Biomolecules: Structure, Function, and Reactions

Overview and Chemical Foundations of Biomolecules

• Definition of Biomolecules:

  • Biomolecules are organic molecules synthesized by living organisms that serve as the fundamental building blocks of life.

  • They are constructed through the bonding of smaller repeating subunits known as monomers, which undergo precise chemical reactions to form higher-order macromolecular architectures capable of specialized biochemical functions.

  • Biomolecules constitute the cellular machinery responsible for energy transduction, structural support, enzymatic catalysis, and hereditary continuity.

• Elemental and Chemical Composition:

  • Biomolecules are primarily composed of carbon ($\text{C}$), hydrogen ($\text{H}$), and oxygen ($\text{O}$), frequently incorporating nitrogen ($\text{N}$), phosphorus ($\text{P}$), and sulphur ($\text{S}$).

  • Carbon represents the most abundant and versatile element in biological systems due to its tetravalent nature, allowing the formation of stable, diverse linear, branched, and cyclic carbon skeletons.

  • Carbohydrates (specifically cellulose) represent the most abundant class of organic molecules in the biosphere.

• Key Functional Groups:

  • Hydroxyl Group ($-\text{OH}$): Found in alcohols and carbohydrates; highly polar and capable of forming hydrogen bonds with aqueous solvent molecules.

  • Carbonyl Group ($>\text{C}=\text{O}$): Composed of a carbon atom double-bonded to an oxygen atom. Divided into two main classes based on position:

  • Aldehydes: The carbonyl group is positioned at the terminal end of the carbon skeleton ($\text{R}-\text{CHO}$, e.g., in glucose).

  • Ketones: The carbonyl group is situated within the interior of the carbon skeleton ($\text{R}-\text{CO}-\text{R}'$, e.g., in fructose).

  • Carboxyl Group ($-\text{COOH}$): Acidic group capable of dissociating a proton to yield a negatively charged carboxylate ion ($-\text{COO}^-$); found in amino acids and fatty acids.

  • Amino Group ($-\text{NH}_2$): Basic group capable of accepting a proton to form a positively charged ammonium ion ($-\text{NH}_3^+$); found in amino acids and nitrogenous bases.

  • Phosphate Group ($-\text{OPO}_3^{2-}$): Acidic, negatively charged functional group derived from phosphoric acid; found in nucleic acid backbones, adenosine triphosphate ($\text{ATP}$), and phospholipids.

  • Sulfhydryl Group ($-\text{SH}$): Known as a thiol group; found in select amino acids such as cysteine, where it participates in covalent cross-linking.

• Levels of Molecular Organization:

  • Monomers: Small, basic, low-molecular-weight chemical subunits that link together covalently to produce larger polymeric structures (e.g., monosaccharides such as glucose).

  • Polymers: High-molecular-weight assemblies formed by linking repeating monomeric units via repetitive covalent bonds (e.g., starch, a polymer of glucose).

  • Macromolecules: Giant molecular complexes possessing exceptionally high molecular weights ($\text{MW}$), encompassing complex polysaccharides, functional proteins, and polynucleic acids ($\text{DNA}$ and $\text{RNA}$).

• Universal Biochemical Reactions:

  • Condensation Reaction (Dehydration Synthesis):

  • Definition: An anabolic chemical reaction in which two molecules are covalently combined to form a larger molecule, accompanied by the simultaneous elimination of a small byproduct molecule (most commonly water, $\text{H}_2\text{O}$, though ammonia or alcohol can also be released in certain chemical contexts).

  • Thermodynamics & Biological Role: Generally endergonic (requiring an input of chemical energy); essential for assembling complex macromolecules including proteins, glycogen, starch, and nucleic acids.

  • Hydrolysis Reaction:

  • Definition: A catabolic chemical reaction in which a covalent bond linking two subunits within a macromolecule is cleaved by the addition of a water molecule ($\text{H}_2\text{O}$), with a proton ($\text{H}^+$) joining one subunit and a hydroxyl group ($-\text{OH}$) joining the other.

  • Thermodynamics & Biological Role: Generally exergonic (releasing energy); fundamental to biological digestion, the intracellular breakdown of polymeric energy reserves (e.g., glycogenolysis), and the hydrolytic cleavage of phosphoanhydride bonds in $\text{ATP}$.

Carbohydrates: Monosaccharides, Disaccharides, and Polysaccharides

• General Principles and Classification:

  • Carbohydrates (saccharides) derive their name from "carbon hydrate" and feature the general empirical formula $(\text{CH}_2\text{O})_n$, maintaining a constant carbon-to-hydrogen-to-oxygen ratio of $1:2:1$.

  • Categorized based on monomer count:

  • Monosaccharides: Single sugar units ($\text{mono}$ = one).

  • Disaccharides: Two monosaccharides joined by a covalent glycosidic bond ($\text{di}$ = two).

