PROTEIN

Proteins are essential biomolecules that play a vital role in virtually all biological processes. Here’s a comprehensive overview of proteins, their types, structure, and functions:


1. What Are Proteins?

  • Definition: Proteins are large, complex molecules made up of chains of amino acids. They are vital for the structure, function, and regulation of the body’s tissues and organs.
  • Composition: Proteins are composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sometimes sulfur (S).
  • Building Blocks: Proteins are made of amino acids (20 standard ones in humans).

2. Types of Proteins

Proteins can be categorized based on their function, structure, and origin:

a) Functional Classification

  1. Enzymes:
    • Catalyze biochemical reactions (e.g., amylase, DNA polymerase).
  2. Structural Proteins:
    • Provide support and shape (e.g., collagen, keratin).
  3. Transport Proteins:
    • Carry molecules across the body (e.g., hemoglobin, albumin).
  4. Signaling Proteins:
    • Transmit signals (e.g., insulin, growth hormone).
  5. Defensive Proteins:
    • Protect the body from pathogens (e.g., antibodies).
  6. Contractile Proteins:
    • Aid in movement (e.g., actin, myosin).
  7. Storage Proteins:
    • Store essential nutrients (e.g., ferritin, casein).

b) Classification Based on Origin

  • Animal Proteins: Found in meat, fish, eggs, and dairy.
  • Plant Proteins: Found in legumes, nuts, seeds, and grains.

c) Classification Based on Composition

  • Simple Proteins: Composed only of amino acids (e.g., albumin).
  • Conjugated Proteins: Contain a non-protein component (e.g., glycoproteins, hemoglobin).

3. Structure of Proteins

Proteins have a hierarchical structure that determines their function:

a) Primary Structure

  • Definition: The sequence of amino acids in a polypeptide chain.
  • Importance: Determines the protein’s overall shape and function.

b) Secondary Structure

  • Definition: Local folding of the polypeptide chain into specific structures.
  • Key Structures:
    1. Alpha Helix (α-helix): A coiled structure stabilized by hydrogen bonds.
    2. Beta Sheet (β-sheet): A sheet-like structure formed by hydrogen bonds between parallel or antiparallel chains.

c) Tertiary Structure

  • Definition: The three-dimensional folding of the entire polypeptide chain.
  • Stabilizing Forces:
    • Hydrogen bonds
    • Ionic bonds
    • Disulfide bridges
    • Hydrophobic interactions

d) Quaternary Structure

  • Definition: The arrangement of multiple polypeptide chains (subunits) into a single functional protein.
  • Example: Hemoglobin, composed of four subunits.

4. Protein Functions

Proteins have diverse roles in the body, including:

  1. Catalysis: Enzymes speed up biochemical reactions.
  2. Structure: Provide mechanical support (e.g., collagen in connective tissues).
  3. Transport: Facilitate movement of substances (e.g., oxygen by hemoglobin).
  4. Communication: Signal transduction and hormone function.
  5. Defense: Antibodies protect against pathogens.
  6. Energy Source: Proteins can be broken down for energy when necessary.

5. Protein Synthesis

  • Steps:
    1. Transcription: DNA is transcribed into messenger RNA (mRNA) in the nucleus.
    2. Translation: mRNA is translated into a polypeptide chain at the ribosome, using transfer RNA (tRNA) and ribosomal RNA (rRNA).

6. Protein Denaturation

  • Definition: Loss of a protein’s structure (and function) due to external factors like heat, pH changes, or chemicals.
  • Examples: Cooking an egg denatures the albumin protein, turning it solid.

7. Protein Deficiency

  • Symptoms: Weakness, fatigue, muscle loss, weakened immunity, and delayed wound healing.
  • Diseases:
    • Kwashiorkor: Severe protein deficiency in children.
    • Marasmus: Deficiency in protein and overall calories.

8. Dietary Proteins

  • Complete Proteins: Contain all nine essential amino acids (e.g., meat, fish, eggs, quinoa).
  • Incomplete Proteins: Lack one or more essential amino acids (e.g., rice, beans). Complementary proteins (e.g., rice + beans) can provide all essential amino acids.

9. Techniques for Protein Study

  • X-ray Crystallography: Determines protein structure.
  • Mass Spectrometry: Identifies and quantifies proteins.
  • Electrophoresis: Separates proteins based on size/charge.

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