Overview
This lecture explains the two main types of proteins—globular and fibrous—highlighting their structures, properties, functions, and key examples relevant for Cambridge A Level Biology.
Types of Proteins
- Proteins are divided into two main classes: globular proteins and fibrous proteins.
- Globular proteins are generally spherical and water-soluble.
- Fibrous proteins are elongated, insoluble, and provide structural support.
Differences in Polypeptide Chains
- Polypeptide chains with mainly hydrophilic (water-attracting) amino acids tend to form globular proteins.
- Chains with mostly hydrophobic (water-repelling) amino acids form fibrous proteins.
- In water, globular proteins fold so hydrophilic amino acids face outwards, making them soluble.
- Fibrous proteins do not dissolve in water and maintain a fiber-like shape.
Globular Proteins
- Water-soluble due to exposed hydrophilic amino acids.
- Spherical (not circular) in shape—must use "spherical" or "ball-shaped" in exams.
- Participate in chemical reactions (e.g., act as enzymes, transporters like hemoglobin, or antibodies).
- Mobility in watery environments allows globular proteins to perform metabolic functions.
Fibrous Proteins
- Water-insoluble, unaffected by water.
- Provide structural roles (e.g., keratin in hair/nails, collagen in skin/blood vessels, elastin in airways).
- Do not participate in chemical reactions due to insolubility and structure.
Example: Hemoglobin (Globular Protein)
- Hemoglobin is a protein found in red blood cells, not the cell itself.
- Composed of two α (alpha) and two β (beta) polypeptide chains, each with a different primary structure.
- Chains fold into secondary (alpha helix), tertiary, and then quaternary structures.
- Each polypeptide includes a heme group (non-protein, contains iron, binds oxygen).
- Hemoglobin is classified as a quaternary protein due to its four interacting chains.
Example: Collagen (Fibrous Protein)
- Collagen consists of three polypeptide chains forming a triple helix (quaternary structure).
- Every third amino acid in collagen is glycine, the smallest amino acid, allowing tight coiling.
- Multiple collagen molecules connect with staggered covalent cross-links to form collagen fibrils.
- Staggered cross-links provide strength, flexibility, and resistance to breaking.
- Collagen fibers have high tensile strength and are important in skin, tendons, and artery walls.
Key Terms & Definitions
- Globular Protein — Spherical, water-soluble protein involved in metabolic reactions.
- Fibrous Protein — Insoluble, elongated protein providing structural support.
- Hydrophilic Amino Acid — Amino acid that interacts with water (polar).
- Hydrophobic Amino Acid — Amino acid that avoids water (non-polar).
- Quaternary Structure — Protein structure with two or more polypeptide chains interacting.
- Hemoglobin — Oxygen-binding globular protein in red blood cells.
- Collagen — Structural fibrous protein with triple-helix structure and high tensile strength.
- Heme Group — Non-protein component of hemoglobin that binds oxygen.
- Glycine — The smallest amino acid, critical for tight coiling in collagen.
Action Items / Next Steps
- Review and memorize the structural and functional differences between globular and fibrous proteins.
- Learn the details of hemoglobin and collagen structures for upcoming chapters.
- Ensure clear understanding of key terms, especially quaternary structure and the role of glycine in collagen.