B1.2.2 – PROTEIN STRUCTURE AND PROPERTIES
📌Definition Table
| Term | Definition |
|---|---|
| Primary Structure | The sequence of amino acids in a polypeptide chain. |
| Secondary Structure | Local folding of the polypeptide into α-helices or β-pleated sheets, stabilised by hydrogen bonds. |
| Tertiary Structure | The overall 3D shape of a single polypeptide, determined by R-group interactions. |
| Quaternary Structure | The association of two or more polypeptide chains into a functional protein. |
| Denaturation | Loss of protein’s native structure (and function) due to changes in pH, temperature, or chemicals. |
| Globular Protein | Spherical, soluble protein with functional roles (e.g., enzymes). |
| Fibrous Protein | Long, insoluble protein with structural roles (e.g., collagen). |
📌Introduction
Protein structure is hierarchical, progressing from a simple amino acid sequence to complex, functional 3D shapes. These structures are stabilised by various chemical interactions, and even minor changes can alter a protein’s function or cause denaturation. The specific folding of proteins determines their biological role — from catalysis to structural support.
❤️ CAS Link: Create a protein structure model exhibition using materials like clay or 3D-printed parts to illustrate primary through quaternary levels.
📌 Levels of Protein Structure
- Primary: Unique sequence of amino acids; determines higher structures and function.
- Secondary: α-helices and β-pleated sheets formed by hydrogen bonds between backbone atoms.
- Tertiary: 3D folding due to R-group interactions — hydrogen bonds, ionic bonds, hydrophobic interactions, disulphide bridges.
- Quaternary: Multiple polypeptides combining (e.g., haemoglobin with four subunits).
- Each level is critical; mutations in the primary sequence can disrupt the entire structure.
- Proteins can be structural (fibrous) or functional (globular).
🧠 Examiner Tip: In long-answer questions, describe at least one type of bond stabilising each structural level for full marks.
📌 Factors Affecting Protein Stability
- Temperature: High heat breaks hydrogen bonds, causing denaturation.
- pH: Changes alter ionic bonds and disrupt folding.
- Chemical Agents: Organic solvents, detergents can disrupt hydrophobic interactions.
- Salt Concentration: High salt can precipitate proteins by disrupting water interactions.
- Some proteins can refold after mild denaturation; others cannot.
- Extremophiles possess proteins stable in extreme conditions.
🌍 Real-World Connection: Enzymes in hot-spring bacteria (e.g., Taq polymerase) are stable at high temperatures and vital for PCR technology.
📌Globular vs. Fibrous Proteins
- Globular: Compact, soluble, dynamic roles (enzymes, hormones, transport). Example: insulin, haemoglobin.
- Fibrous: Long, insoluble, structural roles (collagen in connective tissue, spider silk).
- Differences arise from amino acid sequences and folding patterns.
- Fibrous proteins often have repetitive sequences for strength.
- Globular proteins often have hydrophobic cores and hydrophilic surfaces.
- Structural type relates directly to biological role.
🌐 EE Focus: Investigate thermal stability of different protein types, comparing globular vs fibrous under lab conditions.
📌 Technology in Protein Study
- X-ray crystallography: Determines atomic structure.
- Cryo-electron microscopy (cryo-EM): Visualises proteins without crystallisation.
- NMR spectroscopy: Used for proteins in solution.
- Advances in tech reveal protein folding pathways and dynamics.
- Structural databases store resolved protein models (e.g., Protein Data Bank).
- Computational modelling predicts folding for unstudied proteins.
🔍 TOK Perspective: How do technological advances shape our “certainty” about protein structure, and could future tools prove current models incomplete?