  • Oligosaccharides: Short chains consisting of $3$ to $10$ linked monosaccharides (e.g., raffinose, a trisaccharide of galactose, glucose, and fructose with formula $\text{C}_{18}\text{H}_{32}\text{O}_{16}$).

  • Polysaccharides: Extended polymers comprising hundreds to thousands of monosaccharide repeating units ($\text{poly}$ = many).

• Monosaccharide Structure and Isomerism:

  • Classified by carbon backbone length:

  • Trioses: $3$-carbon sugars ($\text{C}_3\text{H}_6\text{O}_3$).

  • Pentoses: $5$-carbon sugars ($\text{C}_5\text{H}_{10}\text{O}_5$, e.g., ribose, and $\text{C}_5\text{H}_{10}\text{O}_4$ for deoxyribose).

  • Hexoses: $6$-carbon sugars ($\text{C}_6\text{H}_{12}\text{O}_6$, e.g., glucose, fructose, galactose).

  • $\alpha$-Glucose vs. $\beta$-Glucose Anomers:

  • In aqueous solution, hexose monosaccharides undergo intramolecular cyclization between the carbonyl carbon and a hydroxyl group, forming a pyranose ring.

  • Cyclization produces a new chiral center at carbon-1 ($\text{C}_1$), known as the anomeric carbon.

  • In $\alpha$-glucose, the hydroxyl group ($-\text{OH}$) attached to $\text{C}_1$ is oriented downward relative to the plane of the sugar ring (trans to the $\text{CH}_2\text{OH}$ group at $\text{C}_5$).

  • In $\beta$-glucose, the hydroxyl group ($-\text{OH}$) attached to $\text{C}_1$ is oriented upward relative to the plane of the ring (cis to the $\text{CH}_2\text{OH}$ group at $\text{C}_5$).

• $\text{D}$-Glucose vs. $\text{L}$-Glucose Enantiomers:

  • $\text{D}$- and $\text{L}$-glucose represent stereoisomers that are non-superimposable mirror images of one another (enantiomers).

  • The designation depends on the spatial configuration around the penultimate chiral carbon (carbon-5, $\text{C}_5$) in a Fischer projection:

  • $\text{D}$-Glucose (Dextrorotatory): The $-OH$ group on $\text{C}_5$ is positioned on the right-hand side. This is the naturally occurring, biologically active form synthesized by photoautotrophs. It serves as the obligatory primary glycolytic substrate for $\text{ATP}$ synthesis in cellular respiration, acts as the primary metabolic fuel for the human brain, and forms the monomer for starch, glycogen, and cellulose.

  • $\text{L}$-Glucose (Levorotatory): The $-OH$ group on $\text{C}_5$ is positioned on the left-hand side. It is synthetic, biologically inert in human metabolic pathways (remains chemically locked/unmetabolized), and is utilized clinically as an osmotic agent or diagnostic marker in gastrointestinal permeability tests.

• Glycosidic Bond Formation and Disaccharides:

  • A glycosidic bond is a covalent $\text{C}-\text{O}-\text{C}$ linkage formed when the anomeric hydroxyl group of one sugar reacts with a hydroxyl group of an adjacent sugar via dehydration synthesis, with the concomitant release of a water molecule ($\text{H}_2\text{O}$).

  • Sucrose ($\text{C}_{12}\text{H}_{22}\text{O}_{11}$):

  • Formed via a condensation reaction between $\alpha$-glucose and $\beta$-fructose, forming an $\alpha(1\rightarrow 2)$ glycosidic bond.

  • Serves as the principal non-reducing transport carbohydrate in the phloem of vascular plants.

• Lactose ($\text{C}_{12}\text{H}_{22}\text{O}_{11}$):

  • Formed through a condensation reaction linking galactose and glucose via a $\beta(1\rightarrow 4)$ glycosidic bond; represents the dominant sugar in mammalian milk.

• Maltose ($\text{C}_{12}\text{H}_{22}\text{O}_{11}$):

  • Formed by linking two $\alpha$-glucose units via an $\alpha(1\rightarrow 4)$ glycosidic bond; produced as an intermediate during the enzymatic breakdown of starch.

  • Reducing versus Non-Reducing Sugars:

• Reducing Sugars: Possess a free, unbonded anomeric carbon bearing an aldehyde or ketone group that can open into an open-chain form, allowing the sugar to act as a reducing agent by donating electrons to oxidize metal ions (e.g., reducing $\text{Cu}^{2+}$ to $\text{Cu}^+$). Examples include all monosaccharides (glucose, fructose, galactose) and maltose.

• Non-Reducing Sugars: Have their reactive anomeric carbons locked in glycosidic linkages, preventing ring-opening and electron donation. In sucrose, the $\alpha(1\rightarrow 2)$ bond directly joins the anomeric carbon of glucose ($\text{C}_1$) to the anomeric carbon of fructose ($\text{C}_2$), neutralizing their reducing capacity.

  • Structure and Biological Roles of Polysaccharides:

• Polysaccharides share the general polymeric formula $(\text{C}_6\text{H}_{10}\text{O}_5)_n$.

