Comprehensive Study Guide on Biomolecules: Proteins, Lipids, Carbohydrates, and Glycoconjugates

Introduction to Biochemistry and Biomolecules

• Definition: Biochemistry is the study of chemical structures, functions, and reactions of molecules within living organisms.

• Focus Areas: The discipline focuses on understanding biological processes at the molecular level, including structural properties, physiological functions, and molecular interactions of biomolecules.

• Major Biomolecular Classes: Biological processes rely on four primary classes of biomolecules:

  • Proteins

  • Carbohydrates

  • Lipids

  • Nucleic acids

• Chemical Basis of Life: Biomolecules provide the cellular foundation for structural integrity, energy storage, metabolic catalysts, genetic storage, and intercellular signaling.

Amino Acids and Peptides

• Fundamental Structure of $\alpha$-Amino Acids:

  • An $\alpha$-amino acid consists of a central tetrahedral carbon atom (designated as the $\alpha$-carbon, $\text{C}_\alpha$) bonded to four distinct chemical groups:

  • A carboxylate group ($\text{-COO}^-$)

  • A protonated amino group ($\text{-NH}_3^+$)

  • A hydrogen atom ($\text{-H}$)

  • A variable side-chain group ($\text{-R}$ group)

  • The term $\alpha$ indicates that the amine group is attached to the carbon directly adjacent to the carboxylate carbon atom.

  • At physiological $\text{pH}$ (approximately $7.4$), amino acids exist primarily as zwitterions (dipolar ions containing an equal number of positive and negative charges, resulting in a net neutral molecule).

• Stereochemistry:

  • $19$ out of the $20$ standard amino acids are stereoisomers because the $\alpha$-carbon is a chiral center bonded to four different chemical groups.

  • Glycine is the sole exception, as its $\text{-R}$ group is a hydrogen atom, rendering its $\alpha$-carbon achiral.

• Amino Acid Classification by Side-Chain Polarity at $\text{pH} = 7$:

  • Nonpolar, Aliphatic $\text{R}$ Groups (Hydrophobic):

  • Glycine (Gly, G): $\text{R} = \text{-H}$

  • Alanine (Ala, A): $\text{R} = \text{-CH}_3$

  • Valine (Val, V): $\text{R} = \text{-CH}(\text{CH}_3)_2$

  • Leucine (Leu, L): $\text{R} = \text{-CH}_2\text{CH}(\text{CH}_3)_2$

  • Methionine (Met, M): $\text{R} = \text{-CH}_2\text{CH}_2\text{SCH}_3$ (contains a nonpolar thioether link)

  • Isoleucine (Ile, I): $\text{R} = \text{-CH}(\text{CH}_3)\text{CH}_2\text{CH}_3$

• Aromatic $\text{R}$ Groups (Hydrophobic / Moderately Polar):

  • Phenylalanine (Phe, F): Contains a nonpolar phenyl ring attached via a methylene group ($\text{-CH}_2\text{-C}_6\text{H}_5$).

  • Tyrosine (Tyr, Y): Contains a phenol ring ($\text{-CH}_2\text{-C}_6\text{H}_4\text{-OH}$); the hydroxyl group can form hydrogen bonds.

  • Tryptophan (Trp, W): Contains a bicyclic indole functional group.

• Polar, Uncharged $\text{R}$ Groups (Hydrophilic, Non-ionic at $\text{pH} = 7$):

  • Serine (Ser, S): Contains a primary alcohol group ($\text{-CH}_2\text{OH}$).

  • Threonine (Thr, T): Contains a secondary alcohol group ($\text{-CH}(\text{OH})\text{CH}_3$).

  • Cysteine (Cys, C): Contains a reactive thiol/sulfhydryl group ($\text{-CH}_2\text{SH}$).

  • Proline (Pro, P): Contains a cyclic pyrrolidine ring where the side chain covalently connects back to the backbone $\alpha$-amino group, constraining backbone flexibility.

  • Asparagine (Asn, N): Contains an amide side chain ($\text{-CH}_2\text{CONH}_2$).

  • Glutamine (Gln, Q): Contains an amide side chain ($\text{-CH}_2\text{CH}_2\text{CONH}_2$).

