccharides Summary Table

• Structural Digestion Note: Humans lack $\beta(1\rightarrow 4)$ glucosidase enzymes capable of hydrolyzing the $\beta(1\rightarrow 4)$ linkages of cellobiose and cellulose.

• Non-reducing Nature of Sucrose: Sucrose is non-reducing because its glycosidic bond directly connects the anomeric carbon C1 of glucose to the anomeric carbon C2 of fructose, tying up both free anomeric hydroxyl groups.

Relative Sweetness Scale

• Sucrose serves as the standard reference value ($=100$).

Polysaccharides

• General Features: High molecular weight polymers containing more than 10 monosaccharide units joined by glycosidic bonds. Non-sweet, amorphous, non-reducing white compounds.

Storage Polysaccharides

• Starch: Major storage polysaccharide in plants, consisting of two $\alpha\text{-D-glucose}$ polymers:

  • Amylose ($10\text{--}20\%$ of starch): Linear, unbranched polymer of $\alpha\text{-D-glucose}$ units connected by $\alpha(1\rightarrow 4)$ glycosidic bonds (200 to 20,000 units). Steric constraints force the chain into a helical coil secondary structure.

    • Iodine Test for Starch: Polyiodide ions ($I_3^-$) insert into the hollow core of the amylose helix, forming a deep-blue complex (qualitative diagnostic test for starch).

  • Amylopectin ($80\text{--}90\%$ of starch): Branched polymer containing $\alpha(1\rightarrow 4)$ linear glucose chains with $\alpha(1\rightarrow 6)$ branch points occurring every 24 to 30 glucose residues.

• Glycogen ("Animal Starch"):

  • Primary carbohydrate storage molecule in animals, concentrated in liver and skeletal muscle.

  • Glucose homopolymer structured similarly to amylopectin but significantly more densely branched, with $\alpha(1\rightarrow 6)$ branch points occurring every 8 to 12 glucose residues.

  • Glucose chains expand globularly around a central protein core anchor called Glycogenin.

  • High branching frequency provides multiple terminal ends for rapid enzymatic cleavage by glycogen phosphorylase during blood glucose maintenance.

Structural Polysaccharides

• Cellulose:

  • Most abundant naturally occurring organic polymer on Earth; primary structural component of plant cell walls.

  • Unbranched linear homopolymer of D-glucose linked by $\beta(1\rightarrow 4)$ glycosidic bonds.

  • Alternating 180-degree flipped glucose residue orientations produce extended straight chains stabilized by extensive intra- and inter-chain hydrogen bonding.

  • Indigestible by human digestive enzymes; functions as dietary fiber (recommended intake $20\text{--}35\,g/\text{day}$) to retain water and promote healthy intestinal peristalsis.

• Chitin:

  • Structural exoskeleton component of crustaceans, insects, and fungal cell walls.

  • Unbranched linear homopolymer of N-acetyl-D-glucosamine (GlcNAc) linked by $\beta(1\rightarrow 4)$ glycosidic bonds. Structurally identical to cellulose except the C2 hydroxyl group is replaced by an acetylated amino group.

Heteropolysaccharides and Glycosaminoglycans (GAGs)

• Definition: Unbranched heteropolysaccharide chains composed of repeating disaccharide units: [Amino Sugar + Uronic Acid] (or galactose in keratan sulfate).

• Polyanionic Nature: Highly negatively charged due to carboxyl groups ($-COO^-$) and sulfate groups ($-SO_4^{2-}$) at positions C4 or C6.

• Viscoelasticity and Resilience: High negative charge repulsion forces GAG chains to adopt extended linear conformations that trap large volumes of water ($H_2O$). Compression squeezes water out, and pressure release reabsorbs water, providing mechanical shock absorption and lubrication in joint synovial fluid and ocular vitreous humor.

Six Major Types of Glycosaminoglycans (GAGs)

1. Hyaluronic Acid (Hyaluronate): Glucuronic Acid + N-Acetylglucosamine connected by $\beta(1\rightarrow 3)$ intra-disaccharide and $\beta(1\rightarrow 4)$ inter-disaccharide linkages. Simplest GAG, unsulfated, non-covalently linked to proteins, found in synovial fluid, vitreous humor, and loose connective tissue. Also produced by bacteria.

2. Chondroitin Sulfate: Glucuronic Acid + N-Acetylgalactosamine connected by $\beta(1\rightarrow 3)$ intra-disaccharide and $\beta(1\rightarrow 4)$ inter-disaccharide linkages. Sulfated at C4 or C6 of GalNAc. Most abundant GAG in the body (cartilage, bone, tendons, cornea).