• Starch:

  • The principal carbohydrate storage polymer of plants, concentrated in tubers and seeds (potatoes, rice, wheat, maize).

  • Composed of two distinct structural entities:

    • Amylose ($10\text{--}20\%$): An unbranched, linear chain of $\alpha$-D-glucose monomers linked entirely by $\alpha(1\rightarrow 4)$ glycosidic bonds. The linkage geometry causes the chain to coil into a compact, helical architecture optimal for dense energy storage.

    • Amylopectin ($80\text{--}90\%$): A highly branched polysaccharide containing $\alpha(1\rightarrow 4)$ glycosidic linkages along the linear backbones and $\alpha(1\rightarrow 6)$ glycosidic linkages at branch points occurring approximately every $24\text{--}30$ glucose residues.

• Chemical Detection of Starch: Detected using triiodide/iodine solution ($\text{I}_2/\text{KI}$), which exhibits a yellow/brown coloration. Iodine molecules slip inside the helical core of amylose, inducing a charge-transfer complex that yields an intense blue/black color.

  • Glycogen:

• The primary short-term carbohydrate energy storage polymer in animals and humans, deposited predominantly in hepatocytes (liver) and skeletal muscle tissue.

• Structurally resembles amylopectin but features much higher branching density ($\alpha(1\rightarrow 6)$ branches every $8\text{--}12$ glucose residues). This high branching provides numerous free non-reducing terminal ends, facilitating rapid enzymatic cleavage by glycogen phosphorylase to release glucose into the bloodstream during fasting or intense muscular exertion.

  • Cellulose:

• The predominant structural constituent of plant primary cell walls, wood, and cotton fibers; exploited industrially to manufacture paperboard and paper products.

• Formed entirely from $\beta$-glucose subunits linked together by $\beta(1\rightarrow 4)$ glycosidic bonds.

• Because the $\text{C}_1$ hydroxyl group is oriented upward, each alternating $\beta$-glucose monomer must rotate $180^\circ$ relative to its neighbor to accommodate the linkage. This produces a completely straight, unbranched, extended ribbon-like chain.

• Parallel cellulose chains align side-by-side, forming arrays of extensive inter- and intra-molecular hydrogen bonds between exposed hydroxyl groups. These bundles aggregate into dense, rigid crystalline structures known as cellulose microfibrils, conferring high tensile strength and structural rigidity to plant cell walls.

• Humans and other non-ruminant mammals lack cellulase enzymes capable of hydrolyzing $\beta(1\rightarrow 4)$ glycosidic bonds, rendering cellulose indigestible. It passes through the human digestive tract as roughage (insoluble dietary fiber).

Lipids: Classification, Molecular Structures, and Functions

• Fundamental Properties:

  • Lipids constitute a heterogeneous collection of non-polar, hydrophobic organic biomolecules characterized by insolubility in water and high solubility in organic non-polar solvents (e.g., ethanol, ether, chloroform).

  • Composed of carbon, hydrogen, and oxygen, lipids have a markedly higher ratio of carbon-hydrogen ($\text{C}-\text{H}$) covalent bonds to oxygen atoms compared to carbohydrates.

  • Because $\text{C}-\text{H}$ bonds involve equal sharing of electrons with negligible electronegativity differences, lipid hydrocarbon chains lack permanent dipole moments, preventing favorable electrostatic interactions with dipolar water molecules.

  • Energetically dense, lipids yield approximately $9\,\text{kcal/g}$ upon aerobic oxidation, which is more than double the energy density provided by carbohydrates ($4\,\text{kcal/g}$) or proteins ($4\,\text{kcal/g}$).

• Comprehensive Classification of Lipids:

• 1. Simple Lipids (Storage Lipids):

  • Esters of fatty acids with various alcohols.

  • Fats and Oils (Triglycerides / Neutral Fats): Esters of glycerol and three fatty acid chains. Classified as simple if all three fatty acids are identical, or mixed if they differ. Solid at room temperature ($20^\circ\text{C}$) = fats; liquid at room temperature = oils.

  • Waxes: Esters composed of high-molecular-weight, long-chain fatty acids bonded to long-chain monohydric alcohols; serve protective and waterproofing roles (e.g., honeycomb beeswax, earwax/cerumen, plant cuticle wax).

• 2. Compound (Conjugated) Lipids:

  • Esters of fatty acids containing non-lipid prosthetic groups such as phosphate, carbohydrates, or proteins.

  • Phospholipids: Lipids containing a phosphate group; subdivided into glycerophospholipids (containing a glycerol backbone, such as lecithin, cephalin, and plasmalogen) and sphingophospholipids (containing a sphingosine backbone, such as sphingomyelin).

  • Glycolipids: Lipids covalently bonded to carbohydrate moieties; critical components of nervous tissue and cell-surface recognition complexes (e.g., cerebrosides, gangliosides, sulfolipids).

