Concept: Transfer, Flow, and Manipulation of Genetic Information in Living Systems

This section explores how genetic information is expressed and manipulated in living systems, focusing on the flow of information from DNA to RNA to protein, and how mutations can affect protein structure and function.

Learning Objectives

• Explain the flow of genetic information from DNA to mRNA to protein.

• Describe how expression of genetic information leads to protein synthesis.

• Discuss how mutations in genetic information can result in changes in protein structure and function, and their evolutionary significance.

Proteins in Living Systems

Roles of Proteins

Proteins are essential macromolecules that perform a wide variety of functions in living organisms.

• Enzymatic Activity: Many proteins act as enzymes, catalyzing biochemical reactions.

• Structural Support: Proteins like collagen and keratin provide structural integrity to cells and tissues.

• Transport: Hemoglobin transports oxygen in the blood; membrane proteins transport molecules across cell membranes.

• Signaling: Hormones and receptors are proteins involved in cell communication.

• Defense: Antibodies are proteins that help defend against pathogens.

• Movement: Actin and myosin are proteins involved in muscle contraction.

Protein Diversity: Structure and Function

Proteins can have diverse structural features and functions due to differences in their amino acid sequences and structures.

• Primary Structure: The unique sequence of amino acids in a polypeptide chain.

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

• Tertiary Structure: The overall three-dimensional shape of a single polypeptide, determined by interactions among R groups.

• Quaternary Structure: The association of multiple polypeptide chains into a functional protein complex.

• Level Responsible for Diversity: The primary structure (amino acid sequence) is responsible for the diversity of protein structure and function, as it determines all higher levels of structure.

Amino Acids and Protein Structure

General Structure of an Amino Acid

Amino acids are the building blocks of proteins. Each amino acid has a central carbon (alpha carbon) bonded to:

• An amino group (-NH2)

• A carboxyl group (-COOH)

• A hydrogen atom

• A variable side chain (R group) that determines the properties of the amino acid

Example: The structures of valine and alanine differ in their R groups:

• Valine: R group is -CH(CH3)2

• Alanine: R group is -CH3

Classification of Amino Acids

Amino acids can be classified based on the properties of their R groups:

• Nonpolar (hydrophobic)

• Polar (hydrophilic)

• Acidic (negatively charged)

• Basic (positively charged)

Levels of Protein Structure

Primary Structure

The primary structure of a protein is its unique sequence of amino acids, linked by peptide bonds.

• Peptide Bond: A covalent bond formed between the carboxyl group of one amino acid and the amino group of another, releasing water (a condensation reaction).

Equation for Peptide Bond Formation:

Secondary Structure

The secondary structure refers to local folding of the polypeptide chain into structures stabilized by hydrogen bonds.

• Alpha Helix (α-helix): A coiled structure stabilized by hydrogen bonds between every fourth amino acid.

• Beta Pleated Sheet (β-sheet): Sheet-like structure formed by hydrogen bonds between parallel or antiparallel strands.

• Bonds Involved: Hydrogen bonds between the carbonyl oxygen and amide hydrogen of the polypeptide backbone.

Tertiary Structure

The tertiary structure is the overall three-dimensional shape of a polypeptide, determined by interactions among R groups (side chains).

• Types of Interactions:

  • Hydrophobic interactions

  • Hydrogen bonds

  • Ionic bonds

  • Disulfide bridges (covalent bonds between cysteine residues)

  • Van der Waals interactions

• Role of R Groups: The chemical properties of R groups determine the types of interactions and the final shape of the protein.

Quaternary Structure

Some proteins consist of more than one polypeptide chain. The quaternary structure is the arrangement of these subunits into a functional protein complex.

• Example: Hemoglobin is composed of four polypeptide subunits.

Bonds and Interactions in Protein Structure

Types of Bonds/Interactions

• Hydrophobic Bonds: Nonpolar side chains cluster away from water.

• Hydrogen Bonds: Form between polar side chains or backbone atoms.

• Ionic Bonds: Form between oppositely charged side chains (acidic and basic amino acids).

• Disulfide Bridges: Covalent bonds between sulfur atoms of cysteine residues.

Properties of Amino Acid Side Chains

• Hydrophobic Amino Acids: Side chains are nonpolar and tend to be buried inside the protein.

• Hydrophilic Amino Acids: Side chains are polar or charged and tend to be on the protein surface.

Mutations and Protein Function

Effect of Mutations on Protein Structure

Mutations in the DNA sequence can lead to changes in the amino acid sequence of a protein, potentially altering its structure and function.

• Example: Sickle Cell Anemia

  • A single amino acid substitution (valine for glutamic acid) in the beta chain of hemoglobin causes the protein to aggregate, distorting red blood cell shape.

  • This leads to impaired oxygen transport and various health complications.

Additional info: Not all mutations are harmful; some can be neutral or even beneficial, contributing to adaptation and evolution.