Proteins

1. Importance of Proteins

• Structural Role – Collagen, keratin (skin, hair, nails)
• Enzymatic Function – Catalyze biochemical reactions (e.g., amylase)
• Transport & Storage – Hemoglobin (O? transport), ferritin (iron storage)
• Defense & Immunity – Antibodies (immune response)
• Signaling & Regulation – Hormones (insulin, glucagon)
• Movement – Actin, myosin (muscle contraction)

2. Classification of Proteins

A. Based on Structure

• Fibrous Proteins – Structural, insoluble (e.g., collagen, keratin)
• Globular Proteins – Functional, soluble (e.g., enzymes, hemoglobin)

B. Based on Composition

• Simple Proteins – Only amino acids (e.g., albumin)
• Conjugated Proteins – Amino acids + prosthetic group (e.g., glycoproteins, hemoproteins)

C. Based on Function

• Structural (collagen)
• Enzymes (lipase)
• Hormonal (insulin)
• Transport (hemoglobin)
• Defense (antibodies)

3. Amino Acids: Structure, Titration & Zwitterion Nature

• Basic Structure: Central α-carbon, amino (-NH?), carboxyl (-COOH), hydrogen, and R-group (side chain)
• Titration:
  • At low pH: Protonated form (+ve charge)
  • At high pH: Deprotonated form (-ve charge)
  • Isoelectric point (pI): pH where net charge = 0
• Zwitterion Nature:
  • Exists as a dipolar ion with both + and - charges in neutral pH

4. Structural Organization of Proteins

A. Primary Structure

• Linear sequence of amino acids
• Held by peptide bonds

B. Secondary Structure

• α-Helix – Coiled structure stabilized by H-bonds
• β-Pleated Sheet – Sheet-like structure stabilized by H-bonds

C. Tertiary Structure

• 3D folding due to hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bonds

D. Quaternary Structure

• Multiple polypeptide chains forming a functional protein (e.g., hemoglobin)