Proteins are categorized into three primary groups:

1. Simple proteins (e.g., albumin, globulins) contain only amino acids
2. Conjugated proteins (e.g., hemoglobin, glycoproteins) have prosthetic groups
3. Derived proteins (e.g., peptides, peptones) form via protein breakdown

This system helps biochemists analyze protein roles in metabolic pathways and disease mechanisms.

Globular proteins:

• Spherical, water-soluble structures (e.g., insulin, antibodies)
• Function as enzymes, transporters, and messengers

Fibrous proteins:

• Rod-like, insoluble formations (e.g., keratin, fibrin)
• Provide mechanical support to cells and tissues

Their distinct shapes directly correlate with their biological roles.

Conjugated proteins combine amino acids with non-protein components:

• Hemoproteins (heme group) – Oxygen transport (e.g., hemoglobin)
• Glycoproteins (carbohydrates) – Cell recognition (e.g., ABO blood groups)
• Nucleoproteins (DNA/RNA) – Chromosome structure (e.g., histones)

These hybrid molecules enable complex cellular functions impossible for simple proteins.

Key structural proteins include:

1. Collagen (25-35% of body protein): Triple helix structure in skin, bones
2. Keratin (hair/nails): Contains disulfide bonds for toughness
3. Elastin (lungs/arteries): Provides tissue elasticity

Defects in these proteins cause Ehlers-Danlos syndrome and other connective tissue disorders.

Practical applications include:

1. Drug development: Targeting enzyme active sites
2. Diagnostics: Detecting abnormal immunoglobulins
3. Gene therapy: Correcting defective structural proteins
4. Vaccines: Engineering viral surface proteins

For example, insulin therapy relies on understanding protein structure-function relationships.

Special categories include:

• Prions (infectious proteins causing mad cow disease)
• Chromoproteins (light-sensitive opsins in vision)
• Motor proteins (myosin for muscle contraction)

These demonstrate protein diversity beyond traditional classifications.