• Positively Charged (Basic) $\text{R}$ Groups at $\text{pH} = 7$:

  • Lysine (Lys, K): Contains a primary amino group at the $\epsilon$-position ($\text{-(CH}_2)_4\text{NH}_3^+$).

  • Arginine (Arg, R): Contains a positively charged guanidino group ($\text{-(CH}_2)_3\text{NHC}(=\text{NH}_2^+)\text{NH}_2$).

  • Histidine (His, H): Contains an imidazole ring capable of acting as a physiological buffer near neutral $\text{pH}$.

• Negatively Charged (Acidic) $\text{R}$ Groups at $\text{pH} = 7$:

  • Aspartate (Asp, D): Contains an ionized $\beta$-carboxylate group ($\text{-CH}_2\text{COO}^-$).

  • Glutamate (Glu, E): Contains an ionized $\gamma$-carboxylate group ($\text{-CH}_2\text{CH}_2\text{COO}^-$).

• Hydrophobic vs. Hydrophilic Behavior:

  • Nonpolar amino acid side chains are hydrophobic; they fold into the protein interior to avoid contact with the aqueous environment.

  • Polar, acidic, and basic amino acid side chains are hydrophilic; they align along the exterior surface of globular proteins to form hydrogen bonds and ionic interactions with surrounding water, enhancing protein solubility.

Protein Structure and Functions

• Biological Role and Etymology:

  • The word protein is derived from the Greek proteios, meaning "of first importance".

  • Proteins are linear, unbranched polymers built from $\alpha$-amino acids.

  • Key functional roles include:

  • Enzymes: Biological catalysts that accelerate metabolic reaction rates.

  • Antibodies: Defense proteins that recognize and neutralize foreign pathochemical antigens.

  • Transport Proteins: Molecules that carry ions or small molecules across membranes or through physiological fluids (e.g., hemoglobin).

  • Regulatory Proteins: Hormones and signal transduction proteins that modulate metabolic pathways.

  • Structural Proteins: Macromolecules that provide mechanical rigidity and framework to cells and tissues (e.g., collagen, keratin).

• The Four Hierarchy Levels of Protein Structure:

  • Primary Structure:

  • Defined as the linear sequence of amino acid residues in a polypeptide chain.

  • Maintained exclusively by covalent peptide (amide) bonds between adjacent $\alpha$-carboxyl and $\alpha$-amino groups.

  • Example: The Human Immunodeficiency Virus 1 (HIV-1) Tat protein sequence possesses specific functional domains across its primary sequence:

    • Acidic region: Residues $2\text{--}11$

    • Cysteine-rich region: Residues $22\text{--}37$

    • Core region: Residues $38\text{--}48$

    • Basic region: Residues $49\text{--}57$

    • Glutamine-rich region: Residues $58\text{--}72$

    • RGD motif: Residues $78\text{--}80$ (Arg-Gly-Asp)

• Secondary Structure:

  • Localized, regularly repeating 3D spatial arrangements of backbone segments.

  • Maintained by hydrogen bonding between peptide backbone amide hydrogens ($\text{-N-H}$) and carbonyl oxygens ($\text{-C=O}$).

  • Major structures include:

    • $\alpha$--Helix: A coiled helical conformation stabilized by intrachain hydrogen bonds parallel to the helical axis.

    • $\beta$--Pleated Sheet: A sheet-like arrangement where extended polypeptide segments are linked side-by-side by interstrand hydrogen bonds.

• Tertiary Structure:

  • The overall three-dimensional folded conformation of a single polypeptide chain.

  • Arises from long-range interactions between side-chain $\text{-R}$ groups as well as interactions between $\text{-R}$ groups and the peptide backbone.

• Quaternary Structure:

  • The spatial organization and noncovalent or covalent association of two or more individual polypeptide chains (subunits) into a oligomeric complex.

  • Example: Hemoglobin, a heterotetramer consisting of two $\alpha$-globin and two $\beta$-globin chains held together by noncovalent interactions.

Forces and Interactions Governing Protein Conformation

• Shape-Determining Noncovalent and Covalent Interactions:

  • Backbone Hydrogen Bonding: Hydrogen bonds between partial positive hydrogen atoms ($\delta^+ \text{H}$) on amide groups and partial negative oxygen atoms ($\delta^- \text{O}$) on carbonyl groups along the peptide backbone hold secondary structural elements together.