  • Lipoproteins: Macromolecular lipid-protein complexes that facilitate the transport of hydrophobic lipids through aqueous circulatory systems.

• 3. Derived Lipids:

  • Chemical substances produced via the hydrolytic degradation of simple and compound lipids, or synthesized from lipid precursors.

  • Free Fatty Acids and Alcohols (e.g., glycerol).

  • Steroids and Sterols: Characterized by a core four-fused-carbon-ring cyclopentanoperhydrophenanthrene structure. Sterols possess an alcoholic $-OH$ group (e.g., cholesterol, ergosterol). Derivatives include bile acids, vitamin D precursors, and steroid hormones (e.g., testosterone, estrogen, progesterone, corticosteroids).

  • Isoprenoids / Terpenoids / Carotenoids: Pigmented lipid hydrocarbons composed of repeating isoprene units (e.g., carotene, xanthophylls).

  • Triglycerides: Structure and Esterification:

• Formed from one molecule of glycerol (propane-1,2,3-triol, a trihydric alcohol with three hydroxyl groups) and three fatty acid chains.

• Ester Bond Formation: An ester linkage ($-\text{COO}-$) is created via a condensation reaction between a hydroxyl group ($-\text{OH}$) of the glycerol backbone and the terminal carboxyl group ($-\text{COOH}$) of a fatty acid, eliminating one water molecule per bond. Complete synthesis of a triglyceride requires three condensation reactions, yielding three ester bonds and releasing three water molecules ($3\,\text{H}_2\text{O}$).

• Biological Roles of Triglycerides:

  • Compact Energy Storage: Concentrated in adipose tissue; because they are hydrophobic, triglycerides are stored in an anhydrous form without associated water of hydration, avoiding any adverse osmotic effects on intracellular fluid balance.

  • Thermal Insulation: Subcutaneous adipose deposits diminish conductive heat loss, aiding core body temperature homeostasis in endotherms.

  • Mechanical Cushioning: Protects delicate internal organs (such as kidneys and heart) from mechanical trauma.

  • Fat-Soluble Vitamin Absorption: Serves as an obligatory solvent vehicle for absorbing dietary fat-soluble vitamins ($\text{A}$, $\text{D}$, $\text{E}$, and $\text{K}$).

  • Saturated versus Unsaturated Fatty Acids:

• Saturated Fatty Acids:

  • The hydrocarbon tail contains exclusively single carbon-carbon bonds ($\text{C}-\text{C}$), with all remaining valence electrons saturated with hydrogen atoms.

  • Forms straight, linear hydrocarbon chains that pack tightly together via uniform van der Waals interactions.

  • Possess higher melting points; typically solid at room temperature.

  • Predominantly sourced from animal products (butter, cheese, milk, lard, red meat) and tropical oils. Elevated dietary intake correlates with increased circulating low-density lipoprotein (LDL/bad) cholesterol and cardiovascular risk.

  • Examples: Stearic acid ($18\,\text{C}$), palmitic acid ($16\,\text{C}$).

• Unsaturated Fatty Acids:

  • The hydrocarbon chain contains one or more carbon-carbon double bonds ($\text{C}=\text{C}$).

  • Classified as monounsaturated (one double bond, e.g., oleic acid) or polyunsaturated (two or more double bonds, e.g., linoleic acid, linolenic acid).

  • The presence of cis double bonds introduces structural kinks (bends) into the hydrocarbon tail, preventing tight intermolecular packing and decreasing melting points. Consequently, they remain liquid at room temperature (oils).

  • Sourced from plant and marine sources (olive oil, walnuts, flaxseed, avocados, sunflower oil).

  • Essential Fatty Acids: Polyunsaturated fatty acids that cannot be synthesized de novo by animal cells due to the lack of specific desaturase enzymes and must be obtained through the diet (specifically Omega-3 and Omega-6 fatty acids).

  • Phospholipids and Membrane Bilayers:

• Amphipathic Molecular Structure:

  • Phospholipids possess dual chemical properties, featuring an asymmetric distribution of polar and non-polar moieties.

  • Hydrophilic Polar Head: Composed of a glycerol molecule esterified at its third carbon to a negatively charged phosphate group, typically coupled to a charged nitrogenous base such as choline ($-\text{CH}_2-\text{CH}_2-\text{N}^+(\text{CH}_3)_3$). Highly electronegative and readily interacts electrostatically with surrounding water dipoles.

  • Hydrophobic Non-polar Tails: Composed of the two remaining fatty acid hydrocarbon chains esterified to carbons 1 and 2 of the glycerol backbone. One chain is typically saturated (straight), while the other is unsaturated (kinked due to a cis double bond).

• Spontaneous Assembly into Bilayers:

  • In aqueous media, phospholipids spontaneously organize into stable bimolecular sheets (phospholipid bilayers) driven by the hydrophobic effect.

  • The polar hydrophilic head groups orient outward toward the aqueous extracellular environment and the internal cellular cytosol.

