Comprehensive Biochemistry, Cell Biology, and Molecular Genetics Study Guide

Enzyme Catalysis and Lock-and-Key Mechanism

Enzyme Function and Specificity:
- Enzymes act as biological catalysts that accelerate chemical reaction rates without being permanently consumed or structurally altered by the reaction.
- The interaction between an enzyme and its substrate follows a lock-and-key model:
  - The lock represents the enzyme (specifically its active site).
  - The key represents the substrate molecule with a specific three-dimensional spatial conformation.
  - The substrate must fit precisely into the active site of the enzyme to form an enzyme-substrate complex, enabling the chemical reaction to proceed.
Enzyme Reusability and Saturation:
- Upon completion of a chemical reaction, the enzyme dissociates unchanged from the resulting products.
- Once dissociated, the enzyme is immediately available to bind new substrate molecules and catalyze subsequent reactions.
- If substrate concentration is exceptionally high, enzymes can become temporarily saturated or exhausted at a given moment, but their structural integrity remains intact for continued catalytic cycles once dissociated.
Activation Energy Reduction:
- Enzymes increase reaction rates by lowering the activation energy ( $E_a$ ) required for a reaction to occur.
- Thermodynamic Example: If a metabolic system possesses $100$ units of energy, a reaction lacking an enzyme might require an activation energy of $100$ units, allowing only $1$ reaction to proceed. With an enzyme present, the required activation energy may drop to $10$ units, allowing the cell to complete $10$ times as many reactions using the exact same pool of available energy.

Biomolecules, Functional Groups, and High-Energy Compounds

Organic Compound Fundamentals:
- Organic molecules contain Carbon ( $C$ ), Hydrogen ( $H$ ), and typically Oxygen ( $O$ ).
- Carbon atoms form long chains or rings linked together by strong covalent bonds.
- Covalent bonds store potential chemical energy. Cleaving these covalent bonds releases usable energy sequentially ( $\text{Energy } 1$ , $\text{Energy } 2$ , $\text{Energy } 3$ ), rendering organic polymers critical cellular energy stores.
- Most organic macromolecules are soluble in water (hydrophilic), with the notable exception of lipids.
Key Functional Groups:
- Amino Group ( $-NH_2$ ):
  - Found predominantly in amino acids.
  - Acts as a biological base because it can accept a hydrogen ion ( $H^+$ ).
- Carboxyl Group ( $-COOH$ ):
  - Found in amino acids and fatty acids.
  - Acts as an acid because it can dissociate to release a hydrogen ion ( $H^+$ ).
- Hydroxyl Group ( $-OH$ ):
  - Found in alcohols, amino acids, fatty acids, and carbohydrates.
  - Plays a primary role in hydrogen bonding and acid-base interactions.
- Phosphate Group ( $-PO_4^{2-}$ ):
  - Consists of a central phosphorus atom bonded to four oxygen atoms.
  - Found in nucleic acids, phospholipids, and high-energy compounds.
  - Imparts a negative charge and polar solubility properties to molecules.
Adenosine Triphosphate (ATP) and Phosphorylated Compounds:
- Adenosine Monophosphate (AMP): Consists of adenosine bound to $1$ phosphate group.
- Adenosine Diphosphate (ADP): Consists of adenosine bound to $2$ phosphate groups.
- Adenosine Triphosphate (ATP): Consists of an adenosine molecule (adenine base attached to a ribose sugar) bound to $3$ high-energy phosphate groups.
- ATP represents the primary energy currency of the cell. Covalent bonds connecting the terminal phosphate groups store high potential energy.
- Hydrolysis of ATP releases usable cellular energy:

$\text{ATP} \rightleftharpoons \text{ADP} + P_i + \text{Energy}$

$\text{ADP} \rightleftharpoons \text{AMP} + P_i + \text{Energy}$

*   ATP synthesis occurs in the mitochondria during cellular respiration, using energy derived from the breakdown of glucose.

