Section 1.6: Proteins and Nucleic Acids

Section 1.6 – Proteins and Nucleic Acids

Biological macromolecules are essential for life, and among them, proteins and nucleic acids are arguably the most functionally significant. Proteins carry out the majority of cellular functions, acting as enzymes, structural elements, transporters, and regulators, while nucleic acids store, transmit, and express hereditary information. Together, they form the molecular framework for all biological processes.

1. Monomers and the Importance of Polymers

Before diving into proteins and nucleic acids, it’s useful to recall the basic theme of macromolecules: monomers join to form polymers through dehydration synthesis (condensation reactions), and they can be broken back into monomers by hydrolysis.

• Proteins are polymers of amino acids.
• Nucleic acids (DNA and RNA) are polymers of nucleotides.

This repeating-unit structure explains how a small set of building blocks can generate massive molecular diversity — a theme central to life’s complexity.

2. Proteins

2.1 Structure of Amino Acids

Amino acids are the monomers of proteins. Each amino acid has a general structure:

• A central carbon atom (α-carbon).
• A hydrogen atom.
• An amino group (-NH₂).
• A carboxyl group (-COOH).
• A variable R-group (side chain), which determines the chemical properties of the amino acid.

There are 20 standard amino acids in nature. Their side chains may be nonpolar (hydrophobic), polar (hydrophilic), acidic (negatively charged), or basic (positively charged). This diversity in side chain chemistry allows proteins to fold into intricate 3D shapes and perform specialized functions.

2.2 Peptide Bonds and Polypeptides

Amino acids link together through peptide bonds, formed during dehydration synthesis between the carboxyl group of one amino acid and the amino group of another. The resulting chain of amino acids is called a polypeptide.

• Primary structure = the specific amino acid sequence.

Polypeptides fold into complex shapes stabilized by chemical interactions, leading to functional proteins.

2.3 Levels of Protein Structure

Proteins exhibit four levels of structural organization:

• Primary structure – This is the unique amino acid sequence (like letters in a word). A change in even one amino acid can drastically alter function (e.g., sickle cell anemia caused by a single substitution in hemoglobin).
• Secondary structure – These are regular folding patterns such as α-helices and β-pleated sheets, stabilized by hydrogen bonds between backbone atoms.
• Tertiary structure – This is the overall 3D shape of a single polypeptide, formed by interactions between side chains (hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges).
• Quaternary structure – This is the association of multiple polypeptide chains into a functional protein (e.g., hemoglobin, made of four subunits).

2.4 Protein Functions

Proteins are the most versatile biomolecules. Key roles include:

• Enzymatic activity: Enzymes catalyze biochemical reactions by lowering activation energy.
• Structural support: Collagen in connective tissue, keratin in hair and nails.
• Transport: Hemoglobin carries oxygen; membrane proteins transport ions and molecules.
• Defense: Antibodies recognize and neutralize pathogens.
• Movement: Actin and myosin enable muscle contraction.
• Signaling: Hormones like insulin regulate processes; receptor proteins detect signals.

2.5 Protein Denaturation

Proteins are sensitive to environmental conditions. Extreme pH, high temperature, or certain chemicals can denature a protein, disrupting its structure and function. While primary structure may remain intact, secondary and tertiary structures unravel, rendering the protein inactive.

3. Nucleic Acids

3.1 Nucleotides: The Monomers of Nucleic Acids

Nucleotides are the building blocks of nucleic acids. Each nucleotide has three parts:

• A phosphate group.
• A five-carbon sugar (ribose in RNA, deoxyribose in DNA).
• A nitrogenous base.

The bases are categorized into two groups:

• Purines: Adenine (A), Guanine (G) – larger, double-ring structures.
• Pyrimidines: Cytosine (C), Thymine (T, found in DNA), and Uracil (U, found in RNA) – smaller, single-ring structures.

3.2 DNA vs. RNA

DNA (Deoxyribonucleic acid): Stores hereditary information in a double-stranded, antiparallel helix.

• Bases pair specifically (A–T, G–C) via hydrogen bonds.
• Provides instructions for protein synthesis.

• mRNA (messenger RNA): Carries the genetic message to ribosomes.
• tRNA (transfer RNA): Brings amino acids during translation.
• rRNA (ribosomal RNA): Structural and catalytic role in ribosomes.

3.3 Formation of Nucleic Acids

Nucleotides are linked by phosphodiester bonds between the phosphate group of one nucleotide and the sugar of another. This forms a sugar-phosphate backbone with nitrogenous bases extending like “rungs.”

3.4 The Central Dogma of Molecular Biology

Proteins and nucleic acids are linked conceptually through the central dogma:
DNA → RNA → Protein

• Replication: DNA makes copies of itself.
• Transcription: DNA sequence is transcribed into RNA.
• Translation: RNA directs the synthesis of proteins at ribosomes.

This flow of information underlies all cellular function and heredity.

3.5 Nucleic Acids and Evolution

Nucleic acids provide a molecular basis for inheritance. DNA sequences are passed from generation to generation, while mutations introduce genetic variation, fueling evolution by natural selection.

4. Proteins and Nucleic Acids: An Interdependent Relationship

Proteins and nucleic acids are not isolated in their roles; they depend on one another:

• DNA encodes the instructions to make proteins.
• Proteins (such as DNA polymerase, RNA polymerase, ribosomal proteins) are required to replicate and express DNA.

This circular dependency reflects the deep co-evolution of life’s molecules.

5. Summary

• Proteins are polymers of amino acids with four structural levels and diverse functions. Their structure is sensitive to environmental changes.
• Nucleic acids (DNA and RNA) are polymers of nucleotides, storing and transmitting genetic information.
• The central dogma connects the two: DNA encodes proteins, and proteins carry out cellular work.

Together, proteins and nucleic acids represent the molecular foundation for heredity, regulation, structure, and metabolism — making them essential to understanding life at the molecular level.