  • Side-Chain ($\text{R}$ Group) Hydrogen Bonding: Hydrogen bonding between polar side chains (such as Serine $\text{-OH}$ groups) or between side chains and backbone atoms establishes internal stabilizing links.

  • Salt Bridges (Ionic Interactions): Electrostatic attractions between oppositely charged side chains at physiological $\text{pH}$. For instance, the negatively charged side-chain carboxylate of Glutamate ($\text{Glu-COO}^-$) or Aspartate attracts the positively charged guanidino/amino group of Arginine ($\text{Arg-NH}_2^+$) or Lysine.

  • Hydrophobic Interactions: Nonpolar side chains (such as leucine, isoleucine, and valine) aggregate internally due to the entropic drive of water exclusion and Van der Waals dispersion forces, forming a dry hydrophobic interior core.

  • Covalent Disulfide Bridges:

  • Cysteine residues possess a sulfhydryl group ($\text{-SH}$) that can undergo reversible oxidation to yield a covalent sulfur-sulfur bond (disulfide bridge, $\text{-S-S-}$), forming a cystine residue.

  • Reaction Equation:

$2\,\text{Cysteine-SH} \underset{\text{Reduction}}{\overset{\text{Oxidation}}{\rightleftharpoons}} \text{Cystine (S-S)} + 2\,\text{H}^+ + 2\,e^-$

Specialized Structural Proteins: Collagen

• Abundance: Collagen is the single most abundant protein in mammals, accounting for approximately $30\%$ of total body protein mass.

• Tissue Distribution: Forms the primary structural component of skin, tendons, bones, blood vessels, cartilage, and other connective tissues.

• Tropocollagen Structure:

  • Basic structural unit is tropocollagen, a triple helix formed by three polypeptide chains, each containing approximately $1000$ amino acid residues.

  • Interchain hydrogen bonds stabilize the triple helix assembly.

  • Glycine content: Glycine occurs at every third position in the repeating amino acid sequence motif ($\text{Gly-X-Y}$). Because Glycine is the smallest amino acid ($\text{R} = \text{-H}$), its small size allows the three chains to pack tightly into the restricted central axis of the triple coil.

Protein Denaturation and Coagulation

• Denaturation Concept:

  • Denaturation is the process in which a protein loses its native 3D organized conformation (secondary, tertiary, and quaternary structure), causing a complete or partial loss of biological activity.

  • Denaturation does not disrupt covalent peptide bonds and therefore leaves the primary amino acid sequence intact.

• Common Denaturing Agents and Mechanisms:

  • Thermal Energy (Heat): Increases molecular kinetic energy, breaking weak hydrogen bonds and hydrophobic interactions (e.g., heat denaturation of egg white ovalbumin during cooking).

  • $\text{pH}$ Extremes: Alters the protonation states of acidic and basic side chains, breaking essential ionic salt bridges (e.g., acid curdling of milk proteins).

  • Organic Solvents & Detergents: Disrupt internal hydrophobic core interactions.

  • Heavy Metals ($\text{Pb}^{2+}$, $\text{Hg}^{2+}$): Form strong ionic bonds or coordinate complexes with carboxylates and sulfhydryl groups.

  • Mechanical Stress: Physical shearing uncoils folded chains.

• Coagulation: Unfolded denatured protein chains cross-link and aggregate into solid, insoluble precipitates.

Lipids: Classification and Biological Functions

• Definition: Lipids are a diverse group of nonpolar organic molecules that are insoluble in water but soluble in nonpolar organic solvents.

• Caloric Energy Yield: Lipids provide $9\,\text{kcal\,g}^{-1}$ ($37\,\text{kJ\,g}^{-1}$) of metabolizable energy, which is more than double the energy density of carbohydrates or proteins ($4\,\text{kcal\,g}^{-1}$).

• Biological Functions:

  • Long-Term Energy Storage: Triacylglycerols stored inside specialized adipocytes.

  • Cell Membrane Architecture: Phosphoglycerides, sphingolipids, and sterols form the lipid bilayer matrix.