  • The non-polar hydrophobic tails point inward, secluded from water, creating a continuous hydrophobic core that acts as a selectively permeable barrier to hydrophilic solutes, ions, and large polar macromolecules.

Nucleic Acids: Molecular Architecture, Energetics, and Protein Synthesis

• Biological Importance of Genetic Material:

  • A functional genetic molecule must fulfill two mandatory criteria:

  • Accurate Information Storage: Possess a stable chemical capacity to encode exhaustive biological blueprints required for cellular functioning and development.

  • High-Fidelity Self-Replication: Direct its own precise enzymatic copying during mitotic and meiotic divisions, ensuring error-free hereditary transmission to daughter cells.

  • Deoxyribonucleic acid ($\text{DNA}$) and ribonucleic acid ($\text{RNA}$) are high-molecular-weight polynucleotide macromolecules originally isolated from the eukaryotic cell nucleus.

• Nucleotide Subunit Architecture:

  • The nucleotide represents the monomeric subunit of all nucleic acid polymers. It consists of three covalently bonded components:

  • A five-carbon pentose sugar.

  • A nitrogenous ring base.

  • An inorganic phosphate group.

  • Nucleoside versus Nucleotide:

  • Nucleoside: Formed solely by the condensation of a nitrogenous base to the $1'$ carbon of a pentose sugar via an $\text{N}$-glycosidic bond (Base + Sugar).

  • Nucleotide: A nucleoside phosphorylated at its $5'$ carbon via a phosphoester linkage (Base + Sugar + Phosphate).

• Pentose Sugar Derivatives:

  • Ribose ($\text{C}_5\text{H}_{10}\text{O}_5$): Present in $\text{RNA}$; possesses hydroxyl groups ($-\text{OH}$) on both the $2'$ and $3'$ carbon positions of the furanose ring.

  • 2-Deoxyribose ($\text{C}_5\text{H}_{10}\text{O}_4$): Present in $\text{DNA}$; lacks an oxygen atom at the $2'$ position, carrying a hydrogen atom ($-\text{H}$) instead. The absence of the $2'$ hydroxyl group enhances the chemical stability of $\text{DNA}$ by preventing spontaneous alkaline hydrolysis.

• Nitrogenous Bases:

  • Purines (Double-Ring Heterocycles): Composed of a fused six-membered pyrimidine ring and a five-membered imidazole ring. Consist of Adenine ($\text{A}$) and Guanine ($\text{G}$).

  • Pyrimidines (Single-Ring Heterocycles): Composed of a single six-membered aromatic ring. Consist of Cytosine ($\text{C}$), Thymine ($\text{T}$) (restricted to $\text{DNA}$), and Uracil ($\text{U}$) (replaces thymine in $\text{RNA}$).

• The Phosphate Group and Chemical Acidity:

  • Esterified to the $5'$ carbon of the pentose sugar. It links consecutive nucleotides together by reacting with the $3'$ hydroxyl group of an adjacent sugar, forming a strong covalent phosphodiester bond.

  • Retains ionizable hydroxyl groups with low $\text{p}K_a$ values. In physiological fluids ($\text{pH} \approx 7.4$), these groups dissociate their protons ($\text{H}^+$), leaving a net negative charge on each phosphate diester unit, imparting polyanionic and strongly acidic characteristics to nucleic acids.

  • Biological Roles of Nucleotides Beyond Heredity:

• Universal Energy Currency: Adenosine triphosphate ($\text{ATP}$) stores metabolic energy in two terminal high-energy phosphoanhydride bonds linking the $\alpha$-, $\beta$-, and $\gamma$-phosphate groups. Hydrolytic cleavage of these bonds drives endergonic cellular processes.

• Cellular Energetic Processes Powered by $\text{ATP}$: Muscle contraction (driving actin-myosin cross-bridge cycling), active membrane transport (pumping ions against electrochemical gradients), and macromolecular biosynthesis (anabolism of proteins, glycogen, and nucleic acids).

• Other High-Energy Nucleotides: Guanosine triphosphate ($\text{GTP}$, drives translation elongation and G-protein signaling), Cytidine triphosphate ($\text{CTP}$, drives phospholipid synthesis), Uridine triphosphate ($\text{UTP}$, drives glycogen synthesis), and Adenosine monophosphate ($\text{AMP}$).

• Coenzyme and Redox Mediators: Nucleotide structural motifs are integrated into metabolic electron carriers including nicotinamide adenine dinucleotide ($\text{NAD}^+/\text{NADH}$) and nicotinamide adenine dinucleotide phosphate ($\text{NADP}^+/\text{NADPH}$).

• Intracellular Second Messengers: Cyclic adenosine monophosphate ($\text{cAMP}$), generated from $\text{ATP}$ by adenylyl cyclase, acts as an intracellular messenger regulating metabolic responses to extracellular hormones.

  • Double-Helical Structure of $\text{DNA}$:

• Formed by two antiparallel polynucleotide strands running in opposite directions: one strand oriented $5'\rightarrow 3'$ and the complementary strand oriented $3'\rightarrow 5'$.