3. Dermatan Sulfate: L-Iduronic Acid (or Glucuronic Acid) + N-Acetylgalactosamine connected by $\alpha(1\rightarrow 3)$ or $\beta(1\rightarrow 3)$ linkages. Located in skin, blood vessels, and heart valves; exhibits minimal anticoagulant activity.

4. Keratan Sulfate: Galactose + N-Acetylglucosamine connected by $\beta(1\rightarrow 4)$ intra-disaccharide and $\beta(1\rightarrow 3)$ inter-disaccharide linkages. Only GAG lacking a sugar acid. KS I occurs in cornea; KS II occurs in cartilage.

5. Heparin: Glucosamine + Glucuronic or L-Iduronic Acid connected by $\alpha(1\rightarrow 4)$ intra-disaccharide and $\beta(1\rightarrow 4)$ inter-disaccharide linkages. Highest negative charge density of any biological molecule. Stored INTRACELLULARLY inside mast cell granules; acts as a potent clinical anticoagulant.

6. Heparan Sulfate: Extracellular cell-surface and basement membrane GAG containing the same basic backbone as heparin, but possesses fewer sulfate groups and more N-acetylated glucosamine residues.

Mucopolysaccharidoses (MPS)

• Rare hereditary lysosomal storage disorders caused by deficiencies in specific lysosomal hydrolase enzymes responsible for degrading GAGs.

• Leads to progressive intracellular and extracellular GAG accumulation, causing skeletal deformities, coarse facial features, hepatosplenomegaly, and mental retardation.

• Inherited as autosomal recessive traits, except Hunter's Syndrome (MPS II), which is X-linked recessive.

Glycoproteins

• Definition: Proteins conjugated to oligosaccharides where the protein component predominates ($>70\text{--}90\%$ protein, $<10\text{--}30\%$ carbohydrate). Carbohydrate chains are relatively short, branched, and structurally heterogeneous.

• Comparison: Glycoproteins vs. Proteoglycans:

• O-Linked Glycoproteins:

  • The anomeric C1 hydroxyl of the sugar (e.g., GalNAc) is covalently bound to the hydroxyl side chain oxygen of Serine, Threonine, or Hydroxylysine residues.

  • Found in cell surface mucins, blood group antigens, and viral envelope proteins.

• N-Linked Glycoproteins:

  • The anomeric C1 of N-acetylglucosamine is covalently bound to the amide nitrogen side chain of Asparagine (Asn).

  • Recognizes consensus sequence: $Asn-X-Ser$ or $Asn-X-Thr$ (where $X$ is any amino acid except Proline).

  • 5 times ($5\times$) more abundant than O-linked glycoproteins (e.g., Immunoglobulins IgG/IgM, ovalbumin, peptide hormones).

• Core Pentasaccharide of N-Linked Glycoproteins:

  • All N-linked glycoproteins share a conserved core pentasaccharide structure consisting of 2 N-acetylglucosamine (GlcNAc) units and 3 branching Mannose (Man) units: (GlcNAc)_2(Man)_3$.\n\n\n\n\n# Proteoglycans\n\n* **Definition:** Glycoconjugate complexes consisting of a core protein covalently bound to long, unbranched glycosaminoglycan (GAG) chains, where carbohydrates constitute \sim 95\% of the total weight.\n* **Tetrasaccharide Linkage Bridge:** GAG chains attach to core protein Serine residues via a specific tetrasaccharide linkage bridge: `-Xylose-Galactose-Galactose-Glucuronic Acid-` (`-Xyl-Gal-Gal-GlcA-`).\n* **Physiological Roles:** Extracellular matrix structural support, tissue hydration, selective filtration barriers, growth factor binding, cell adhesion.\n* **Integral Membrane Proteoglycans:**\n * *Syndecan:* Transmembrane protein core conjugated with heparan sulfate and chondroitin sulfate chains.\n * *Glypican:* Membrane protein attached to the outer plasma membrane via a glycosylphosphatidylinositol (GPI) anchor, conjugated with heparan sulfate.\n\n\n\n* **ECM Proteoglycan Aggrecan:**\n * Massive "bottle-brush" aggregate present in cartilage tissue.\n * Structure: Long central hyaluronan (hyaluronic acid) backbone non-covalently bound via link proteins to hundreds of aggrecan core proteins, each loaded with covalently bound chondroitin sulfate and keratan sulfate GAG chains.\n\n\n\n\n# Peptidoglycans\n\n* **Definition:** Structural glycoconjugate polymer constituting bacterial cell walls, consisting of linear polysaccharide chains cross-linked by short oligopeptides (<10 amino acids).\n* **Carbohydrate Backbone:** Alternating repeating disaccharides of N-Acetylglucosamine (NAG / GlcNAc) and N-Acetylmuramic Acid (NAM / MurNAc) joined by \beta(1\rightarrow 4) glycosidic bonds.\n* **Gram-Positive Bacterial Cell Wall:**\n * Thick peptidoglycan layer (20\text{--}80\,nm) surrounding the plasma membrane; retains Crystal Violet primary stain.\n * NAM tetrapeptide side chains (L-Ala - D-Glu - L-Lys - D-Ala$) are cross-linked via a **Pentaglycine bridge** ($5\times\text{Glycine}).\n * Contains teichoic acid and lipoteichoic acid polymers conferring antigenic specificity.\n* **Gram-Negative Bacterial Cell Wall:**\n * Thin peptidoglycan layer (2\text{--}7\,nm) located within the periplasmic space between inner plasma membrane and outer membrane; loses primary stain and accepts Safranin counterstain.\n * Tetrapeptides cross-link **DIRECTLY** to each other without a pentaglycine bridge.\n * Lacks teichoic acid; outer membrane contains Lipopolysaccharides (LPS / endotoxin) anchored to peptidoglycan via Braun's lipoprotein.\n\n\n\n\n# Practice Exercises and Discussion\n\n* **Practice Exercise 1: Monosaccharide Classification**\n * *Structure A (C_6 with C2 ketone):* Ketohexose\n * *Structure B (C_6 with C1 aldehyde):* Aldohexose\n * *Structure C (C_4 with C1 aldehyde):* Aldotetrose\n * *Structure D (C_5 with C2 ketone):* Ketopentose\n* **Practice Exercise 2: Specific Monosaccharide Traits**\n * *Question:* Which sugar is a pentose? **Ribose**\n * *Question:* Which sugar is a ketose? **Fructose**\n * *Question:* Which cyclic form contains a 6-membered ring? **Glucose**\n * *Question:* Which cyclic form has two carbon atoms outside the ring? **Fructose**\n* **Practice Exercise 3: Chiral Center Identification**\n * *Compound a (CH_3-CH_2-CH_2-OH):* Achiral (C bonded to 2 H atoms).\n * *Compound b (CH_3-CH(CH_3)-CH_2-CH_2-CH_3):* Achiral (C bonded to 2 identical methyl groups).\n * *Compound c (CH_3-CH(OH)-CH_2-CH_3):* Chiral (C bonded to 4 distinct groups: H, OH, methyl, ethyl).\n * *Compound d (1,4-dibromocyclohexane C4):* Achiral (possesses internal plane of symmetry).\n* **Practice Exercise 4: Disaccharide Properties**\n * *Question:* Contains two different monosaccharide units? **Lactose** (galactose + glucose) and **Sucrose** (glucose + fructose).\n * *Question:* Yields only monosaccharides upon complete hydrolysis? **All disaccharides** (lactose, maltose, cellobiose, sucrose).\n * *Question:* Is a non-reducing sugar? **Sucrose**.\n* **Practice Exercise 5: Polysaccharide Traits**\n * *Question:* Contains \alpha(1\rightarrow 4) glycosidic linkages? **Amylopectin**, **Amylose**, **Glycogen**.\n * *Question:* All glycosidic linkages present are of the same type? **Amylose** (\alpha 1\rightarrow 4$), **Cellulose** ($\beta 1\rightarrow 4$), **Chitin** ($\beta 1\rightarrow 4).\n * *Question:* Chain is unbranched? **Amylose**, **Cellulose**, **Chitin**.\n * *Question:* Repeating monomer unit is not glucose? **Chitin** (monomer is N-acetyl-D-glucosamine).\n* **Practice Exercise 6: Glycosaminoglycan Traits**\n * *Question:* Unsulfated GAG? **Hyaluronic acid**.\n * *Question:* Contains amino sugar + sugar acid? **Hyaluronic acid**, **Chondroitin sulfate**, **Dermatan sulfate**, **Heparin**, **Heparan sulfate** (all except Keratan sulfate, which contains Galactose instead of sugar acid).\n * *Question:* Monosaccharides joined by \alpha(1\rightarrow 4) linkage? **Heparin**, **Heparan sulfate**.\n * *Question:* Repeating disaccharides joined by \beta(1\rightarrow 3)$$ linkage? Keratan sulfate.

• Discussion Riddle:

  • Riddle: What is the sugar that couldn't breathe?

  • Answer: A 9-carbon monosaccharide, because it is a "nonose" (no nose).