  • Endocrine Signaling: Steroid hormones function as critical intercellular chemical messengers.

  • Carrier Function: Dietary fats transport lipid-soluble vitamins ($\text{A}$, $\text{D}$, $\text{E}$, and $\text{K}$) across intestinal cell membranes.

  • Protection: Provides mechanical shock absorption and thermal insulation around vital organs.

• Four Main Lipid Categories:

  • Fatty Acids (Saturated and Unsaturated)

  • Glycerides (Glycerol-containing esters: monoglycerides, diglycerides, triglycerides)

  • Nonglyceride Lipids (Sphingolipids, Steroids, Waxes)

  • Complex Lipids (Lipoproteins)

Fatty Acids: Structure, Saturation, and Physical Properties

• Structure: Long, straight-chain carboxylic acids without branching. Standard natural fatty acid chain lengths range from $10$ to $20$ carbon atoms, usually containing an even number of carbons inclusive of the carboxyl carbon atom.

• Essential Fatty Acids: Fatty acids that cannot be synthesized de novo by human metabolic pathways and must be obtained directly from dietary sources.

• Saturated vs. Unsaturated Fatty Acids:

  • Saturated Fatty Acids: Carbon chains containing no carbon-carbon double bonds ($\text{C=C}$). Hydrocarbon chains are fully saturated with hydrogen atoms, allowing linear packing, strong intermolecular Van der Waals forces, and high melting points (solid at room temperature, e.g., stearic acid, $18\text{ C}$).

  • Unsaturated Fatty Acids: Carbon chains containing one or more double bonds ($\text{C=C}$), predominantly in the cis conformation. A cis double bond introduces a permanent rigid kink in the hydrocarbon chain. This kink prevents tight molecular packing, lowering Van der Waals forces and decreasing melting points (liquid at room temperature, e.g., oleic acid, $18\text{ C}$ with one cis double bond).

  • Physical State Examples: Unsaturated plant oils (such as olive oil) remain liquid at room temperature, whereas saturated animal fats (such as bacon fat) remain solid.

Glycerides and Triacylglycerols

• Structure of Glycerides:

  • Glycerides are lipid esters produced by the esterification of the triol alcohol glycerol ($\text{C}_3\text{H}_8\text{O}_3$) with fatty acids.

  • Esterification can occur at one, two, or all three hydroxyl positions, forming monoglycerides, diglycerides, or triglycerides (triacylglycerols).

• Triacylglycerols (Triglycerides):

  • Neutral triesters composed of three fatty acid molecules esterified to one glycerol backbone.

  • Function as the primary concentrated form of energy storage in animal adipose tissue.

  • Reaction Equation:

$\text{Glycerol} + 3\,\text{Fatty Acids} \rightarrow \text{Triacylglycerol} + 3\,\text{H}_2\text{O}$

• Melting Point Comparison (Fats vs. Oils):

  • Fats: Solid triacylglycerol mixtures containing high proportions of saturated fatty acids.

  • Oils: Liquid triacylglycerol mixtures containing high proportions of unsaturated fatty acids with lowering melting points.

Chemical Reactions of Triacylglycerols

• Catalytic Hydrogenation:

  • Addition reaction where hydrogen gas ($\text{H}_2$) is added across the carbon-carbon double bonds ($\text{C=C}$) of unsaturated fatty acids in the presence of a transition metal catalyst (e.g., Nickel, $\text{Ni}$).

  • Industrial application: Converts liquid unsaturated vegetable oils into solid or semi-solid saturated fats (e.g., commercial production of shortening such as Crisco or margarine).

  • Chemical Equation:

$\text{R-CH=CH-R'} + \text{H}_2 \xrightarrow{\text{Ni}} \text{R-CH}_2\text{-CH}_2\text{-R'}$

• Saponification:

  • Base-catalyzed ester hydrolysis of triacylglycerols using a strong base such as sodium hydroxide ($\text{NaOH}$) or potassium hydroxide ($\text{KOH}$).

  • Cleaves ester linkages to yield glycerol and three alkali metal salts of fatty acids (soap molecules).