• The structural backbone on the helix exterior is made of alternating deoxyribose sugars and phosphate groups linked covalently by phosphodiester bonds.

• Complementary Base Pairing (Chargaff’s Rules):

  • The planar nitrogenous bases project inward into the central axis, pairing selectively via hydrogen bonds.

  • Adenine ($\text{A}$) pairs exclusively with Thymine ($\text{T}$) via two hydrogen bonds ($\text{A}=\text{T}$).

  • Guanine ($\text{G}$) pairs exclusively with Cytosine ($\text{C}$) via three hydrogen bonds ($\text{G}\equiv\text{C}$).

• Spatial Dimensions of the Double Helix:

  • Diameter of the double helix: $2.0\,\text{nm}$.

  • Distance between adjacent planar base pairs: $0.34\,\text{nm}$.

  • Length of one complete helical turn: $3.4\,\text{nm}$, which corresponds to exactly $10$ base pairs per turn.

  • Helical geometry creates alternating major grooves and minor grooves along the outer surface, serving as sequence-specific docking sites for regulatory proteins.

• Higher-Order Genomic Packaging:

  • Chromosomes: Discrete structures composed of tightly condensed nuclear $\text{DNA}$ and histone proteins ($23$ homologous pairs in diploid human somatic cells).

  • Centromere: Specialized chromosomal region that constricts the chromosome and serves as an assembly platform for kinetochores during spindle fiber attachment.

  • Genes: Defined linear sequences of $\text{DNA}$ nucleotides that encode functional proteins or structural $\text{RNA}$ molecules.

  • Alleles: Alternative nucleotide sequence variations of a given gene located at the same genetic locus.

  • Telomeres: Repetitive nucleoprotein caps positioned at terminal chromosome ends that protect against degradation, end-to-end chromosomal fusions, and the loss of coding sequences during incomplete terminal replication.

• Classes and Functions of Ribonucleic Acid ($\text{RNA}$):

• Messenger RNA ($\text{mRNA}$):

  • Abundance & Structure: Constitutes approximately $5\%$ of total cellular $\text{RNA}$. Synthesized as a linear, single-stranded transcript complementary to the template strand of genomic $\text{DNA}$.

  • Eukaryotic Modifications: Possesses an enzymatic $5'$ cap ($7$-methylguanosine) that protects against exonucleolytic degradation and promotes ribosome binding, alongside a $3'$ poly-A tail added during post-transcriptional processing.

  • Function: Transports the genetic message from the nucleus through nuclear pores into the cytoplasm. Genetic information is organized into successive non-overlapping triplet nucleotide units termed codons, each specifying an individual amino acid.

  • Stability: Highly transient, exhibiting a relatively short operational half-life.

• Transfer RNA ($\text{tRNA}$):

  • Abundance & Size: Comprises roughly $15\%$ of total cellular $\text{RNA}$; smallest $\text{RNA}$ molecule, containing $70\text{--}90$ nucleotides with approximately $60$ distinct species.

  • Structural Conformation: Single-stranded polynucleotide stabilized by complementary base pairing into a four-armed "cloverleaf" secondary structure, which folds into a compact L-shaped tertiary structure.

  • Key Structural Motifs:

  • Acceptor Stem ($3'$ End): Terminates in an unbonded, universal sequence of three bases: $\text{CCA}-3'$, which serves as the covalent attachment site for a specific amino acid.

  • Anticodon Loop: A flexible structural loop bearing a unique three-base anticodon that binds complementarily to the matching codon on the $\text{mRNA}$ strand.

  • $5'$ Terminal End: Characteristically terminates with a guanine nucleotide.

  • Function: Delivers designated amino acids from the cytoplasmic pool to the translating ribosome, matching its anticodon to the $\text{mRNA}$ codon sequence.

• Ribosomal RNA ($\text{rRNA}$):

  • Abundance & Origin: The most abundant $\text{RNA}$ class, representing roughly $80\%$ of all cellular $\text{RNA}$. Transcribed from nucleolar organizer regions containing ribosomal $\text{DNA}$ ($\text{rDNA}$).

  • Structure & Function: Highly folded, stable, globular molecule that complexes with ribosomal proteins to assemble the small and large subunits of ribosomes.

  • Prokaryotic Ribosome Architecture ($70\text{S}$):

  • Small Subunit ($30\text{S}$): Contains $16\text{S}\;\text{rRNA}$ and associated proteins; aligns and decodes $\text{mRNA}$.

  • Large Subunit ($50\text{S}$): Contains $23\text{S}\;\text{rRNA}$, $5\text{S}\;\text{rRNA}$, and proteins.

  • Catalytic Activity (Ribozyme): The $23\text{S}\;\text{rRNA}$ functions as a catalytic ribozyme within the Peptidyl Transferase Center (PTC) of the large subunit, catalyzing peptide bond synthesis between adjacent amino acids.

