Proteins: Classification and Types

Proteins: Classification and Types

• Proteins are classified based on their chemical composition and properties into three main groups: Simple proteins, Conjugated proteins, and Derived proteins.
• Simple proteins yield only amino acids or their derivatives upon hydrolysis.
• Albumins are a type of simple protein with the following characteristics:
  • They have a low molecular weight (around 68,000).
  • They are soluble in pure water.
  • They are heat-coagulable.
  • Found mainly in animal tissues.
• Globulins are another type of simple protein with the following characteristics:
  • They have a higher molecular weight (approximately 150,000).
  • They are insoluble in pure water but soluble in diluted solutions of salts, alkalis, and acids.
  • They are heat-coagulable.
  • Found in both animal and plant tissues.
• Glutelins are plant proteins that are insoluble in neutral solvents but soluble in very dilute acids and alkalis.
• Prolamins (or alcohol-soluble proteins) are found in plant seeds and are soluble in 70-80% alcohol but insoluble in water, neutral solvents, or absolute alcohol.
• Histones are basic proteins that are positively charged and bind to negatively charged nucleic acids.
• Protamins are also basic proteins and are rich in arginine. They often occur in tissues in combination with nucleic acids.
• Albuminoids (or scleroproteins) are a group of highly insoluble proteins that make up structural components of the body. Examples include keratin, collagen, and elastin.
  • Keratin is found in hair, fingernails, and skin and is rich in cysteine residues, forming cysteine cross-links.
  • Collagen is the most abundant protein in vertebrates and is a major component of bone, skin, tendons, and cartilage.

Collagen: Structure and Function

• Collagen is a fibrous protein that provides structural support to tissues.
• The basic unit of collagen is the tropocollagen molecule, which is a triple helix composed of three polypeptide chains.
• Each chain has about 1000 amino acids and is characterized by a repeating tripeptide sequence: Gly-X-Y.
  • X is often proline.
  • Y is usually proline or hydroxyproline.