Biochemistry Survey: Chemical Foundations, Molecular Interactions, and the Hydrophobic Effect

Introduction to Biochemistry and Course Survey

• Definition and Scope of STEM Fields:

  • Chemistry: The study of matter, its properties, and how it is converted from one form to another.
  • Physics: The study of energy and how it can be converted from one form to another.
  • Biology: The study of matter inside living organisms and its conversion processes.
  • Biochemistry: Focuses specifically on matter in living organisms and its transformations. It incorporates principles of physics (specifically bioenergetics) to understand how energy is derived and utilized in biomaterial processes (e.g., protein folding).

• Course Structure (The Survey/Crash Course):

  • This is a 5-week survey class designed to summarize two semesters of biochemistry into one intensive semester.
  • Part 1: Basic chemical principles including non-covalent and covalent interactions, water properties, ionization equilibria ($pH$), and a concise overview of thermodynamics.
  • Part 2: Biological molecules, focusing on proteins, nucleic acids, sugars, and lipids.
  • Part 3: Protein functions, specifically the role of proteins as enzymes, catalysis, and regulation.
  • Part 4: Metabolism, applying concepts from the first three parts.
  • Part 5: Molecular Biology, summarizing the field into approximately three topics covered over two days.

Cellular Organization and Elemental Composition

• Hierarchy of Biological Complexity:

  1. Organelles: The specialized structures within cells.
  2. Supramolecular Complexes: Large complexes like ribosomes (made of proteins and $RNA$) and the cytoskeleton.
  3. Biological Macromolecules: Large molecules such as proteins and nucleic acids.
  4. Building Blocks: Small molecule precursors like amino acids (e.g., glycine).
  5. Inorganic Precursors and Metabolites: Fundamental inorganic elements and metabolic intermediates.

• Elemental Abundance in the Human Body:

  • The four most abundant elements in living organisms are Hydrogen ($H$), Carbon ($C$), Oxygen ($O$), and Nitrogen ($N$).
  • Order of Abundance: $H > O > C > N$.
  • Macronutrients: Highly abundant elements and ions including Phosphorus ($P$), Sulfur ($S$), Sodium ($Na^+$), Magnesium ($Mg^{2+}$), Potassium ($K^+$), Calcium ($Ca^{2+}$), and Chloride ($Cl^-$).
  • Micronutrients: Essential trace elements including Iron ($Fe$), Zinc ($Zn$), Copper ($Cu$), Manganese ($Mn$), and Molybdenum ($Mo$).

Chemical Bonding and Energies

• Covalent Bonds (Intramolecular):

  • Biological molecules are formed through covalent bonds between elements like Carbon, Nitrogen, Oxygen, and Hydrogen.
  • Bond Ranges and Energies:
    • Single Bonds (Sigma Bonds): Standard range of $200 \times 400\,kJ\,mol^{-1}$.
    • Double Bonds: Standard range of $400 \times 600\,kJ\,mol^{-1}$.
    • Triple Bonds: Standard range of $600 \times 1000\,kJ\,mol^{-1}$.

• Non-Covalent Interactions (Intermolecular):

  • Ionic (Coulombic) Interactions: Interactions between two ions (e.g., $K^+$ and $NH_4^+$). Strength is approximately $20\,kJ\,mol^{-1}$.
  • Ion-Dipole Interactions: Occur when a full ion interacts with a molecule possessing a dipole (partial charges). Weaker than pure ionic interactions.
  • Dipole-Dipole Interactions: Interactions between two polar molecules. The electron cloud moves toward the more electronegative atom (e.g., $O$ in water), creating partial negative ($\delta^-$) and partial positive ($\delta^+$) regions.
  • Hydrogen Bonding: A specialized, directional dipole-dipole interaction.
    • Donor: Always a hydrogen atom connected to a highly electronegative atom (Nitrogen, Oxygen, or Fluorine).
    • Acceptor: An electronegative atom (Nitrogen, Oxygen, Fluorine; sometimes Sulfur).
    • Energy: Typically $10 \times 20\,kJ\,mol^{-1}$ but can vary based on charge (e.g., charged groups can reach $4\,kcal\,mol^{-1}$ or higher).
    • Biological Role: Essential for maintaining protein secondary structures (alpha helices and beta sheets) and DNA base pairing ($G \rightleftharpoons C$ has 3 bonds; $A \rightleftharpoons T$ has 2 bonds).
  • Van der Waals Interactions (London Dispersion Forces):
    • Weak, short-range, non-directional interactions caused by temporary induced dipoles.
    • They are significant when they occur in large numbers over a large surface area (e.g., "cling wrap" effect).
    • Molecular Recognition: These forces allow for structural complementarity. Since protein surfaces are rugged, high surface contact maximize Van der Waals forces, facilitating specific binding between partners.