  • Binding Sites: Coordinates three functional ribosomal sites: A (aminoacyl site, docks incoming charged $\text{tRNA}$), P (peptidyl site, retains growing peptidyl-$\text{tRNA}$), and E (exit site, releases deacylated $\text{tRNA}$).

Proteins: Amino Acids, Higher-Order Folding, and Functional Classes

• Biological Importance of Proteins:

  • Proteins constitute more than $50\%$ of the total dry mass of most living cells and mediate virtually all specialized cellular operations.

  • Cellular Catalysis: All biological enzymes are proteins (though not all proteins are enzymes).

  • Signaling & Hormonal Control: Select regulatory hormones are proteins, including insulin and glucagon.

  • Membrane Integrity & Transport: Integral proteins mediate selective transport across lipid bilayers.

  • Gas Transport: Specialized metalloproteins (hemoglobin and myoglobin) bind and deliver molecular oxygen ($\text{O}_2$).

  • Immunological Defense: Antibodies (immunoglobulins) neutralize foreign pathogens.

  • Mechanical Motion: Cytoskeletal motor proteins (actin and myosin) power cellular locomotion and muscular contraction.

  • Structural Scaffolding: Collagen reinforces bones and arterial walls; keratin provides durability to hair, nails, and epidermis.

  • Nutrient Storage: Ovalbumin in egg white and casein in milk provide essential amino acid reserves.

• Amino Acid Monomers and Peptide Bond Synthesis:

  • Proteins are unbranched polymers built from twenty standard amino acid monomers.

  • General Structure of an Amino Acid:

  • Formed around a central, asymmetric alpha-carbon ($\text{C}_\alpha$) bonded to four distinct substituents:

    • An amino group ($-\text{NH}_2$).

    • A carboxylic acid group ($-\text{COOH}$).

    • A hydrogen atom ($-\text{H}$).

    • A variable chemical side chain designated as the $\text{R}$ group.

  • The $\text{R}$ group confers distinct chemical properties (polar, non-polar, acidic, basic, or aromatic) to each amino acid.

  • The simplest amino acid is glycine, where the $\text{R}$ group is a single hydrogen atom ($-\text{H}$).

• The Peptide Bond:

  • Formed by a condensation reaction between the alpha-carboxyl group of one amino acid and the alpha-amino group of an adjacent amino acid, releasing one water molecule ($\text{H}_2\text{O}$).

  • Produces a covalent carbon-nitrogen ($\text{C}-\text{N}$) linkage known as a peptide bond ($-\text{CONH}-$).

  • The resulting dipeptide possesses directional polarity, terminating with an unreacted amino group at one end (the $\text{N}$-terminus, $-\text{NH}_3^+$) and an unreacted carboxyl group at the opposite end (the $\text{C}$-terminus, $-\text{COO}^-$).

  • Repetitive condensation reactions generate an elongated polypeptide chain. In vivo, peptide bonds are cleaved via enzymatically catalyzed hydrolysis during digestion in the stomach and small intestine.

• The Four Levels of Protein Structural Hierarchy:

• 1. Primary Structure:

  • The unique, linear sequence of amino acids joined exclusively by covalent peptide bonds along the polypeptide backbone.

  • Encoded directly by the nucleotide sequence of the corresponding structural gene.

  • Dictates all subsequent higher-order folding patterns; substitution of a single amino acid residue can alter a protein's functional properties.

• 2. Secondary Structure:

  • Regular, localized folding or coiling of the polypeptide backbone, stabilized exclusively by hydrogen bonds formed between carbonyl oxygen atoms ($>\text{C}=\text{O}$) and amino hydrogen atoms ($-\text{NH}-$) of peptide bonds.

  • Does not involve $\text{R}$-group interactions.

  • $\alpha$-Helix: A right-handed helical spiral where each turn encompasses approximately $3.6$ amino acids. Every carbonyl oxygen forms a hydrogen bond with the amino hydrogen of the residue positioned four steps along the chain.

  • $\beta$-Pleated Sheet: Formed when two or more extended polypeptide segments (termed $\beta$-strands) align side-by-side either in a parallel or antiparallel orientation, linked by transverse hydrogen bonds that generate a pleated sheet conformation.

• 3. Tertiary Structure:

  • The overall, comprehensive three-dimensional compact conformation of an entire polypeptide chain, produced by the folding and coiling of secondary structural motifs.

  • Maintained entirely by diverse intramolecular interactions occurring between the variable amino acid $\text{R}$ groups:

  • Hydrogen Bonds: Form between strongly electronegative atoms in polar $\text{R}$ groups (e.g., between hydroxyl groups $-\text{OH}$ and carbonyl $>\text{C}=\text{O}$ or amino $-\text{NH}-$ groups).

  • Ionic Bonds (Salt Bridges): Form between oppositely charged, ionized $\text{R}$ groups, specifically between basic amino groups ($-\text{NH}_3^+$) and acidic carboxylate groups ($-\text{COO}^-$). Readily disrupted by changes in ambient $\text{pH}$.