  • Chemical Equation:

$\text{Triglyceride} + 3\,\text{NaOH} \rightarrow \text{Glycerol} + 3\,\text{R-COO}^-\text{Na}^+$

Soaps, Detergents, Micelles, and Hard Water

• Soap Mechanism of Action:

  • Soap molecules are amphipathic containing two distinct structural regions:

  • Hydrophilic Ionic Head: Carboxylate salt end ($\text{-COO}^-\text{Na}^+$) dissolves in polar water.

  • Hydrophobic Nonpolar Tail: Long hydrocarbon chain ($\text{-R}$) dissolves in nonpolar grease and oils.

  • Micelle Aggregation: In water, soap molecules assemble into spherical aggregates called micelles. Hydrophobic hydrocarbon tails cluster inward to sequester nonpolar grease inside the core, while hydrophilic carboxylate heads project outward into the water.

• Hard Water Interference:

  • Hard water contains high concentrations of divalent cations, specifically Calcium ($\text{Ca}^{2+}$) and Magnesium ($\text{Mg}^{2+}$).

  • Divalent cations react with soluble soap carboxylate anions, precipitating insoluble carboxylate salts:

$2\,\text{R-COO}^- + \text{Ca}^{2+} \rightarrow (\text{R-COO}^-)_2\text{Ca}^{2+}(s)$

• This precipitation removes active soap from solution, destroys emulsifying capacity, and leaves an insoluble scum precipitate.

Supramolecular Lipid Structures: Micelles, Liposomes, and Bilayers

• Self-Assembled Amphipathic Lipid Assemblies:

  • Micelle: Single-layered spherical aggregate formed by single-tailed lipids (e.g., fatty acid salts, bile salts) with hydrophobic tails facing inward.

  • Liposome: Spherical double-layered vesicle enclosing an aqueous central core, used artificially for targeted drug delivery.

  • Bilayer Sheet: Two-dimensional sheet composed of two opposed leaflets of double-tailed phospholipids; forms the basic framework of cell membranes.

Phospholipids, Sphingolipids, and Membrane Architecture

• Phospholipids:

  • Lipids containing a alcohol backbone (glycerol or sphingosine) esterified to fatty acids and a phosphoric acid residue linked to an alcohol head group.

  • Amphipathic structure consists of a hydrophilic polar phosphate head and two hydrophobic nonpolar hydrocarbon tails.

  • Example: Phosphatidylcholine (lecithin).

• Sphingolipids:

  • Membrane lipids built on the 18-carbon amino alcohol sphingosine backbone rather than glycerol.

  • Sphingomyelin contains a fatty acid bound via an amide bond to the sphingosine amino group and a phosphocholine head group.

• Cell Membrane Fluid Mosaic Model:

  • Consists of a phospholipid bilayer matrix containing embedded integral membrane proteins, peripheral membrane proteins, glycolipids, and sterols.

  • Integral proteins span the membrane (frequently via hydrophobic $\alpha$-helical segments), whereas peripheral proteins associate with outer or inner surfaces.

Sterols, Steroid Hormones, and Bile Acids

• Sterane Nucleus: All sterols contain a characteristic tetracyclic fused ring nucleus comprising three 6-membered cyclohexane rings (A, B, C) and one 5-membered cyclopentane ring (D), termed the cyclopentanoperhydrophenanthrene or sterane nucleus.

• Cholesterol:

  • Major sterol present in human biological membranes.

  • Structure: Amphipathic molecule containing a rigid sterane nucleus, a nonpolar hydrocarbon tail, and a single polar hydroxyl ($\text{-OH}$) group at C-3.

  • Essential Functions:

  • Structural membrane component: Inserts between membrane phospholipids, regulating membrane fluidity and increasing mechanical stability.

  • Metabolic precursor: Serves as the starting substrate for the hepatic synthesis of bile acids and steroid hormones.

  • Origin: Obtained from dietary sources and synthesized de novo in the liver.

• Bile Acids and Bile Salts:

  • Synthesized in the liver from cholesterol, stored in the gallbladder, and secreted into the small intestine.

  • Examples: Cholic acid, Chenodeoxycholic acid.

  • Function: Act as biological detergents, forming mixed micelles with dietary lipids to emulsify fats and enhance pancreatic lipase activity.

• Steroid Hormones:

  • Mineralocorticoids: Regulate renal electrolyte balance and blood pressure ($\text{Na}^+$ retention and $\text{K}^+$ excretion); e.g., Aldosterone.