Carbohydrate Structure, Classification, and Metabolism

Overview and Abundance:
- Carbohydrates are organic polymers composed of carbon, hydrogen, and oxygen.
- Account for approximately $1.5\,\%$ of total human body weight.
- Functions primarily as a readily accessible energy source for cellular ATP production.
Monosaccharides (Simple Sugars):
- Single-unit sugar monomers, typically adopting ring structures in aqueous solutions.
- Classified by the total number of carbon atoms:
  - Triose: $3$ carbons.
  - Tetrose: $4$ carbons.
  - Pentose: $5$ carbons (e.g., ribose, deoxyribose).
  - Hexose: $6$ carbons (e.g., glucose, fructose, galactose).
  - Heptose: $7$ carbons.
- Glucose: A hexose containing $6$ carbon atoms and multiple hydroxyl ( $-OH$ ) groups. It serves as the principal substrate for glycolysis, breaking down into two $3$ -carbon pyruvate molecules to power mitochondrial ATP synthesis.
Disaccharides:
- Formed when $2$ monosaccharides are joined together via dehydration synthesis (removal of a water molecule, $H_2O$ , involving an $-OH$ group from one sugar and an $-H$ from another).
- Broken down into constituent monosaccharides via hydrolysis (addition of a water molecule).
- Carbohydrates are polar and hydrophilic, dissolving easily in water.
- Three major disaccharides:
  - Sucrose (table sugar) = Glucose + Fructose
  - Lactose (milk sugar) = Glucose + Galactose
  - Maltose = Glucose + Glucose
Polysaccharides:
- Complex macromolecular polymers consisting of long chains of monosaccharide units joined by covalent bonds.
- Glycogen: Known as "animal starch." Highly branched polymer of glucose stored in liver and skeletal muscle tissues. Broken down back into glucose when energy demands rise.
- Starch: Storage polysaccharide synthesized by plants.

Lipid Types, Fatty Acids, and Biological Roles

General Properties and Body Composition:
- Composed of carbon, hydrogen, and oxygen with a carbon-to-hydrogen ratio of approximately $1:2$
- May contain phosphorus ( $P$ ), nitrogen ( $N$ ), or sulfur ( $S$ ).
- Nonpolar and hydrophobic (insoluble in water); require specialized transport mechanisms (lipoproteins) to travel through the bloodstream.
- Yield significantly more energy per gram upon metabolic breakdown than carbohydrates.
- Proportion of total body weight:
  - Males: $12\,\%$ to $18\,\%$
  - Females: $18\,\%$ to $24\,\%$
Five Major Classes of Lipids:
1. Fatty Acids:
  - Long carbon chains ending in a carboxyl group ( $-COOH$ ).
  - Saturated Fatty Acids: Lack carbon-carbon double bonds ( $C=C$ ); all carbon atoms are fully saturated with hydrogen atoms. Typically solid at room temperature; abundant in meat and dairy products.
  - Unsaturated Fatty Acids: Contain one or more double carbon-carbon bonds ( $C=C$ ), introducing kinks into the hydrocarbon tail. Contain fewer hydrogen atoms. Generally liquid at room temperature (plant oils) and associated with reduced risk of cardiovascular disease.
2. Glycerides:
  - Esters formed by binding fatty acid chains to a $3$ -carbon glycerol backbone.
  - Monoglyceride: $1$ glycerol + $1$ fatty acid tail.
  - Diglyceride: $1$ glycerol + $2$ fatty acid tails.
  - Triglyceride: $1$ glycerol + $3$ fatty acid tails (the maximum capacity of glycerol).
  - Formed via dehydration synthesis (releasing $3$ molecules of $H_2O$ to form a full triglyceride). Cleaved sequentially via hydrolysis reactions (adding $H_2O$ yields a diglyceride plus a free fatty acid, then a monoglyceride).
3. Eicosanoids:
  - Lipids derived from arachidonic acid (a $20$ -carbon fatty acid).
  - Function as local chemical messengers between cells during tissue injury or disease.
  - Prostaglandins: Subclass released by damaged tissues; induce pain sensations and act as chemotactic attractors recruiting immune cells to injury sites.
  - Leukotrienes: Participate in immune and inflammatory responses.
4. Steroids:
  - Large lipid molecules composed of a distinct four-ring carbon framework.
  - Cholesterol: Essential structural component of eukaryotic cell membranes that regulates membrane fluidity, flexibility, and integrity. Necessary for cell growth and division.
  - Steroid Hormones: Derived from cholesterol. Include sex hormones (estrogen and testosterone) that control reproductive and metabolic functions, as well as hormones regulating bone density and erythropoiesis.
5. Phospholipids and Glycolipids:
  - Phospholipid: Diglyceride attached to a phosphate group and a non-lipid nitrogenous head.
  - Glycolipid: Diglyceride attached to a carbohydrate chain.
  - Amphipathic Nature: Contain both hydrophobic regions (nonpolar fatty acid tails) and hydrophilic regions (polar phosphate/carbohydrate heads).
  - In aqueous environments, phospholipids spontaneously assemble into spherical micelles or continuous phospholipid bilayers, forming the structural basis of cellular membranes.

