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 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 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 Cell Wall and Plasma Membrane→.
| 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 |
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).