Molecular Interactions in Human Physiology: Biomolecules, Bonds, and Protein Function

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Chapter 2: Molecular Interactions

Overview

This chapter introduces the foundational concepts of molecular interactions in human physiology, focusing on the structure and function of biomolecules, types of chemical bonds, and the principles of protein interactions. Understanding these topics is essential for grasping how biological molecules contribute to cellular processes and overall physiological function.

Molecules and Bonds

Organic Molecules and Biomolecules

Organic molecules are compounds that contain carbon and are fundamental to living organisms. Biomolecules are organic molecules found in living systems and are classified into four major groups:

Carbohydrates
Lipids
Proteins
Nucleotides

Most biomolecules contain carbon (C), hydrogen (H), and oxygen (O). Their basic functions include:

Energy storage and transfer
Serving as building blocks for cellular structures

Biochemistry of Lipids

Fatty Acids

Fatty acids are long hydrocarbon chains with a carboxyl group (-COOH) at one end. They are classified based on the presence and number of double bonds:

Saturated fatty acids: No double bonds; fully saturated with hydrogen. Example: Palmitic acid.
Monounsaturated fatty acids: One double bond. Example: Oleic acid.
Polyunsaturated fatty acids: Two or more double bonds. Example: Linolenic acid.

Formation of Lipids

Lipids are formed by combining glycerol (a 3-carbon molecule) with fatty acids:

Monoglyceride: Glycerol + 1 fatty acid
Diglyceride: Glycerol + 2 fatty acids
Triglyceride: Glycerol + 3 fatty acids

Lipid-Related Molecules

Some molecules are related to lipids but differ in structure and function:

Type	Structure	Function/Example
Eicosanoids	Modified fatty acids with carbon rings	Prostaglandins, thromboxanes (regulate physiological functions)
Steroids	Four linked carbon rings	Cholesterol (primary steroid in humans), cortisol
Phospholipids	Glycerol + 2 fatty acids + phosphate group	Main component of cell membranes

Biochemistry of Carbohydrates

Monosaccharides

Monosaccharides are the simplest carbohydrates and serve as building blocks for more complex sugars. They are classified by the number of carbons:

Type	Example	Formula	Function
Pentoses (5C)	Ribose, Deoxyribose	C5H10O5	RNA and DNA backbone
Hexoses (6C)	Glucose, Fructose, Galactose	C6H12O6	Energy source

Disaccharides and Polysaccharides

Disaccharides are formed by joining two monosaccharides (e.g., sucrose = glucose + fructose). Polysaccharides are long chains of monosaccharides (e.g., glycogen, starch, cellulose).

Biochemistry of Proteins

Amino Acids and Protein Structure

Proteins are polymers of amino acids. There are 20 standard amino acids that combine in various sequences to form peptides and proteins. Protein structure is hierarchical:

Primary structure: Sequence of amino acids in a polypeptide chain
Secondary structure: Local folding patterns (α-helix, β-sheet) stabilized by hydrogen bonds
Tertiary structure: Overall 3D shape of a single polypeptide, including interactions like disulfide bonds
Quaternary structure: Assembly of multiple polypeptide subunits (e.g., hemoglobin)

Biochemistry of Nucleotides and Nucleic Acids

Nucleotides

Nucleotides are the building blocks of nucleic acids (DNA and RNA). Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group.

Nucleic Acids

DNA and RNA are polymers of nucleotides. Base pairing occurs via hydrogen bonds:

Purines: Adenine (A), Guanine (G)
Pyrimidines: Cytosine (C), Thymine (T), Uracil (U)
Base pairing rules: A pairs with T (or U in RNA), G pairs with C

Chemical Bonds and Interactions

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms. They are strong and require energy to form or break. Types include single, double, and triple bonds. Molecules can be polar (unequal sharing) or nonpolar (equal sharing).

Ionic Bonds

Ionic bonds result from electrostatic attraction between oppositely charged ions. Atoms gain or lose electrons to form cations (+) and anions (-).

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom and an electronegative atom (O, N, or F). They are important in water's properties and in stabilizing biological molecules.

Van der Waals Forces

These are weak, nonspecific attractions between atoms due to transient charge distributions.

Functional Groups

Functional groups are specific combinations of atoms that confer characteristic chemical properties to molecules. They often move between molecules as a single unit and are critical in biochemical reactions.

Important Ions in the Body

Cations	Anions
Na+ (Sodium)	Cl- (Chloride)
K+ (Potassium)	HCO3- (Bicarbonate)
Ca2+ (Calcium)	HPO42- (Phosphate)
H+ (Hydrogen)	SO42- (Sulfate)
Mg2+ (Magnesium)

Solutions and Solubility

Biological Solutions

Biological solutions are typically aqueous (water-based). Solutes are dissolved in solvents, and solubility depends on the chemical nature of the solute:

Hydrophilic: Water-soluble (polar or ionic molecules)
Hydrophobic: Not water-soluble (nonpolar molecules)

Concentration

Concentration is defined as the amount of solute per unit volume of solution:

Formula:

Molecular Shape and Function

Molecular bonds determine the shape of molecules, which in turn affects their biological function. Proteins exhibit complex shapes due to their hierarchical structure, and changes in pH or hydrogen ion concentration can alter molecular conformation.

pH and Buffers

pH Scale

pH measures the concentration of free hydrogen ions (H+) in solution:

pH < 7: Acidic
pH > 7: Alkaline (basic)

Acids release H+ in water; buffers (e.g., bicarbonate) moderate changes in pH.

Protein Interactions

Types of Protein Functions

Enzymes: Catalyze biochemical reactions
Membrane transporters: Move substances across cell membranes
Signal molecules: Communicate between cells
Receptors: Bind ligands and initiate cellular responses
Binding proteins: Carry substances in the body
Immunoglobulins: Antibodies for immune defense
Regulatory proteins: Control cell processes

Protein-Ligand Binding

Proteins are selective in binding ligands due to molecular complementarity. The induced-fit model describes how proteins change shape to accommodate ligands.

Binding Reactions and Law of Mass Action

Protein-ligand binding is reversible and described by equilibrium constants:

Equilibrium constant (Keq):
Dissociation constant (Kd):

Binding reactions obey the law of mass action, which states that the rate of a reaction is proportional to the product of the concentrations of the reactants.

Factors Affecting Protein Binding

Isoforms: Closely related proteins with similar function but different affinity
Activation: Proteolytic activation, cofactors (ions or small organic groups)
Modulation: Chemical modulators (competitive inhibitors, irreversible antagonists, allosteric and covalent modulators)
Physical modulators: Temperature, pH
Regulation: Up-regulation and down-regulation of protein amount
Saturation: Maximum reaction rate when all binding sites are occupied

Summary Table: Key Biomolecules and Their Functions

Biomolecule	Building Blocks	Main Functions
Carbohydrates	Monosaccharides	Energy, cell structure
Lipids	Fatty acids, glycerol	Energy storage, membranes, signaling
Proteins	Amino acids	Enzymes, structure, transport, signaling
Nucleic acids	Nucleotides	Genetic information, energy transfer

Additional info: Some explanations and table entries were expanded for clarity and completeness based on standard academic knowledge.