Protein Structure, Amino Acids, and Peptide Bond Formation

General Characteristics and Functions:
- Proteins account for approximately $20\,\%$ of total human body weight.
- Composed of carbon, hydrogen, oxygen, nitrogen, and frequently sulfur or phosphorus.
- Serve diverse physiological roles: structural support, cellular movement, molecular transport, pH buffering, metabolic regulation (enzymes), hormonal coordination, immunological defense (antibodies), and blood clotting.
- Contractile and structural examples: Actin, myoglobin, and tubulin drive muscle fiber contraction and relaxation in skeletal muscle.
- Enzymatic example: Lipase features a specifically shaped catalytic pocket that binds triglycerides between the glycerol backbone and fatty acids to catalyze lipid hydrolysis.
- Hormonal example: Thyroxine ( $T_4$ ) secreted by the thyroid gland regulates neuronal growth in the brain, hepatic cholesterol synthesis, fat metabolism, bone elongation, and cardiac rate/hypertrophy.
Amino Acid Monomers:
- Proteins are constructed from $20$ standard amino acids.
- Every amino acid shares a central alpha-carbon ( $C_\alpha$ ) bonded to four groups:
  1. An Amino group ( $-NH_2$ )
  2. A Carboxyl group ( $-COOH$ )
  3. A Hydrogen atom ( $-H$ )
  4. A variable Side chain / R group ( $-R$ )
- The unique chemical identity and polarity of each amino acid are determined exclusively by its R group:
  - Glycine: R group is a single hydrogen atom ( $-H$ ).
  - Alanine: R group is a methyl group ( $-CH_3$ ).
  - Valine: R group is $-CH(CH_3)_2$
  - Nonpolar / Hydrophobic Amino Acids: Glycine, alanine, valine, cysteine, proline, leucine, isoleucine, methionine, tryptophan, phenylalanine.
Peptide Bond Formation:
- Amino acids link together via dehydration synthesis to form covalent peptide bonds.
- Reaction Mechanism: The hydroxyl group ( $-OH$ ) from the carboxyl group of the first amino acid combines with a hydrogen atom ( $-H$ ) from the amino group of the second amino acid, releasing a molecule of water ( $H_2O$ ).
- Dipeptide: $2$ amino acids linked by $1$ peptide bond.
- Polypeptide: A chain of $3$ or more amino acids linked by peptide bonds.
- A linear sequence of over $100$ amino acids typically constitutes a protein, but linear chains are non-functional until folded into precise three-dimensional conformations.

Levels of Protein Organization and Denaturation

Four Hierarchical Levels of Protein Conformation:
1. Primary Structure:
  - The linear sequence of amino acids linked by peptide bonds along the polypeptide backbone.
  - Possesses an N-terminus (amino end) and a C-terminus (carboxyl end).
  - Lacks biological activity or catalytic capability in this uncoiled state.
2. Secondary Structure:
  - Local structural patterns created by hydrogen bonding between peptide backbone atoms.
  - Alpha-Helix ( $\alpha$ -helix): Spiral, coiled helical conformation.
  - Beta-Pleated Sheet ( $\beta$ -sheet): Flat, pleated sheet-like arrangement.
3. Tertiary Structure:
  - Complex three-dimensional folding and coiling of the polypeptide chain into a compact, functional globular or fibrous shape.
  - Stabilized by R-group interactions (hydrophobic interactions, ionic bonds, disulfide bridges, hydrogen bonds).
  - Example: Individual alpha-subunit of hemoglobin containing a central heme group.
4. Quaternary Structure:
  - The spatial arrangement and interaction of two or more independent polypeptide subunits functioning as a single macromolecular complex.
  - Hemoglobin: Consists of $4$ globular polypeptide subunits ( $2$ alpha-chains and $2$ beta-chains), each containing an iron-bearing heme group capable of binding oxygen ( $O_2$ ) with high affinity.
  - Collagen: Fibrous structural protein composed of triple-intertwined helical polypeptide chains.
Protein Denaturation:
- An irreversible structural alteration wherein extreme physical or chemical conditions (high temperatures exceeding $110^\circ\text{F}$ or severe pH deviations) disrupt secondary, tertiary, and quaternary structures.
- Destroys the biological activity and catalytic function of the protein.
- Example: Thermal denaturation of ovalbumin in egg whites converts a clear, water-soluble liquid into an opaque, insoluble solid due to irreversible cross-linking of sulfhydryl ( $-SH$ ) groups.
- Pathological Point Mutations: Substituting a single amino acid in a sequence can ruin protein folding and abolish activity. For example, point mutations in the presenilin protein (e.g., at position $140$ , $75$ , or $70$ ) alter cleavage of amyloid-beta ( $\text{A}\beta$ ), leading to toxic protein aggregation and familial early-onset Alzheimer's disease. Sickle cell anemia similarly results from a single amino acid substitution in the beta-chain of hemoglobin.
Functional Categories of Proteins:
- Catalytic: Enzymes (e.g., lipase, ATP synthase).
- Structural: Collagen (bone, cartilage, teeth, skin), keratin (hair, nails).
- Communication: Peptide hormones, membrane-bound signaling receptors.
- Transport: Transmembrane channels, carrier proteins, hemoglobin.
- Recognition / Identity: Surface glycoproteins act as self-identity markers. Host cells displaying foreign viral or bacterial glycoproteins are targeted and destroyed by immune cells.
- Defense: Antibodies (immunoglobulins) and clotting proteins.
- Motor / Contractile: Actin, myosin, tubulin (constitutes cilia and flagella driving cellular motility).
- Adhesion: Cell Adhesion Molecules (CAMs) lock adjacent cells together to maintain tissue barrier integrity.