  • Glucocorticoids: Regulate carbohydrate metabolism and anti-inflammatory pathways; e.g., Hydrocortisone (Cortisol).

  • Sex Hormones: Control developmental secondary sexual characteristics:

  • Androgens: e.g., Testosterone (systematically $\Delta^4\text{-androstene 17 ol, 3one}$).

  • Estrogens: e.g., Estradiol (systematically $1,3,5\text{-estratriene-3,17, diol}$).

Plasma Lipoproteins and Cholesterol Transport

• Lipoprotein Structure:

  • Water-soluble macromolecular complexes that transport hydrophobic lipids through blood plasma.

  • Consists of a nonpolar neutral lipid core (triacylglycerols and cholesterol esters) encapsulated by a monolayer shell of amphipathic phospholipids, unesterified cholesterol, and apolipoproteins.

• Four Major Lipoprotein Classes and Percentage Compositions:

  • Chylomicrons:

  • Largest size, lowest density.

  • Function: Transport dietary triacylglycerols from intestine to adipose and muscle tissue.

  • Composition: $90\%$ Triglyceride, $5\%$ Cholesterol, $4\%$ Phospholipid, $1\%$ Protein.

  • Very Low-Density Lipoprotein (VLDL):

  • Synthesized in the liver.

  • Function: Transport endogenously synthesized lipids to peripheral tissues.

  • Composition: $60\%$ Triglyceride, $18\%$ Phospholipid, $14\%$ Cholesterol, $8\%$ Protein.

  • Low-Density Lipoprotein (LDL):

  • Derived from VLDL metabolism in bloodstream.

  • Function: Delivers cholesterol to peripheral tissues ("bad cholesterol").

  • Composition: $45\%$ Cholesterol, $25\%$ Protein, $20\%$ Phospholipid, $10\%$ Triglyceride.

  • High-Density Lipoprotein (HDL):

  • Synthesized in the liver.

  • Function: Scavenges excess cholesterol from peripheral tissues and returns it to the liver for excretion ("good cholesterol" via reverse cholesterol transport).

  • Composition: $45\%$ Protein, $30\%$ Phospholipid, $20\%$ Cholesterol, $5\%$ Triglyceride.

Receptor-Mediated Endocytosis and Disease Mechanisms

• LDL Receptor Pathway:

  • Discovered during investigations of familial hypercholesterolemia.

  • When peripheral cells require cholesterol, they express LDL cell-surface receptors localized in specialized membrane pits.

  • Circulating LDL particles bind specifically to the LDL receptors.

  • The membrane invaginates, internalizing the receptor-bound LDL via endocytosis into an endocytotic vesicle.

  • Pathological Consequence: Genetic failure to synthesize functional LDL receptors leads to severe hypercholesterolemia, systemic plaque accumulation, and premature atherosclerosis.

Carbohydrates: Classification and Nomenclature

• Definition: Polyhydroxy aldehydes or polyhydroxy ketones, or substances that yield these compounds upon hydrolysis.

