Classification of proteins

In organic chemistry and biochemistry, proteins are classified based on their structure, function, and composition. Here’s a broad overview of these classifications:

By Structure:
- Primary Structure: The sequence of amino acids in a polypeptide chain.
- Secondary Structure: The local folding of the polypeptide chain into structures like alpha-helices and beta-sheets, stabilized by hydrogen bonds.
- Tertiary Structure: The overall three-dimensional shape of a single polypeptide chain, formed by interactions among secondary structures.
- Quaternary Structure: The arrangement of multiple polypeptide chains (subunits) into a functional protein complex.
By Function:
- Enzymes: Proteins that act as biological catalysts to speed up chemical reactions (e.g., amylase, which breaks down starch).
- Structural Proteins: Provide support and shape to cells and tissues (e.g., collagen, keratin).
- Transport Proteins: Carry molecules across cell membranes or through the bloodstream (e.g., hemoglobin, which carries oxygen in the blood).
- Regulatory Proteins: Involved in regulating physiological processes (e.g., insulin, which regulates blood sugar levels).
- Defense Proteins: Protect against disease (e.g., antibodies, which identify and neutralize pathogens).
- Storage Proteins: Store essential nutrients (e.g., casein in milk, which provides amino acids).
By Composition:
- Simple Proteins: Consists only of amino acids (e.g., albumins, globulins).
- Conjugated Proteins: Contain non-amino acid components in addition to amino acids (e.g., glycoproteins, which have carbohydrate groups attached; lipoproteins, which have lipid components).
By Solubility:
- Globular Proteins: Generally soluble in water and have a compact, spherical shape (e.g., enzymes, antibodies).
- Fibrous Proteins: Typically insoluble in water and have elongated, fibrous shapes (e.g., collagen, silk).