Molecular Interactions: Biomolecules, Bonds, and Protein Function

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Chapter 2: Molecular Interactions

Molecules and Bonds

Molecular interactions are fundamental to physiology, as they determine the structure and function of cells and tissues. Most biomolecules are composed of carbon, hydrogen, and oxygen, and their interactions are governed by various types of chemical bonds.

Organic molecules: Compounds containing both carbon and hydrogen. Biomolecules are a subset of organic molecules essential for life.
Biomolecules: The four major classes are carbohydrates, proteins, lipids, and nucleic acids.

Carbohydrates

General formula: CH2O
Monosaccharides: Single sugar units, typically in ring structures. Glucose is the primary monosaccharide used for energy (e.g., in ATP production). Fructose is another monosaccharide, metabolized mainly by the liver. Excessive intake (e.g., high fructose corn syrup) can lead to non-alcoholic fatty liver disease.
Disaccharides: Two monosaccharides joined by a covalent bond. Examples: sucrose, lactose, maltose.
Polysaccharides: Long chains of monosaccharides. Starch (plants) and glycogen (animals) are used for energy storage.
Simple carbohydrates: Mono-, di-, and some polysaccharides that are rapidly digested and absorbed, causing blood glucose spikes (e.g., white rice, white bread).
Complex carbohydrates: Retain bran/fiber, slow digestion and glucose absorption, and provide dietary fiber (e.g., whole grains).

Proteins

Polymers of amino acids linked by peptide bonds.
Structural proteins: e.g., myosin, actin, collagen; generally do not denature.
Functional proteins: e.g., enzymes; can denature with high temperature or extreme pH.

Lipids

Nonpolar biomolecules, insoluble in water.
Triglycerides: Glycerol backbone with three fatty acid chains.
Saturated fats: All single bonds between carbons; solid at room temperature; typically from animal sources.
Unsaturated fats: At least one double bond; liquid at room temperature; usually from plant sources.
Trans fats: Man-made by hydrogenating unsaturated fats; associated with health risks.
Phospholipids: Glycerol backbone, two fatty acids, and a phosphate group; amphipathic (hydrophilic and hydrophobic ends); major component of cell membranes.
Steroids: Derived from cholesterol; include hormones.

Nucleic Acids

DNA, RNA, and ATP are nucleic acids.
Composed of a nucleotide base, a sugar, and a phosphate group.

Atoms and Chemical Bonds

Atoms are the smallest units of matter retaining the properties of an element. They consist of a nucleus (protons and neutrons) and electrons in orbitals.

Electrons: Play four key biological roles:
- Covalent bonds: Electrons are shared between atoms.
- Ions: Atoms that have gained or lost electrons, becoming charged.
- High-energy electrons: Store energy (e.g., in ATP).
- Free radicals: Molecules with unpaired electrons; can cause cellular damage; neutralized by antioxidants (Vitamins A, C, E).

Covalent Bonds

Formed by sharing electrons between atoms.
Polar molecules: Unequal sharing of electrons; partial charges develop.
Nonpolar molecules: Equal sharing; no partial charges.

Noncovalent Bonds

Facilitate reversible interactions.
Ionic bonds: Transfer of electrons; attraction between oppositely charged ions (anions = negative, cations = positive).
Hydrogen bonds: Weak bonds between partial charges; important for water properties and protein structure; contribute to surface tension.
Van der Waals forces: Very weak attractions between atoms due to transient charges.

Noncovalent Interactions and Biological Solutions

Hydrophilic interactions: Allow formation of biological solutions (e.g., cytoplasm).
Molecular shape: Determines molecular function, especially in proteins.

Protein Structure

Primary structure: Sequence of amino acids.
Secondary structure: Local folding (alpha helix, beta strands).
Tertiary structure: 3D folding; includes globular proteins (functional, can denature) and fibrous proteins (structural, do not denature).

Hydrogen Ions and pH

pH: Measures concentration of free protons (H+).
Acids: Proton donors.
Bases: Proton acceptors.
Buffer: Prevents large fluctuations in pH.

Protein Interactions

Proteins interact with other molecules to perform a wide range of functions in the body.

Enzymes: Catalyze (speed up) chemical reactions without being consumed.
Membrane transporters: Move substances across cell membranes (channels, carriers).
Signal molecules: Hormones, neurotransmitters.
Receptors: Bind signal molecules and initiate cellular responses.
Binding proteins: Transport molecules (e.g., hemoglobin).
Immunoglobulins: Antibodies.
Regulatory proteins: Control cellular processes (e.g., transcription factors).

Protein Binding

Binding (active) site: Region on a protein where a molecule binds.
Ligand: Any molecule that binds to another molecule.
Substrate: Specific ligand for enzymes and transporters.
Specificity: Ability to bind certain ligands or groups of related ligands.
Induced fit model: Enzyme active site can change shape to accommodate related molecules.

Reversibility and Affinity

Protein-ligand binding is often reversible and reaches equilibrium (reactants = products).
Affinity: Degree of attraction between protein and ligand.

Law of Mass Action

If the ratio of reactants to products is altered, the reaction shifts to restore equilibrium.

Competition and Modulation

Competitors: Related ligands competing for the same binding site (e.g., epinephrine and norepinephrine).
Agonists: Mimic the action of a substrate (e.g., nicotine and acetylcholine).
Antagonists: Inhibit activity by blocking binding or function.
Competitive inhibitors: Compete for the same binding site; can be reversible.
Allosteric modulators: Bind elsewhere on the protein, altering the binding site.
Covalent modulators: Add or remove functional groups (e.g., phosphate) to alter protein activity.

Isoforms and Activation

Isoforms: Related proteins with similar functions but different affinities (e.g., fetal vs. adult hemoglobin).
Activation: Some proteins require modification to become active (e.g., pepsinogen to pepsin).
Cofactor: Inorganic ion or small organic molecule (coenzyme) required for protein function.

Physical Factors Affecting Proteins

Temperature and pH can alter protein structure and function.

Regulation of Protein Amount

Up-regulation: Increases protein production.
Down-regulation: Decreases protein production.

Saturation and Reaction Rate

When all proteins are bound to substrate, the reaction rate reaches a maximum (saturation).

Summary Table: Types of Chemical Bonds and Interactions

Bond/Interaction	Description	Strength	Example
Covalent	Electrons shared between atoms	Strong	Peptide bonds in proteins
Ionic	Transfer of electrons; attraction between ions	Moderate	NaCl (table salt)
Hydrogen	Attraction between partial charges	Weak	Between water molecules
Van der Waals	Transient, weak attractions	Very weak	Between nonpolar molecules

Key Equations

Law of Mass Action:
pH Calculation:

Example: Enzyme-Substrate Interaction

Enzymes such as amylase bind to starch (substrate) and catalyze its breakdown into maltose. The enzyme's active site is specific for the substrate, and the reaction rate increases with substrate concentration until saturation is reached.

Additional info: Some details, such as the induced fit model and the distinction between globular and fibrous proteins, were expanded for clarity and completeness.