Proteins are classified in several ways, depending on their structure, composition, and function. Here are some key classifications:
1. Based on Composition:
1.1 Simple Proteins:
These proteins yield only amino acids upon hydrolysis. Examples include:
- Albumins: Found in blood, egg whites (e.g., ovalbumin).
- Globulins: Present in blood serum and other tissues.
- Glutelins: Found in plant seeds (e.g., gluten).
- Albuminoids: Structural proteins like keratin (hair, nails) and collagen (connective tissue).
- Histones: Associated with DNA in chromosomes.
- Protamines: Found in sperm cells.
1.2 Conjugated Proteins:
These proteins consist of a simple protein combined with a non-protein component (prosthetic group). Examples include:
- Nucleoproteins: Protein + nucleic acid (e.g., ribosomes).
- Glycoproteins: Protein + carbohydrate (e.g., antibodies).
- Phosphoproteins: Protein + phosphate group (e.g., casein in milk).
- Hemoglobin: Protein + heme group (oxygen transport).
- Lipoproteins: Protein + lipid (e.g., transport cholesterol).
2. Based on Structure:
Proteins are further categorized based on their three-dimensional structure, often utilizing databases like SCOP (Structural Classification of Proteins) and CATH (Class, Architecture, Topology, Homologous superfamily). These databases classify proteins based on structural similarities, reflecting their evolutionary relationships. The Structural Classification of Proteins (SCOP) database, for instance, provides a detailed and comprehensive description of the structural and evolutionary relationships of proteins with known structures. Similarly, NCBI's Conserved Domain Database (CDD) annotates biomolecular sequences, identifying evolutionarily conserved protein domain footprints, aiding in functional classification.
3. Based on Function:
Proteins perform a vast array of functions within an organism, leading to functional classification. Databases such as InterPro assist in this process by classifying proteins into families and predicting domains and sites critical for their functions. For example, enzymes are proteins that catalyze biochemical reactions, while structural proteins provide support and shape to cells and tissues.
The information from the provided references supports these classifications. The references highlight examples of simple and conjugated proteins, the role of databases in structural classification (like SCOP and CDD), and the importance of functional classification.