• Empirical Formula: Simple carbohydrates have the general empirical formula $(\text{CH}_2\text{O})_n$, where n = 3\text{--}7$.\n- Synthesis: Produced in photosynthetic plants and stored as energy reserves (starch).\n- Basic Types:\n - Monosaccharides: Single sugar monomer (e.g., Glucose, Fructose).\n - Disaccharides: Two monosaccharide units linked by a covalent glycosidic bond (e.g., Sucrose, Lactose, Maltose).\n - Oligosaccharides: Chains of 3$to$10 covalently bound monosaccharide units.\n - Polysaccharides: High-molecular-weight polymers containing dozens to thousands of monosaccharide units (e.g., Starch, Glycogen, Cellulose, Chitin).\n- Monosaccharide Nomenclature Rules:\n - Based on functional group:\n - Aldose: Monosaccharide containing an aldehyde carbonyl group (\text{-CHO}) at C-1.\n - Ketose: Monosaccharide containing a ketone carbonyl group (\text{-C=O}) usually at C-2.\n - Based on carbon chain length:\n - Triose (3\text{ C}$), Tetrose ($4\text{ C}$), Pentose ($5\text{ C}$), Hexose ($6\text{ C}).\n - Combined Naming:\n - D-Glucose is an **aldohexose** (6\text{-carbon} aldose).\n - D-Fructose is a **ketohexose** (6\text{-carbon} ketose).\n\n# Monosaccharide Stereochemistry and Modifications\n\n- Functional Modifications of Monosaccharides:\n - Amino Sugars: Hydroxyl group (\text{-OH}$) at C-2 is replaced by an amino group ($\text{-NH}_2); e.g., Glucosamine.\n - $N$-Acetylated Sugars: Addition of an acetyl group to the amino substituent; e.g., $N$-acetylglucosamine (GlcNAc).\n - Uronic Acids: Primary alcohol group at C-6 is oxidized to a carboxylic acid group (\text{-COOH}); e.g., D-Glucuronic acid (GlcA).\n\n# Disaccharides and Glycosidic Bonds\n\n- Glycosidic Bond Formation:\n - An $O$-glycosidic bond is a covalent link formed when the anomeric hydroxyl group of a monosaccharide reacts with a hydroxyl group of another molecule (sugar, lipid, or protein) with the elimination of a water molecule (\text{H}_2\text{O}).\n - Condensation forms glycosidic linkages; hydrolysis breaks glycosidic linkages.\n - Pharmacological Glycosides: Digitoxin, derived from the foxglove plant (*Digitalis*), is a cardiac glycoside composed of a steroid core bound to sugar residues, prescribed for heart heart failure and irregular heartbeat.\n\n\n\n\n\n- Key Disaccharides:\n - Sucrose ("Table sugar"):\n - Composed of \alpha\text{-D-glucose}$and$\beta\text{-D-fructose}$connected via an$\alpha(1\leftrightarrow 2)\beta$glycosidic bond between both anomeric carbon atoms (designated$\text{Glc}(\alpha 1\leftrightarrow 2\beta)\text{Fru}$, systematically$\alpha\text{-D-glucopyranosyl }\beta\text{-D-fructofuranoside}).\n - Non-reducing sugar because both anomeric carbons participate in the glycosidic bond.\n - Extracted commercially from sugarcane and sugar beets.\n\n\n\n - Lactose ("Milk sugar"):\n - Major disaccharide in mammalian milk.\n - Composed of \beta\text{-D-galactose}$and$\beta\text{-D-glucose}$linked by a$\beta(1\rightarrow 4)$glycosidic bond (designated$\text{Gal}(\beta 1\rightarrow 4)\text{Glc}$, systematically$\beta\text{-D-galactopyranosyl-}(1\rightarrow 4)\text{-}\beta\text{-D-glucopyranose}).\n - Intestinal lactase deficiency prevents hydrolysis of lactose, leading to **lactose intolerance**.\n - Pathology Note: Lack of epimerase enzyme prevents galactose conversion to glucose, causing **galactosemia** (elevated galactose causing cataracts and mental retardation).\n\n\n\n - Maltose ("Malt sugar"):\n - Produced by starch hydrolysis.\n - Composed of two \alpha\text{-D-glucose}$monomers linked by an$\alpha(1\rightarrow 4) glycosidic bond.\n - Mammals produce \alpha$--glycosidases capable of digesting$\alpha$--linkages, but generally cannot digest$\beta--linkages.\n\n\n\n# Polysaccharides: Structural and Energy Storage Roles\n\n- General Classification:\n - Homopolysaccharides: Polymers consisting of only one type of monosaccharide monomer.\n - Heteropolysaccharides: Polymers consisting of two or more different monosaccharide monomer types.