Properties of Water as a Biological Solvent

• Molecular Structure: Water ($H_2O$) has a bent structure with a permanent dipole. Oxygen is electronegative whereas hydrogens are electropositive.
• Solvation: Water is a universal solvent because it can form ion-dipole and hydrogen bond interactions with solutes.
  • Dielectric Constant: Water has a high relative dielectric constant ($\epsilon_r \approx 78$), which allows it to decrease the attractive force between ions, facilitating their dissolution.
• Hydrogen Bonding Network:
  • Each water molecule has two hydrogen bond donors and two hydrogen bond acceptors.
  • A single water molecule can form up to 4 hydrogen bonds.
  • States of Water:
    • Ice: Maximum of 4 hydrogen bonds per molecule; highly structured.
    • Liquid Water: Dynamic network where molecules form 1-2 short-lived bonds (duration in picoseconds) while moving.
    • Gaseous Water (Steam): No hydrogen bonds; high kinetic energy leads to total dissociation.

The Hydrophobic Effect

• Definition: The observation that nonpolar (hydrophobic) molecules tend to aggregate in aqueous solution to exclude water.
• Mechanism and Entropy:
  • Water molecules form a rigid, "cage-like" (clathrate) structure around individual nonpolar tails (e.g., fatty acids) to maximize their own hydrogen bonding, which is entropically unfavorable (decreases disorder).
  • When two nonpolar molecules interact/cluster, the surface area exposed to water decreases.
  • Water molecules from the "cages" are released back into the bulk liquid.
  • The release of water increases the disorder of the system, thus increasing entropy ($\Delta S > 0$).
• Thermodynamic Favorability: The driving force is NOT the attraction between the nonpolar molecules themselves, but the favorable increase in entropy of the solvent (water).
• Biological Applications:
  • Micelle and membrane formation.
  • Protein folding (hydrophobic cores).
  • Ligand/substrate binding to enzyme active sites.

Ionization of Water and the pH Scale

• Equilibrium of Water Ionization:
  • $H_2O \rightleftharpoons H^+ + OH^-$
  • Equilibrium Constant ($K_{eq}$):$K_{eq} = \frac{[H^+][OH^-]}{[H_2O]}$
  • $[H_2O]$ is approximately $55.5\,mol\,dm^{-3}$.
  • $K_{eq} = 1.8 \times 10^{-16}$.
• Ion Product of Water ($K_w$):
  • $K_w = [H^+][OH^-] = 10^{-14}$ (at $25\,^{\circ}C$).
  • In pure water, $[H^+] = [OH^-] = 10^{-7}\,mol\,dm^{-3}$.
• The Logarithmic pH Scale:
  • The "p" operator represents a negative base-10 logarithm ($-\log_{10}$).
  • $pH = -\log_{10}([H^+])$
  • $pOH = -\log_{10}([OH^-])$
  • $pH + pOH = 14$
  • A change of 1 $pH$ unit represents a 10-fold change in the concentration of hydrogen ions.

Biological pH Values and Enzyme Activity

• Standard Biological Fluids:
  • Gastric Juice: $pH\,1.2 \times 3.0$
  • Blood Plasma: $pH\,7.4$
  • Pancreatic Fluid: $pH\,7.8 \times 8.0$
  • Urine: $pH\,5.5 \times 8.0$
  • Saliva: $pH\,6.6$
  • Intracellular pH: Typically around $6.9 \times 7.6$ (e.g., Liver is $6.9$, Muscle is $6.1$).
• Enzymatic Optimization: Enzymes function optimally at the $pH$ of their natural environment.
  • Pepsin: Found in the stomach; optimal activity at $pH\,3.0$.
  • Fumarase (Fumarate Enzyme): Found in the TCA cycle; optimal activity at $pH\,7.6$.
  • Lysozyme: Found in tears/saliva; optimal activity at $pH\,5.0$.

Questions & Discussion

• Q: Are those energy numbers (bond breaking) required for the exam?
  • A: No, do not memorize unique numbers. You must know the trends (single < double < triple) and the approximate ranges ($200-400$ vs $600-1000$) to answer questions intelligently.
• Q: What is the significance of the dielectric constant?
  • A: It refers to the solvent's ability to diminish the electrostatic force between charges. Water's high dielectric constant ($78$) means ionic interactions are much weaker in water than in a vacuum, allowing ions to separate.
• Q: If we have a hydroxide ion instead of a partial charge, is it still hydrogen bonding?
  • A: It could be considered an ion-dipole interaction, but hydrogen bonding is distinct because of its high directionality. The donor and acceptor must align to form a stable interaction.

Academic Advice and Study Strategies

• The "Supercomputer" Brain Metaphor: The brain stores information everywhere, but unlike a computer, it doesn't automatically attach a retrieval address. To store and retrieve information effectively (especially for exams like the MCAT), you must train the brain by retrieving the stored data multiple times.
• Active Recall Strategy:
  1. Close the notes.
  2. Write down everything remembered on a blank sheet of paper.
  3. Compare the writing with the original notes for accuracy.
  4. Repeat this process after several hours (e.g., 5-6 hours).
• Storytelling in Metabolism: Large amounts of information are easier to remember when framed as a story. This was the historical method of information transfer before books.
• Structural Memorization (Anatomy Metaphor): When memorizing a large body of work (like anatomy), start with the structural "bones" (major topics), then fill in the organs (concepts), veins/nerves (connectivity), and finally the "makeup" (minor details). Do not try to memorize everything at once without a core structure.