Proteins: Structure, Sources, and Vital Functions

Posted on May 22, 2025 in Human Nutrition and Dietetics

Understanding Proteins: Functions and Classification

Key Protein Functions

  • Fatty Acid Synthesis: Enzymes catalyze the synthesis of fatty acids.
  • Reserve Proteins: Egg white ovalbumin serves as a reserve protein.
  • Transport: Hemoglobin transports oxygen in the blood.
  • Hormonal Regulation: The hormone insulin regulates glucose metabolism.

Major Dietary Protein Sources

  • Cereals: (e.g., rice, oats, corn, wheat)
  • Legumes: (e.g., beans, lentils, soybeans, peas)
  • Dairy: (e.g., milk, cheese, yogurt)
  • Nuts and Seeds: (e.g., sesame, almonds, amaranth, peanuts)
  • Meats

Benefits of Plant Proteins Compared to Animal Proteins

  • Less Acidifying: They are less acidifying to the blood, as they are accompanied by more minerals.
  • Fewer Purines: Contain fewer purines and are more easily eliminated.
  • Healthier Fats: Contain less saturated fat, are unsaturated, and contain no cholesterol.
  • Rich in Fiber: They provide dietary fiber.
  • Organ Friendly: Less burden on the liver and kidneys.
  • Easy to Digest: Generally easier to digest.
  • Economical & Sustainable: More economical for our budget and the planet.

Protein Classification

Simple Proteins (Holoproteins)

  • Globular Proteins:
    • Prolamins: Zein (maize), Gliadin (wheat), Hordein (barley)
    • Glutenins: Glutenin (wheat), Oryzanin (rice)
    • Albumins: Serum Albumin (blood), Ovalbumin (egg), Lactalbumin (milk)
    • Hormones: Insulin, Growth Hormone, Prolactin, Thyrotropin
    • Enzymes: Hydrolases, Oxidases, Ligases, Lyases, Transferases
  • Fibrous Proteins:
    • Collagen: Found in connective tissues and cartilage.
    • Keratins: Present in epidermal formations like hair, nails, feathers, and horns.
    • Elastin: Found in tendons and blood vessels.
    • Fibroin: Present in silk threads (from spiders, insects).

Heteroproteins (Conjugated Proteins)

  • Glycoproteins: (Non-protein component: Monosaccharides)
    • Ribonuclease
    • Mucoprotein
    • Antibodies
    • Luteinizing Hormone
  • Lipoproteins: (Non-protein component: Lipids)
    • High-density, low-density, and very low-density lipoproteins, which transport lipids in the blood.
  • Nucleoproteins: (Non-protein component: Nucleic Acids)
    • Chromatin (Nucleosomes)
    • Ribosomes
  • Metalloproteins: (Non-protein component: Metals)
    • Hemoglobin, Hemocyanin, Myoglobin (carry oxygen)
    • Cytochromes (transport electrons)
  • Phosphoproteins: (Non-protein component: Phosphate Groups)

Essential Amino Acids and Their Functions

  1. Isoleucine: Along with Leucine and Growth Hormone, involved in the formation and repair of muscle tissue.
  2. Leucine: Together with Isoleucine and Growth Hormone (HGH), involved in the formation and repair of muscle tissue.
  3. Lysine: One of the most important amino acids, involved in various functions such as growth, tissue repair, immune system antibody production, and hormone synthesis.
  4. Methionine: Assists in protein synthesis and is a primary limiting factor in dietary protein.
  5. Phenylalanine: Involved in the production of collagen (primarily for skin and connective tissue structure) and the formation of various neurohormones.
  6. Threonine: Along with Methionine and L-Aspartic acid, assists the liver in its detoxification functions.
  7. Tryptophan: Involved in growth and hormone production, especially adrenal gland secretion. Also involved in the synthesis of serotonin, a neurohormone linked to relaxation and sleep.
  8. Valine: Stimulates growth and tissue repair, maintains various systems, and supports nitrogen balance.

Key Properties of Proteins

  • Solubility: Proteins maintain solubility due to strong and weak bonds. However, increasing temperature and extreme pH levels can lead to a loss of solubility (denaturation).
  • Specificity: Each protein has a specific function determined by its primary structure.
  • Electrolytic Capacity: Determined through electrolysis: if proteins migrate to the positive pole, their radical is negatively charged, and vice versa.