\n\n\n\n- Structural Polysaccharides:\n - Chitin:\n - Homopolysaccharide of $N$-acetylglucosamine linked by \beta(1\rightarrow 4) glycosidic bonds.\n - Interchain hydrogen bonding produces exceptional mechanical strength.\n - Constitutes exoskeletons of insects, lobsters, shrimp, and cell walls of algae, fungi, and yeast.\n\n\n\n\n\n - Cellulose:\n - Major structural framework of plant cell walls and wood fiber.\n - Unbranched linear homopolysaccharide of D-glucose linked by \beta(1\rightarrow 4) glycosidic bonds.\n - Alternating residues are flipped 180^\circ, allowing linear chains to form rigid hydrogen-bonded sheets.\n - Animals lack cellulase enzymes to hydrolyze \beta(1\rightarrow 4) bonds. Symbiotic cellulase-producing bacteria in termites and grazing ruminants (cattle, horses) allow cellulose digestion.\n\n\n\n\n\n- Storage Polysaccharides:\n - Starch:\n - Plant energy storage homopolysaccharide composed of D-glucose linked by \alpha--linkages.\n - Contains two structural forms:\n - Amylose (10\text{--}30\%$): Unbranched linear polymer of D-glucose linked by$\alpha(1\rightarrow 4)$bonds, forming a helical coil ($6$residues per turn) that traps iodine ($\text{I}_2).\n - Amylopectin (70\text{--}90\%$): Branched polymer of D-glucose with$\alpha(1\rightarrow 4)$main chain bonds and$\alpha(1\rightarrow 6)$branch points occurring every$12$to$25 residues.\n\n\n\n - Glycogen:\n - Animal energy reserve polymer stored as glycogen granules in liver and muscle cells.\n - Highly branched homopolysaccharide of D-glucose with \alpha(1\rightarrow 4)$main chains and$\alpha(1\rightarrow 6)$branch points occurring every$8$to$10 residues.\n - High branching density provides numerous non-reducing ends for rapid enzymatic cleavage by glycogen phosphorylase.\n\n\n\n\n\n# Glycoconjugates: Proteoglycans, Glycoproteins, and Glycolipids\n\n- Glycoconjugate Definitions:\n - Proteoglycans: Macromolecules consisting of a core protein covalently bound to sulfated glycosaminoglycan (GAG) chains; major components of cell surfaces and extracellular matrix.\n - Glycoproteins: Proteins covalently linked to branched, structurally diverse oligosaccharides.\n - Glycolipids: Membrane lipids covalently bound to oligosaccharide heads.\n- Proteoglycans and Matrix Architecture:\n - GAG chains (e.g., chondroitin sulfate, keratan sulfate) attach to serine residues of core proteins via a tetrasaccharide bridge (\text{Ser-Xyl-Gal-Gal-GlcA}).\n - Cell-surface proteoglycans: Syndecan, Glypican.\n - Matrix proteoglycans: Aggrecan, Perlecan, Decorin, Biglycan.\n - Cartilage Extracellular Matrix (ECM): Aggrecan monomers attach noncovalently along a long hyaluronan backbone via link proteins within a collagen network.\n\n\n\n\n\n\n\n- Glycoproteins & Linkage Types:\n - $O$-linked: Carbohydrate bound via an $O-glycosidic bond to the hydroxyl group of Serine (Ser) or Threonine (Thr).\n - N$-linked: Carbohydrate bound via an $N-glycosidic bond to the amide nitrogen of Asparagine (Asn).\n\n\n\n- ABO Blood Group Antigenic Determinants:\n - Oligosaccharide chains on red blood cell membranes act as antigenic determinants:\n - Type A antigen: Terminal N$-acetylgalactosamine (\beta\text{-}N\text{-acetylgalactosamine}).\n - Type B antigen: Terminal galactose (\alpha\text{-galactose}$).</p><ul><li><p>Type AB: Displays both A and B antigens (universal recipient).</p></li><li><p>Type O: Contains neither terminal sugar (universal donor).</p></li><li><p>Incompatible blood transfusion causes antibody binding, immune cross-linking, and agglutination (clumping).</p></li></ul></li></ul><img src="https://assets.knowt.com/pdf-flow-prod/0e72a2a5-d925-4d91-834f-7d65247d4363-figures/96.jpg" data-width="50$n \ge 10$$) determining bacterial serotype.

• Connective Tissue Glycosaminoglycans & Heparin:

  • Hyaluronate (Hyaluronic Acid): Unsulfated GAG of repeating D-glucuronate and $N$-acetylglucosamine; functions as a biological joint lubricant.

  • Chondroitin 6-sulfate: Sulfated GAG in cartilage.

  • Heparin: Highly sulfated, negatively charged glycosaminoglycan that functions as a potent anticoagulant by binding clotting factors.