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

Biomolecules and Their Importance

Overview of Biomolecules

Biomolecules are organic molecules that are fundamental to the structure and function of living organisms. They are primarily composed of carbon and hydrogen, and often include oxygen, nitrogen, phosphorus, and sulfur. The four major classes of biomolecules are carbohydrates, lipids, proteins, and nucleotides.

• Carbohydrates: Serve as energy sources and structural components.

• Lipids: Function in energy storage, membrane structure, and signaling.

• Proteins: Perform a wide range of functions including catalysis, transport, and structural support.

• Nucleotides: Form the basis of genetic material and energy transfer molecules.

Lipids

Fatty Acids

Fatty acids are long hydrocarbon chains with a terminal carboxyl group. They are classified based on the presence and number of double bonds:

• Saturated fatty acids: No double bonds (e.g., palmitic acid).

• Monounsaturated fatty acids: One double bond (e.g., oleic acid).

• Polyunsaturated fatty acids: Two or more double bonds (e.g., linolenic acid).

Formation of Lipids

Lipids such as triglycerides are formed by the combination of glycerol and fatty acids through dehydration synthesis:

• Monoglyceride: Glycerol + 1 fatty acid

• Diglyceride: Glycerol + 2 fatty acids

• Triglyceride: Glycerol + 3 fatty acids

Lipid-Related Molecules

Some important lipid-related molecules include:

• Eicosanoids: Derived from 20-carbon fatty acids; function as signaling molecules (e.g., prostaglandins).

• Steroids: Characterized by four linked carbon rings (e.g., cholesterol, cortisol).

• Phospholipids: Composed of two fatty acids, a glycerol backbone, and a phosphate group; major components of cell membranes.

Carbohydrates

Monosaccharides

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

Pentose Sugars (5C)	Hexose Sugars (6C)
Ribose, Deoxyribose	Glucose, Fructose, Galactose

Disaccharides and Polysaccharides

Disaccharides are formed by the joining of two monosaccharides, while polysaccharides are long chains of monosaccharide units.

Disaccharide	Constituent Monosaccharides
Sucrose (table sugar)	Glucose + Fructose
Maltose	Glucose + Glucose
Lactose	Glucose + Galactose

Proteins

Amino Acids

Amino acids are the monomers of proteins. Each amino acid contains a central carbon atom bonded to a hydrogen atom, an amino group (-NH2), a carboxyl group (-COOH), and a variable R group (side chain) that determines its properties.

• R group: Determines the chemical nature and reactivity of the amino acid.

• Source of nitrogen: Proteins are a major dietary source of nitrogen.

Levels of Protein Structure

• Primary structure: Sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding patterns such as alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.

• Tertiary structure: Overall three-dimensional shape of a single polypeptide, determined by interactions among R groups.

• Quaternary structure: Association of multiple polypeptide chains.

Protein Types

• Fibrous proteins: Long, insoluble, structural proteins (e.g., collagen).

• Globular proteins: Compact, soluble proteins with diverse functions (e.g., enzymes, hemoglobin).

Nucleotides

Structure of Nucleotides

Nucleotides are composed of three components:

• One or more phosphate groups

• A five-carbon sugar (ribose or deoxyribose)

• A nitrogenous base (purine or pyrimidine)

Purines have a double-ring structure (adenine, guanine), while pyrimidines have a single-ring structure (cytosine, thymine, uracil).

Chemical Bonds and Molecular Interactions

Types of Chemical Bonds

• Covalent bonds: Strong bonds formed by sharing electron pairs between atoms. Can be single, double, or triple bonds.

• Ionic bonds: Formed when atoms gain or lose electrons, resulting in attraction between oppositely charged ions.

• Hydrogen bonds: Weak bonds between a hydrogen atom and an electronegative atom (e.g., oxygen, nitrogen).

• Van der Waals forces: Weak, nonspecific interactions between molecules.

Polarity

• Nonpolar molecules: Even distribution of electrons; no charge separation.

• Polar molecules: Uneven distribution of electrons; regions of partial positive and negative charge.

Ions and Isotopes

• Cations: Positively charged ions (lost electrons).

• Anions: Negatively charged ions (gained electrons).

• Isotopes: Atoms with the same number of protons but different numbers of neutrons.

Functional Groups

Functional groups are specific combinations of atoms that confer characteristic chemical properties to molecules. Common functional groups include:

Functional Group	Structure
Amino	–NH2
Carboxyl (acid)	–COOH
Hydroxyl	–OH
Phosphate	–H2PO4

Aqueous Solutions and Solubility

Definitions

• Solute: Substance that dissolves in a liquid.

• Solvent: The liquid in which solutes dissolve (water is the universal solvent in biology).

• Solution: Combination of solutes dissolved in a solvent.

• Solubility: The ease with which a solute dissolves in a solvent.

Hydrophilic molecules dissolve easily in water, while hydrophobic molecules do not.

Concentration

The concentration of a solution is defined as:

$\text{Concentration} = \frac{\text{amount of solute}}{\text{volume of solution}}$

Molecular Shape and Function

Relationship Between Shape and Function

The shape of a molecule, determined by its chemical bonds, is closely related to its biological function. Proteins, in particular, exhibit complex three-dimensional structures that are essential for their activity.

• Secondary structures: Alpha-helix and beta-pleated sheet.

• Tertiary structure: Overall 3D folding.

• Disulfide bonds (S–S): Covalent bonds that stabilize protein structure.

Protein Interactions and Binding

Protein Binding

Proteins are highly selective in the molecules they bind, a property known as specificity. Binding occurs at a specific region called the binding site and is governed by noncovalent interactions.

• Ligand: Any molecule that binds to a protein.

• Substrate: The specific reactant that an enzyme acts upon.

• Molecular complementarity: The fit between the shape of the binding site and the ligand.

• Induced-fit model: The binding site changes shape to accommodate the ligand.

• Affinity: The strength of the binding interaction.

Reversibility and Equilibrium

Protein-ligand binding is reversible and follows the law of mass action. At equilibrium:

$[P] + [L] \rightleftharpoons [PL]$

The equilibrium constant ($K_{eq}$) describes the ratio of bound to unbound molecules at equilibrium.

Factors Affecting Protein Binding

Factor	Effect
Cofactors	Required for ligand binding at the binding site
Proteolytic activation	Converts inactive to active form by removing part of the molecule
Competitive inhibitor	Competes with ligand for binding site
Irreversible inhibitor	Binds permanently to binding site
Allosteric modulator	Binds away from binding site and changes protein activity
Covalent modulator	Covalently modifies protein, altering activity
pH and temperature	Alter protein shape and function; extreme changes can denature proteins

Regulation of Protein Activity

• Up-regulation: Increase in protein amount or activity.

• Down-regulation: Decrease in protein amount or activity.

• Saturation: Maximum reaction rate is reached when all binding sites are occupied.

The rate of protein-mediated reactions depends on the concentration of both protein and ligand, up to the point of saturation.

Summary Table: Key Biomolecules and Their Functions

Biomolecule	Monomer	Main Functions
Carbohydrates	Monosaccharides	Energy, structure
Lipids	Fatty acids, glycerol	Energy storage, membranes, signaling
Proteins	Amino acids	Catalysis, structure, transport, regulation
Nucleic acids	Nucleotides	Genetic information, energy transfer

Additional info: Some explanations and context have been expanded for clarity and completeness, including the law of mass action, the induced-fit model, and the summary tables.