Protein Structure and Denaturation

Protein Structure

• Each subunit of a protein has its own primary, secondary, and tertiary structure.
• Subunits are held together by hydrogen bonds and van der Waals forces between nonpolar side chains.

Denaturation and Renaturation

• Denaturation is the process of modifying a protein's conformation without rupturing native peptide linkages.
• Denaturation inactivates protein functionality, decreases solubility, destroys biological activity, improves digestibility, and alters water binding ability.
• Denaturation can be achieved by applying heat, treatment with alcohols, heavy metals, or acids/bases.
• Renaturation is the conversion of a denatured protein back into its native 3D structure.
• Renaturation is sometimes reversible, but not as common or easy as denaturation.

Zwitter Ion

• A zwitter ion is a molecule with functional groups that have both positive and negative electrical charges, resulting in a net charge of zero.
• Amino acids are the best-known examples of zwitter ions, containing an amine group (basic) and a carboxylic group (acidic).
• The -NH2 group is the stronger base, picking up H+ from the -COOH group to form a zwitter ion.

Classification of Proteins based on Biological Functions

• Catalytic proteins: catalyze biochemical reactions in cells (e.g., enzymes and co-enzymes).
• Structural proteins: make various structural components of living beings (e.g., collagen in bones, elastin in ligaments, and keratin in hair and nails).
• Nutrient proteins: have nutritional value and provide nutrition when consumed (e.g., casein in milk).
• Regulatory proteins: regulate metabolic and cellular activities in cells and tissues (e.g., hormones).
• Defense proteins: provide defensive mechanisms against pathogens (e.g., antibodies and complement proteins).
• Transport proteins: transport nutrients and other molecules from one organ to another (e.g., hemoglobin).
• Storage proteins: store various molecules and ions in cells (e.g., ferritin storing iron).
• Contractile or mobile proteins: help in movement and locomotion of various body parts (e.g., actin, myosin, and tubulin).
• Toxic proteins: are toxic and can damage tissues (e.g., snake venom and bacterial exotoxins).

Classification of Proteins based on Chemical Composition

• Simple proteins:
  • Fibrous proteins (e.g., collagen).
  • Globular proteins (e.g., albumin).
• Conjugated proteins:
  • Nucleoproteins (protein linked to a nucleic acid, either DNA or RNA).
  • Glycoproteins (carbohydrate covalently linked to protein).
  • Lipoproteins (lipid and protein).
  • Phosphoproteins (protein with a phosphate group attached).
  • Chromoproteins (protein with a pigmented prosthetic group, e.g., hemoglobin).
  • Metalloproteins (enzyme proteins containing metallic elements, e.g., iron, copper, and cobalt).

Derived Proteins

• Derived proteins are derivatives of either simple or complex proteins resulting from the action of heat, enzymes, and chemicals.
• Examples include albumose (derived from albumins).

Non-Standard Amino Acids

• Besides the 20 amino acids present in protein structures, there are several other biologically important amino acids.
• Examples include:
  • 4-hydroxyproline and 5-hydroxylysine in collagen.
  • Methylated, phosphorylated, or acetylated amino acids in histones.
  • γ-Carboxyglutamic acid in certain plasma proteins involved in blood clotting.
  • D-Penicillamine (D-dimethylglycine), a metabolite of penicillin, used in chelation therapy of Wilson's disease.

This quiz covers the structure of proteins, including primary, secondary, and tertiary structures, and the process of denaturation and its effects on protein functionality and solubility.