2.4 Proteins Notes

Introduction

Proteins are essential macromolecules that play critical roles in the structure and function of living organisms. They are composed of long chains of amino acids and are involved in virtually every cellular process. This study note will cover the various aspects of proteins, including their structure, functions, and examples, as outlined in the International Baccalaureate (IB) Biology syllabus.

Amino Acids & Polypeptides

Amino Acids

Amino acids are the building blocks of proteins. There are 20 amino acids commonly found in proteins, each with a general structure that includes:

• A central carbon atom ($\text{C}_{\alpha}$).
• An amine group ($\text{-NH}_2$).
• A carboxylic acid group ($\text{-COOH}$).
• A hydrogen atom ($\text{H}$).
• An R group (side chain) that varies among different amino acids and determines their properties.

General Structure of an Amino Acid

$$ \text{H}_2\text{N}-\text{CHR}-\text{COOH} $$

Peptide Bond

A peptide bond is formed through a dehydration synthesis reaction between the carboxyl group of one amino acid and the amine group of another, releasing a molecule of water ($\text{H}_2\text{O}$). The resulting bond is a covalent linkage known as a peptide bond.

$$ \text{R}_1-\text{CH}-\text{CO-NH}-\text{CH}-\text{R}_2 $$

Polypeptides

Polypeptides are long chains of amino acids linked by peptide bonds. The sequence of amino acids in a polypeptide determines the protein's shape and function.

Functions of Proteins

Proteins have diverse functions in living organisms, including:

1. Enzymes: Catalyze biochemical reactions (e.g., Rubisco).
2. Structural Proteins: Provide support and strength to cells and tissues (e.g., collagen, keratin).
3. Transport Proteins: Carry molecules across cell membranes or through the bloodstream (e.g., hemoglobin).
4. Hormones: Act as signaling molecules to regulate physiological processes (e.g., insulin).
5. Antibodies: Play a role in the immune response (e.g., immunoglobulins).
6. Receptors: Detect and respond to chemical signals (e.g., rhodopsin).
7. Cell Adhesion: Help cells stick together (e.g., integrins).
8. Blood Clotting: Involved in forming blood clots (e.g., fibrinogen).
9. Cytoskeleton: Provide structural support within cells (e.g., actin, tubulin).
10. DNA Compaction: Help compact DNA within the nucleus (e.g., histones).

Examples of Proteins

Rubisco

• Function: Catalyzes the fixation of carbon dioxide during photosynthesis.
• Structure: Composed of 16 polypeptide chains forming a globular protein.
• Significance: The most abundant enzyme on Earth, present in all leaves.

Insulin

• Function: Regulates blood glucose levels by promoting the absorption of glucose from the blood.
• Structure: Composed of 2 polypeptide chains forming a short, globular protein.
• Source: Produced and secreted by β-cells in the pancreas.