Nucleic Acids, Genetic Code, Transcription, and Translation

Nucleic Acid Structure and Components:
- Macromolecules containing carbon, hydrogen, oxygen, nitrogen, and phosphorus.
- Monomer unit: Nucleotide.
- Each nucleotide consists of three subunits:
  1. A Phosphate group ( $-PO_4^{2-}$ )
  2. A Five-carbon pentose sugar (Deoxyribose in DNA; Ribose in RNA)
  3. A Nitrogenous base
Deoxyribonucleic Acid (DNA) vs. Ribonucleic Acid (RNA):
- DNA:
  - Contains deoxyribose sugar.
  - Nitrogenous bases: Adenine ( $A$ ), Guanine ( $G$ ), Cytosine ( $C$ ), and Thymine ( $T$ ).
  - Structure: Double-stranded antiparallel double helix retained within the cell nucleus.
  - Complementary Base-Pairing Rules: Adenine pairs exclusively with Thymine ( $A-T$ ); Cytosine pairs exclusively with Guanine ( $C-G$ ).
- RNA:
  - Contains ribose sugar.
  - Nitrogenous bases: Adenine ( $A$ ), Guanine ( $G$ ), Cytosine ( $C$ ), and Uracil ( $U$ ). Uracil replaces thymine and is never present in DNA.
  - Structure: Single-stranded molecule synthesized in the nucleus that translocates to the cytoplasm.
  - Complementary Base-Pairing Rules: Adenine pairs with Uracil ( $A-U$ ); Cytosine pairs with Guanine ( $C-G$ ).
Functional Classes of RNA:
1. Messenger RNA (mRNA): Single-stranded transcript synthesized from a DNA template that carries genetic instructions to cytoplasm.
2. Transfer RNA (tRNA): Clover-shaped RNA molecules that transport specific amino acids to ribosomes during translation.
3. Ribosomal RNA (rRNA): Structural and enzymatic constituent of ribosomes.
Protein Synthesis: Transcription and Translation:
- Transcription (DNA $\rightarrow$ mRNA):
  - Occurs inside the nucleus.
  - RNA polymerase uses a single strand of DNA as a template to build a complementary single-stranded mRNA molecule.
  - Sequence Conversion Example: If a coding DNA strand contains 5'-ATGA-GTCC-AAGT-3', the complementary DNA strand is 3'-TACT-CAGG-TTCA-5'. Transcribing this sequence yields an mRNA molecule reading 5'-AUGA-GUCC-AAGU-3' (substituting $U$ for $T$ ).
- Translation (mRNA $\rightarrow$ Protein):
  - Occurs in the cytoplasm on ribosomes.
  - The ribosome reads mRNA in linear groups of three nucleotides called codons.
  - Each triplet codon specifies one particular amino acid carried by a tRNA with a complementary anticodon.
  - Start Codon: AUG (mRNA) codes for Methionine.
  - Stop Codons: UAA, UAG, UGA (mRNA) or TAA, TAG, TGA (DNA). Stop codons bind no tRNA amino acids, causing the translation complex to dissociate and release the completed polypeptide. Redundancy in stop codons protects cells against fatal point mutations.

