Introduction to Biochemistry: Chemical Foundations and Biological Macromolecules

Defining Biochemistry

Biochemistry as an Interdisciplinary Science

Biochemistry is the study of the chemical processes and substances that occur within living organisms. It integrates principles from multiple scientific disciplines to understand the molecular basis of life.

• Related Fields: Biochemistry draws from organic chemistry, physical chemistry, biophysics, medical science, cell biology, microbiology, genetics, physiology, and nutrition.

• Applications: Understanding disease mechanisms, drug development, nutrition, and genetic engineering.

Chemical Elements of Cells and Organisms

Major and Minor Elements in Living Systems

Living organisms are primarily composed of a limited set of chemical elements, with a few being especially abundant and essential for life.

• Major Elements: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) are the most prevalent elements in biological molecules.

• Other Essential Elements: Sulfur (S), Phosphorus (P), and ions such as Na+, K+, Mg2+, Ca2+, and Cl− are also vital for cellular function.

Figure: The periodic table highlights elements most relevant to biochemistry, categorized by abundance and biological importance.

Biological Macromolecules

Classes and Structure of Biological Macromolecules

Four major classes of biological macromolecules are essential for the structure and function of living organisms. These macromolecules are polymers (except lipids) made from smaller organic subunits.

• Nucleic Acids: DNA and RNA, polymers of nucleotides, store and transmit genetic information.

• Proteins: Polymers of amino acids, perform a wide range of structural and catalytic functions.

• Polysaccharides: Polymers of monosaccharides (sugars), serve as energy storage and structural materials.

• Lipids: Not true polymers, but large complexes of fatty acids; serve as membrane components, energy stores, and signaling molecules.

Monomers and Linkages

Macromolecule	Monomer	Linkage
Nucleic acids	Nucleotide	Phosphodiester
Protein	Amino acid	Peptide (amide)
Polysaccharide	Monosaccharide	Glycoside (ether)
Lipids (triacylglycerols)	Fatty acids	Ester

Additional info: Lipids are not true polymers but form large complexes through non-covalent interactions.

Examples of Biological Polymers

• Nucleic Acids: DNA and RNA are composed of nucleotide monomers linked by phosphodiester bonds.

• Proteins: Polypeptide chains are formed by amino acids joined via peptide bonds. Example: Tyrosine is one of the 20 standard amino acids.

• Polysaccharides: Cellulose is a polymer of β-glucose units; starch and glycogen are polymers of α-glucose.

• Lipids: Triacylglycerols consist of glycerol esterified with three fatty acids; phospholipids and cholesterol are key membrane components.

Lipids and Membrane Structure

Structure and Function of Lipids

Lipids are hydrophobic or amphipathic molecules that play critical roles in cellular structure and energy storage.

• Structural Role: Major components of biological membranes, forming barriers that compartmentalize cells.

• Energy Storage: Serve as long-term energy reserves.

• Other Functions: Act as hormones and signaling molecules.

Phospholipid Bilayer Formation

Phospholipids spontaneously form bilayers in aqueous environments, creating the fundamental structure of cellular membranes.

• Bilayer Structure: Hydrophilic (polar) head groups face outward, while hydrophobic (nonpolar) tails face inward, away from water.

Structure and Properties of Water

Unique Properties of Water

Water is the universal solvent in biological systems due to its unique chemical and physical properties.

• Hydrogen Bonding: Each water molecule can form up to four hydrogen bonds (two donor, two acceptor sites).

• Permanent Dipole: Water has a bent molecular geometry, resulting in a permanent dipole moment.

• High Heat Capacity: Water can absorb significant amounts of heat with minimal temperature change.

• Density: Liquid water is denser than ice, which is why ice floats.

• High Dielectric Constant: Water can dissolve many ionic and polar substances.

Water as a Molecular Lattice

Water forms an extensive hydrogen-bonded network, which is more ordered in ice (solid) than in liquid water.

• Solid (Ice): Each water molecule forms four hydrogen bonds, creating an open lattice structure.

• Liquid: Hydrogen bonds are transient, allowing for fluidity and higher density.

Amphipathic Molecules in Aqueous Solution

Behavior of Amphipathic Molecules

Amphipathic molecules contain both hydrophilic and hydrophobic regions, influencing their behavior in water.

• Self-Assembly: Can form monolayers, micelles, or bilayers depending on concentration and molecular structure.

• Biological Membranes: The phospholipid bilayer is the primary structural component of cellular membranes.

Acids and Bases: Proton Donors and Acceptors

Brønsted-Lowry Definition and Ionization

The acid-base properties of molecules are fundamental to biochemical reactions, especially in aqueous environments.

• Acids: Proton (H+) donors.

• Bases: Proton acceptors.

• Strong Acids: Dissociate almost completely in water.

• Weak Acids: Dissociate only partially.

• Hydronium Ion: The dissociated proton associates with water to form H3O+.

The pH Scale and the Physiological pH Range

Definition and Importance of pH

pH is a measure of the hydrogen ion concentration in solution and is critical for maintaining proper biochemical function.

• pH Equation:

$\mathrm{pH} = -\log [\mathrm{H}^+]$

• Acidic Solutions: pH < 7

• Basic Solutions: pH > 7

• Physiological pH Range: Most biological reactions occur between pH 6.5 and 8.0.

Effect of pH on Biomolecules

The charge and function of biomolecules can change with pH, affecting their interactions and activity.

• Molecular Charge: The overall charge of proteins and other biomolecules depends on the pH of the environment.

• Example: The surface charge of human ubiquitin varies with pH, influencing its interactions.

Additional info: This principle underlies protein folding, enzyme activity, and molecular recognition in cells.