Proteins can be classified based on their composition into three main types:
1. Simple proteins (Homoproteins)- Composed only of amino acids.
Examples:
• Albumins (e.g., Serum albumin)
• Globulins (e.g., Immunoglobulins)
• Histones (e.g., Nucleoproteins)
• Protamines (e.g., Salmine in fish sperm)
2. Conjugated proteins (Heteroproteins)- Contain both amino acids and a non-protein component (prosthetic group).
Subtypes based on the prosthetic group:
• Glycoproteins – Carbohydrate (e.g., Mucin)
• Lipoproteins – Lipid (e.g., HDL, LDL)
• Metalloproteins – Metal ions (e.g., Hemoglobin with iron)
• Phosphoproteins – Phosphate group (e.g., Casein in milk)
• Nucleoproteins – Nucleic acid (e.g., Ribosomes)
• Chromoproteins – Colored prosthetic group (e.g., Hemoglobin, Cytochromes)
3. Derived proteins- Formed by the partial hydrolysis or degradation of simple or conjugated proteins.
Examples:
• Peptones – Intermediate products of protein digestion
• Proteoses – Smaller fragments of proteins
• Polypeptides – Short chains of amino acids