  • Disulfide Bonds (Bridges): Extremely strong, covalent bonds formed when the sulfhydryl groups ($-\text{SH}$) of two adjacent cysteine residues ($\text{C}_3\text{H}_7\text{NO}_2\text{S}$) undergo an oxidation reaction ($-2\text{H}$) to produce a cystine linkage ($-\text{S}-\text{S}-$). Can be cleaved by reducing agents ($+2\text{H}$).

  • Hydrophobic Interactions: Weak, non-covalent forces driven by the hydrophobic effect, whereby non-polar aliphatic and aromatic $\text{R}$ groups cluster into the internal core of the protein to minimize destabilizing contact with the surrounding aqueous solvent.

• 4. Quaternary Structure:

  • The functional assembly of two or more independent polypeptide chains (termed subunits) into an aggregated multi-subunit protein complex.

  • May incorporate non-protein components known as prosthetic groups (forming conjugated proteins).

  • Subunits are held together by the same non-covalent and covalent interactions found in tertiary structures (hydrophobic forces, ionic bonds, hydrogen bonds, and occasionally inter-chain disulfide bridges).

  • Globular versus Fibrous Proteins:

• Globular Proteins:

  • Conformation & Solubility: Compact, highly folded, spherical or round shapes. They are soluble in water because hydrophobic $\text{R}$ groups are directed inward toward the central core, while polar, hydrophilic $\text{R}$ groups face outward, forming favorable solvation shells with water molecules.

  • Biological Function: Metabolically dynamic functional proteins that operate in aqueous environments (blood plasma, cytoplasm). Examples include regulatory hormones (insulin), carrier molecules (hemoglobin, myoglobin), and catalytic enzymes.

  • Enzyme Architecture: Possess a specialized cleft known as an active site, consisting of a binding site that selectively docks the complementary substrate through non-covalent interactions, and a catalytic site that directly accelerates chemical bond transformations.

  • Albumin: Sourced from egg white; soluble in water due to its compact spherical geometry displaying abundant hydrophilic side chains on its external perimeter.

• Fibrous Proteins:

  • Conformation & Solubility: Extended, repetitive, elongated rope-like or sheet-like structures. Insoluble in water due to the exposure of non-polar hydrophobic amino acid residues along their surfaces and tight intermolecular packing.

  • Biological Function: Highly stable, metabolically inert structural proteins that provide high tensile strength, elasticity, and mechanical support.

  • Collagen: Composed of three polypeptide chains wound tightly together into a triple-helix "rope" reinforced by covalent cross-links, providing tensile strength to bone, cartilage, tendons, and arterial walls.

  • Keratin: Built from microfibrils aggregating into macrofibrils, surrounded by a cortex and protective cuticle within hair and wool. Abundant cysteine residues form numerous intermolecular disulfide cross-links, rendering keratin resistant to mechanical stretching, chemical degradation, and aqueous dissolution.

  • Structure and Pathology of Haemoglobin:

• Quaternary Architecture of Normal Adult Haemoglobin ($\text{HbA}$):

  • A conjugated globular tetramer composed of four distinct globin polypeptide chains: two alpha ($\alpha$) chains ($141$ amino acids each) and two beta ($\beta$) chains ($146$ amino acids each).

  • Each globin subunit encloses an iron-containing haeme prosthetic group held within an internal hydrophobic pocket.

  • The central iron ion ($\text{Fe}^{2+}$) binds one molecule of oxygen ($\text{O}_2$) reversibly, enabling a complete hemoglobin tetramer to transport up to four oxygen molecules ($4\,\text{O}_2$, or $8$ oxygen atoms) simultaneously from pulmonary capillaries to respiring tissues.

  • When oxygenated, the protein adopts a relaxed conformation known as oxyhaemoglobin (bright red); when deoxygenated, it shifts to a tense conformation (dark bluish-red).

  • Normal human erythrocytes are packed with approximately $250\text{ million}$ hemoglobin molecules per cell.

• Molecular Basis of Sickle Cell Anaemia:

  • A hereditary genetic disorder caused by a single point mutation in the gene encoding the $\beta$-globin chain.

  • In the mutant protein, the normal sixth amino acid residue on the surface of the $\beta$-globin chain—glutamic acid (a polar, hydrophilic, charged residue)—is substituted by valine (a non-polar, hydrophobic, aliphatic residue).

  • When deoxygenated, the non-polar valine residue creates a "sticky" hydrophobic patch on the surface of the $\beta$-chain that docks into a complementary hydrophobic receptor on an adjacent hemoglobin molecule.

  • This causes mutant hemoglobin molecules to polymerize into rigid, insoluble crystalline fibers that distort the erythrocyte membrane.

  • The normal flexible, biconcave red blood cells are deformed into rigid, fragile, crescent or sickle shapes, which occlude narrow capillaries, impede microvascular circulation, and lead to hemolysis, tissue ischemia, acute pain crises, and severe anemia.