# 2.15 CLASSIFICATION OF PROTEINS Proteins can be classified into two main groups based on their molecular shape and structure: fibrous and globular. This structural difference is directly related to their function within an organism. ### Fibrous Proteins Fibrous proteins are composed of long polypeptide chains arranged in parallel to form fibers or sheets. They primarily serve structural and contractile roles in the body. * **Shape:** Long, fiber or filament-like * **Structure:** Exist in a secondary structure (e.g., alpha-helix or beta-pleated sheet) during function * **Solubility:** Insoluble in aqueous medium (water) * **Physical Properties:** Elastic in nature * **Crystallization:** Cannot be crystallized * **Examples:** * **Collagen:** A major structural protein in connective tissues * **Fibrinogen:** A blood plasma protein essential for blood clotting * **Actin and Myosin:** Contractile proteins found in muscle cells * **Keratin:** Structural protein found in hair, skin, and nails ### Globular Proteins Globular proteins have polypeptide chains folded into a compact, spherical or globule-like shape. They are typically involved in metabolic, transport, and immune functions. * **Shape:** Spherical or globule-like * **Structure:** Exist in a tertiary or quaternary structure during function * **Solubility:** Soluble in aqueous medium (water) * **Physical Properties:** Inelastic in nature * **Crystallization:** Can be crystallized * **Examples:** * **Enzymes:** Biological catalysts that speed up chemical reactions * **Hormones:** Chemical messengers like insulin * **Antibodies:** Proteins of the immune system that identify and neutralize pathogens * **Channel Proteins:** Proteins embedded in cell membranes that facilitate transport. For more on membrane proteins, see . ### Comparison of Fibrous and Globular Proteins | Feature | Fibrous Proteins | Globular Proteins | | :--- | :--- | :--- | | **Shape** | Long, filament-like | Spherical, globule-like | | **Structure** | Secondary | Tertiary or Quaternary | | **Function** | Structural, contractile | Metabolic, transport, immune | | **Solubility in Water** | Insoluble | Soluble | | **Elasticity** | Elastic | Inelastic | | **Crystallization** | Cannot be crystallized | Can be crystallized | | **Examples** | Collagen, Keratin, Myosin | Enzymes, Hormones, Antibodies | ### Simple, Conjugated, and Derived Proteins Proteins can also be classified based on their chemical composition: * **Simple Proteins:** On hydrolysis, these yield only amino acids (e.g., albumin, globulin). * **Conjugated (Proteids) Proteins:** These consist of a simple protein united with a non-protein group called a prosthetic group (e.g., Glycoproteins, Lipoproteins). * **Derived Proteins:** These are substances formed from simple or conjugated proteins by the action of heat, enzymes, or chemical reagents (e.g., peptones, proteoses).