Cell Theory, Cellular Morphology, and Compartmentalization

Principles of Cell Theory:
- The cell is the structural and functional building block of all living organisms.
- All physiological operations are performed by cellular structures.
- All new cells originate from the division of pre-existing cells.
- The human body comprises over $100\text{ trillion}$ cells categorized into more than $200$ distinct cell types.
- Every somatic cell in an organism descends from a single fertilized ovum (zygote).
Cell Size and Structural Morphology:
- Human cells average less than $0.1\,\text{mm}$ ( $100\,\mu\text{m}$ ) in diameter, requiring light or electron microscopy for visualization.
- Morphological Variations:
  - Ovum: Large spherical cell (~ $100\,\mu\text{m}$ diameter).
  - Neurons: Possess elongated processes (axons) extending up to $1\,\text{m}$ in length.
  - Squamous: Thin, flat, tile-like cells (epithelial surfaces).
  - Cuboidal: Hexagonal cubes (glandular epithelium).
  - Columnar: Tall rectangular columns (gastrointestinal mucosa; often feature apical microvilli or cilia to maximize absorption surface area).
  - Stellate / Polygonal: Star-shaped with projecting processes (neurons, osteocytes, microglia).
  - Spheroidal / Discoid: Round or biconcave discs (erythrocytes).
  - Fusiform: Spindle-shaped tapered cylinders (smooth muscle cells).
  - Fibrous: Long thread-like fibers (skeletal muscle cells).
Eukaryotic vs. Prokaryotic Organization:
- Eukaryotic Cells: Enclose genomic DNA within a membrane-bound nucleus and contain membrane-bound intracellular organelles.
- Prokaryotic Cells: Lack a membrane-bound nucleus; genomic DNA floats directly within the cytoplasm.
Intracellular Compartments and Organelles:
- Extracellular Fluid (ECF): Fluid environment surrounding cells (interstitial fluid, plasma).
- Intracellular Fluid (ICF) / Cytosol: Gel-like internal fluid. Contains higher concentrations of potassium ( $K^+$ ), magnesium ( $Mg^{2+}$ ), phosphate ( $-PO_4^{2-}$ ), and dissolved proteins, but lower concentrations of sodium ( $Na^+$ ) relative to ECF.
- Cytoplasm: Cytosol plus all suspended organelles.
- Non-Membranous Organelles: Lack an outer lipid membrane boundary. Include the cytoskeleton, centrioles, ribosomes, cilia, and microvilli.
- Membranous Organelles: Enclosed by phospholipid bilayer membranes. Include the nucleus, mitochondria, lysosomes, peroxisomes, Golgi apparatus, and endoplasmic reticulum (ER).
- Lysosomal Autolysis: Lysosomes store hydrolytic enzymes. Membrane breakdown releases these enzymes into cytosol, triggering apoptosis (programmed cell death).

Plasma Membrane Structure, Fluid Mosaic Model, and Membrane Proteins

Fluid Mosaic Model:
- The plasma membrane consists of a dynamic phospholipid bilayer incorporated with proteins, steroids (cholesterol), and carbohydrates.
- Fluidity: Phospholipids and embedded proteins move laterally within the plane of the membrane.
- Mosaic: A heterogeneous mosaic pattern of integral proteins, peripheral proteins, and glycolipids.
Membrane Permeability and Functions:
- Provides physical isolation between ICF and ECF.
- Establishes selective permeability, restricting passage based on molecular size, electrical charge, lipid solubility, and structural conformation.
- Regulates nutrient entry, waste elimination, cellular environmental sensing, and structural anchoring.
Classes of Membrane Proteins:
1. Integral (Transmembrane) Proteins:
  - Span the entire width of the phospholipid bilayer.
  - Firmly embedded; cannot be extracted without damaging membrane structure.
  - Act as water-filled ion channels, carrier transport proteins, signaling receptors, or active pumps.
  - Sodium-Potassium ATPase Pump ( $\text{Na}^+/\text{K}^+$ -ATPase): Transmembrane enzymatic pump that hydrolyzes ATP to actively export $3\,\text{Na}^+$ ions and import $2\,\text{K}^+$ ions against concentration gradients.
2. Peripheral Proteins:
  - Bound strictly to the inner or outer surface of the lipid bilayer.
  - Easily dissociated from the membrane.
  - Function primarily as regulatory enzymes, structural anchors, or signaling intermediates (e.g., cytochrome c, NADH dehydrogenase sub-complexes).